User:Drewbie/Macromolecules/Sugars

Sugars

The function of carbohydrates includes energy storage and providing structure. Sugars are carbohydrates, although there are carbohydrates that are not sugars. There are more carbohydrates on Earth than any other type of biomolecule. The simplest type of carbohydrate is a monosaccharide, which among other properties contains carbon, hydrogen, and oxygen, mostly in a ratio of 1:2:1 (generalized formula C_nH_2nO_n, where n is at least 3). Glucose, one of the most important carbohydrates, is an example of a monosaccharide. So is fructose, the sugar that gives fruits their sweet taste. Some carbohydrates (especially after condensation to oligo- and polysaccharides) contain less carbon relative to H and O, which still are present in 2:1 (H:O) ratio. Monosaccharides can be grouped into aldoses (having an aldehyde group at the end of the chain, e. g. glucose) and ketoses (having a keto group in their chain; e. g. fructose). Both aldoses and ketoses occur in an equilibrium between the open-chain forms and (starting with chain lengths of C4) cyclic forms. These are generated by bond formation between one of the hydroxy groups of the sugar chain with the carbon of the aldehyde or keto group in a semiacetal bond. This leads to saturated five-membered (furanoses) or six-membered (pyranoses) heterocyclic rings containing one O as the hetero-atom.

Chemical Reactions

Two monosaccharides can be joined together using dehydration synthesis, in which a hydrogen atom is removed from the end of one molecule and a hydroxyl group (—OH) is removed from the other; the remaining residues are then attached at the sites from which the atoms were removed. The H—OH or H2O is then released as a molecule of water, hence the term dehydration. The new molecule, consisting of two monosaccharides, is called a disaccharide and is conjoined together by a glycosidic or ether bond. The reverse reaction can also occur, using a molecule of water to split up a disaccharide and break the glycosidic bond; this is termed hydrolysis. The most well-known disaccharide is sucrose, ordinary sugar (in scientific contexts, called table sugar or cane sugar to differentiate it from other sugars). Sucrose consists of a glucose molecule and a fructose molecule joined together. Another important disaccharide is lactose, consisting of a glucose molecule and a galactose molecule. As most humans age, the production of lactase, the enzyme that hydrolyzes lactose back into glucose and galactose, typically decreases. This results in lactase deficiency, also called lactose intolerance.

Sugar polymers are characterised by having reducing or non-reducing ends. A reducing end of a carbohydrate is a carbon atom which can be in equilibrium with the open-chain aldehyde or keto form. If the joining of monomers takes place at such a carbon atom, the free hydroxy group of the pyranose or furanose form is exchanged with an OH-side chain of another sugar, yielding a full acetal. This prevents opening of the chain to the aldehyde or keto form and renders the modified residue non-reducing. Lactose contains a reducing end at its glucose moiety, whereas the galactose moiety form a full acetal with the C4-OH group of glucose. Saccharose does not have a reducing end because of full acetal formation between the aldehyde carbon of glucose (C1) and the keto carbon of fructose (C2).

When a few (around three to six) monosaccharides are joined together, it is called an oligosaccharide (oligo- meaning "few"). These molecules tend to be used as markers and signals, as well as having some other uses.

Many monosaccharides joined together make a polysaccharide. They can be joined together in one long linear chain, or they may be branched. Two of the most common polysaccharides are cellulose and glycogen, both consisting of repeating glucose monomers. Cellulose is made by plants and is an important structural component of their cell walls. Humans can neither manufacture nor digest it. Glycogen, on the other hand, is an animal carbohydrate; humans use it as a form of energy storage.

Glucose in Metabolism

Glucose is the major energy source in most life forms; a number of catabolic pathways converge on glucose. For instance, polysaccharides are broken down into their monomers (glycogen phosphorylase removes glucose residues from glycogen). Disaccharides like lactose or sucrose are cleaved into their two component monosaccharides. Glucose is mainly metabolized by a very important and ancient ten-step pathway called glycolysis, the net result of which is to break down one molecule of glucose into two molecules of pyruvate; this also produces a net two molecules of ATP, the energy currency of cells, along with two reducing equivalents in the form of converting NAD+ to NADH. This does not require oxygen; if no oxygen is available (or the cell cannot use oxygen), the NAD is restored by converting the pyruvate to lactate (e. g. in humans) or to ethanol plus carbon dioxide (e. g. in yeast). Other monosaccharides like galactose and fructose can be converted into intermediates of the glycolytic pathway. In aerobic cells with sufficient oxygen, like most human cells, the pyruvate is further metabolized. It is irreversibly converted to acetyl-CoA, giving off one carbon atom as the waste product carbon dioxide, generating another reducing equivalent as NADH. The two molecules acetyl-CoA (from one molecule of glucose) then enter the citric acid cycle, producing two more molecules of ATP, six more NADH molecules and two reduced (ubi)quinones (via FADH2 as enzyme-bound cofactor), and releasing the remaining carbon atoms as carbon dioxide. The produced NADH and quinol molecules then feed into the enzyme complexes of the respiratory chain, an electron transport system transferring the electrons ultimately to oxygen and conserving the released energy in the form of a proton gradient over a membrane (inner mitochondrial membrane in eukaryotes). Thereby, oxygen is reduced to water and the original electron acceptors NAD+ and quinone are regenerated. This is why humans breathe in oxygen and breathe out carbon dioxide. The energy released from transferring the electrons from high-energy states in NADH and quinol is conserved first as proton gradient and converted to ATP via ATP synthase. This generates an additional 28 molecules of ATP (24 from the 8 NADH + 4 from the 2 quinols), totaling to 32 molecules of ATP conserved per degraded glucose (two from glycolysis + two from the citrate cycle). It is clear that using oxygen to completely oxidize glucose provides an organism with far more energy than any oxygen-independent metabolic feature, and this is thought to be the reason why complex life appeared only after Earth's atmosphere accumulated large amounts of oxygen.

In vertebrates, vigorously contracting skeletal muscles (during weightlifting or sprinting, for example) do not receive enough oxygen to meet the energy demand, and so they shift to anaerobic metabolism, converting glucose to lactate (lactic acid). The liver regenerates the glucose, using a process called gluconeogenesis. This process is not quite the opposite of glycolysis, and actually requires three times the amount of energy gained from glycolysis (six molecules of ATP are used, compared to the two gained in glycolysis). Analogous to the above reactions, the glucose produced can then undergo glycolysis in tissues that need energy, be stored as glycogen (or starch in plants), or be converted to other monosaccharides or joined into di- or oligosaccharides.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates. They consist of one sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose, galactose, and ribose. Monosaccharides are the building blocks of disaccharides like sucrose (common sugar) and polysaccharides (such as cellulose and starch). Further, each carbon atom that supports a hydroxyl group (except for the first and last) is chiral, giving rise to a number of isomeric forms all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but they have different chemical and physical properties.

Structure

With few exceptions (e.g. deoxyribose), monosaccharides have the chemical formula (CH₂O)n and the chemical structure H(CHOH)nC=O(CHOH)mH. If n or m is zero, it is an an aldehyde and is termed an aldose, otherwise it is a ketone and is termed a ketose. Monosaccharides contain either a ketone or aldehyde functional group, and hydroxyl groups on most or all of the non-carbonyl carbon atoms.

Cyclic structure

Most monosaccharides form cyclic structures, which predominate in aqueous solution, by forming hemiacetals or hemiketals (depending on whether they are aldoses or ketoses) between an alcohol and the carbonyl group of the same sugar. Glucose, for example, readily forms a hemiacetal linkage between its carbon-1 and the hydroxyl group of its carbon-5. Since such a reaction introduces an additional stereogenic center, two anomers are formed (α-isomer and β-isomer) from each distinct straight-chain monosaccharide. The interconversion between these two forms is called mutarotation.

A common way of representing the cyclic structure of monosaccharides is the Haworth projection.

In Haworth projection, the α-isomer has the OH- of the anomeric carbon under the ring structure, and the β-isomer, has the OH- of the anomeric carbon on top of the ring structure. In chair conformation, the α-isomer has the OH- of the anomeric carbon in an axial position, whereas the β-isomer has the OH- of the anomeric carbon in equatorial position.

Isomerism

The total number of possible stereoisomers of one compound (n) is dependent on the number of stereogenic centers (c) in the molecule. The upper limit for the number of possible stereoisomers is n = 2c. The only carbohydrate without an isomer is dihydroxyacetone or DHA

Classification

Monosaccharides are classified according to three different characteristics: the placement of its carbonyl group, the number of carbon atoms it contains, and its chiral handedness. If the carbonyl group is an aldehyde, the monosaccharide is an aldose; if the carbonyl group is a ketone, the monosaccharide is a ketose. The smallest possible monosaccharide, those with three carbon atoms, are called trioses. Those with four are called tetroses, five are called pentoses, six are hexoses, and so on. These two systems of classification are often combined. For example, glucose is an aldohexose (a six-carbon aldehyde), ribose is an aldopentose (a five-carbon aldehyde), and fructose is a ketohexose (a six-carbon ketone).

Each carbon atom bearing a hydroxyl group (-OH), with the exception of the first and last carbons, are asymmetric, making them stereocenters with two possible configurations each (the -H and -OH may be on either side). Because of this asymmetry, a number of isomers may exist for any given monosaccharide formula. The aldohexose D-glucose, for example, has the formula (C·H2O)6, of which all but two of its six carbons atoms are chiral centers, making D-glucose one of 24 = 16 possible stereoisomers. In the case of a triose, there is one pair of possible stereoisomers, which are enantiomers and epimers. The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar. Because D sugars are biologically far more common, the D is often omitted all together.

Monosaccharides are classified by the number of carbon atoms they contain:

• Triose, 3 carbon atoms
• Tetrose, 4 carbon atoms
• Pentose, 5 carbon atoms
• Hexose, 6 carbon atoms
• Heptose, 7 carbon atoms
• Octose, 8 carbon atoms
• Nonose, 9 carbon atoms

Monosaccharides are classified the type of keto group they contain:

• Aldose, -CHO (aldehyde)
• Ketose, C=O (ketone)

Monosaccharides are classified according to their molecular configuration at carbon 2:

• D, configuration as in D-glyceraldehyde
• L, configuration as in L-glyceraldehyde

All these classifications can be combined, resulting in names like D-aldohexose or ketotriose.

Conformation

Pyran and furan, after which the pyranose and furanose configurations of monosaccharides are named.

The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, forming a heterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called furanose and pyranose forms, respectively, and exist in equilibrium with the straight-chain form.

During the conversion from straight-chain form to cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon, becomes a chiral center with two possible configurations: the oxygen atom may take a position either above or below the plane of the ring. The resulting possible pair of stereoisomers are called anomers. In the α anomer, the -OH substituent on the anomeric carbon rests on the opposite side of the ring from the CH2OH attached to the asymmetric carbon furthest from the anomeric carbon. The alternative form, in which the CH2OH and the anomeric hydroxyl are on the same side of the plane of the ring, is called the β anomer. Because the ring and straight-chain forms readily interconvert, both anomers exist in equilibrium.

List of monosaccharides

This is a list of some common monosaccharides, not all are found in nature - some have been synthesized:

• Trioses:
  • Aldotriose: glyceraldehyde
  • Ketotriose: dihydroxyacetone
• Tetroses:
  • Aldotetrose: erythrose and threose
  • Ketotetrose: erythrulose
• Pentoses:
  • Aldopentoses: arabinose, lyxose, ribose and xylose
  • Ketopentoses: ribulose and xylulose
• Hexoses:
  • Aldohexoses: allose, altrose, galactose, glucose, gulose, idose, mannose and talose
  • Ketohexoses: fructose, psicose, sorbose and tagatose
• Heptoses:
  • Keto-heptoses: mannoheptulose, sedoheptulose
• Octoses: octolose, 2-keto-3-deoxy-manno-octonate
• Nonoses: sialose

Reactions

1. Formation of acetals. 2. Formation of hemiacetals and hemiketals. 3. Formation of ketals.

Glucose

Glucose (Glc), a monosaccharide (or simple sugar), is one of the most important carbohydrates in biology. The cell uses it as a source of energy and metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration in both prokaryotes and eukaryotes.

Two isomers of the aldohexose sugars are known as glucose, only one of which (D-glucose) is biologically active. This form (D-glucose) is often referred to as dextrose (dextrose monohydrate), especially in the food industry. This article deals with the D-form of glucose. The mirror-image of the molecule, L-glucose, cannot be used by cells.

Structure

Glucose (C₆H₁₂O₆) contains six carbon atoms and an aldehyde group and is therefore referred to as an aldohexose. The glucose molecule can exist in an open-chain (acyclic) and ring (cyclic) form (in equilibrium), the latter being the result of an intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an intramolecular hemiacetal. In water solution both forms are in equilibrium, and at pH 7 the cyclic one is the predominant. As the ring contains five carbon atoms and one oxygen atom, which resembles the structure of pyran, the cyclic form of glucose is also referred to as glucopyranose. In this ring, each carbon is linked to an hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH₂OH group.

Isomers

Aldohexose sugars have 4 chiral centers giving 2⁴ = 16 optical stereoisomers. These are split into two groups, L and D, with 8 sugars in each. Glucose is one of these sugars, and L and D-glucose are two of the stereoisomers. Only 7 of these are found in living organisms, of which D-glucose (Glu), D-galactose (Gal) and D-mannose (Man) are the most important. These eight isomers (including glucose itself) are all diastereoisomers in relation to each other and all belong to the D-series.

An additional asymmetric center at C-1 (called the anomeric carbon atom) is created when glucose cyclizes and two ring structures, called anomers, can be formed — α-glucose and β-glucose. They differ structurally in the orientation of the hydroxyl group linked to C-1 in the ring. When D-glucose is drawn as a Haworth projection, the designation α means that the hydroxyl group attached to C-1 is below the plane of the ring, β means it is above. The α and β forms interconvert over a timescale of hours in aqueous solution, to a final stable ratio of α:β 36:64, in a process called mutarotation.

Production

Natural

1. Glucose is one of the products of photosynthesis in plants and some prokaryotes.
2. In animals and fungi, glucose is the result of the breakdown of glycogen, a process known as glycogenolysis. In plants - the breakdown substrate is starch.
3. In animals, glucose is synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate and glycerol, by a process known as gluconeogenesis.

Commercial

Glucose is produced commercially via the enzymatic hydrolysis of starch. Many crops can be used as the source of starch. Maize, rice, wheat, potato, cassava, arrowroot, and sago are all used in various parts of the world. In the United States, cornstarch (from maize) is used almost exclusively.

This enzymatic process has two stages. Over the course of 1-2 hours near 100°C, these enzymes hydrolyze starch into smaller carbohydrates containing on average 5-10 glucose units each. Some variations on this process briefly heat the starch mixture to 130 °C or hotter one or more times. This heat treatment improves the solubility of starch in water, but deactivates the enzyme, and fresh enzyme must be added to the mixture after each heating.

In the second step, known as saccharification, the partially hydrolyzed starch is completely hydrolyzed to glucose using the glucoamylase enzyme from the fungus Aspergillus niger. Typical reaction conditions are pH 4.0–4.5, 60 °C, and a carbohydrate concentration of 30–35% by weight. Under these conditions, starch can be converted to glucose at 96% yield after 1–4 days. Still higher yields can be obtained using more dilute solutions, but this approach requires larger reactors and processing a greater volume of water, and is not generally economical. The resulting glucose solution is then purified by filtration and concentrated in a multiple-effect evaporator. Solid D-glucose is then produced by repeated crystallizations.

Function

We can speculate on the reasons why glucose, and not another monosaccharide such as fructose (Fru) , is so widely used in evolution/the ecosystem/metabolism. Glucose can form from formaldehyde under abiotic conditions, so it may well have been available to primitive biochemical systems. Probably more important to advanced life is the low tendency of glucose, by comparison to other hexose sugars, to non-specifically react with the amino groups of proteins. This reaction (glycation) reduces or destroys the function of many enzymes. The low rate of glycation is due to glucose's preference for the less reactive cyclic isomer. Nevertheless, many of the long-term complications of diabetes (e.g., blindness, kidney failure, and peripheral neuropathy) are probably due to the glycation of proteins or lipids. In contrast, enzyme-regulated addition of glucose to proteins by glycosylation is often essential to their function.

As an energy source

Glucose is a ubiquitous fuel in biology. It is used as an energy source in most organisms, from bacteria to humans. Use of glucose may be by either aerobic or anaerobic respiration (fermentation). Carbohydrates are the human body's key source of energy, through aerobic respiration, providing approximately 4 kilocalories (17 kilojoules) of food energy per gram. Breakdown of carbohydrates (e.g. starch) yields mono- and disaccharides, most of which is glucose. Through glycolysis and later in the reactions of the Citric acid cycle (TCAC), glucose is oxidized to eventually form CO2 and water, yielding energy, mostly in the form of ATP. The insulin reaction, and other mechanisms, regulate the concentration of glucose in the blood. A high fasting blood sugar level is an indication of prediabetic and diabetic conditions.

Glucose in glycolysis

Use of glucose as an energy source in cells is via aerobic or anaerobic respiration. Both of these start with the early steps of the glycolysis metabolic pathway. The first step of this is the phosphorylation of glucose by hexokinase to prepare it for later breakdown to provide energy.

The major reason for the immediate phosphorylation of glucose by a hexokinase is to prevent diffusion out of the cell. The phosphorylation adds a charged phosphate group so the glucose 6-phosphate cannot easily cross the cell membrane. Irreversible first steps of a metabolic pathway are common for regulatory purposes.

As a precursor

Glucose is critical in the production of proteins and in lipid metabolism. Also, in plants and most animals, it is a precursor for vitamin C (ascorbic acid) production. It is modified for use in these processes by the glycolysis pathway.

Glucose is used as a precursor for the synthesis of several important substances. starch soulution Starch, cellulose, and glycogen ("animal starch") are common glucose polymers (polysaccharides). Lactose, the predominant sugar in milk, is a glucose-galactose disaccharide. In sucrose, another important disaccharide, glucose is joined to fructose. These synthesis processes also rely on the phosphorylation of glucose by the first step of glycolysis.

Sources and absorption

All major dietary carbohydrates contain glucose, either as their only building block, as in starch and glycogen, or together with another monosaccharide, as in sucrose and lactose. In the lumen of the duodenum and small intestine the oligo- and polysaccharides are broken down to monosaccharides by the pancreatic and intestinal glycosidases. Glucose is then transported across the apical membrane of the enterocytes by SLC5A1 and later across their basal membrane by SLC2A2 (ref). Some of glucose goes directly to fuel brain cells and erythrocytes, while the rest makes its way to the liver and muscles, where it is stored as glycogen, and to fat cells, where it is stored as fat. Glycogen is the body's auxiliary energy source, tapped and converted back into glucose when there is need for energy.

Fructose

Fructose (or levulose) is a simple sugar (monosaccharide) found in many foods and is one of the three most important blood sugars along with glucose and galactose. Honey; tree fruits; berries; melons; and some root vegetables, such as beets, sweet potatoes, parsnips and onions, contain fructose, usually in combination with sucrose and glucose. Fructose is also derived from the digestion of sucrose, a disaccharide consisting of glucose and fructose that is broken down by enzymes during digestion. Fructose is the sweetest naturally occurring sugar, estimated to be twice as sweet as sucrose.

Structure

Fructose is a levorotatory monosaccharide and an isomer of glucose (C₆H₁₂O₆). Pure fructose has a sweet taste similar to cane sugar, but with a "fruity" aroma. Although fructose is a hexose (6 carbon sugar), it generally exists as a 5-member hemiketal ring (a furanose). This structure is responsible for the long metabolic pathway and high reactivity compared to glucose.

The first -OH points the opposite way from the second and third -OH.

Isomerism

D-Fructose has the same configuration at its penultimate carbon as D-glyceraldehyde. Fructose is sweeter than glucose due to its stereomerism structure.

Health effects

Fructose absoprtion occurs via the GLUT-5 (fructose only) transporter, and the GLUT2 transporter, for which it competes with glucose and galactose. A deficiency of GLUT 5 may result in excess fructose carried into the lower intestine where it provides nutrients for the existing flora, which produce gas. It may also cause water retention in the intestine. These effects may lead to bloating, excessive flatulence, loose stools, and even diarrhea depending on the amounts eaten and other factors.

Fructose has been hypothesized to cause obesity, elevated LDL cholesterol and triglycerides, leading to metabolic syndrome. Unlike animal experiments, some human experiments have failed to show a correlation between fructose consumption and obesity. Short term tests, lack of dietary control, and lack of a non-fructose consuming control group are all confounding factors in human experiments. However, there are now a number of reports showing correlation of fructose consumption to obesity, especially central obesity which is generally regarded as the most dangerous type. (Wylie-Rosett, 2004)(Havel, 2005)(Bray, 2004) (Dennison, 1997)

Fructose also chelates minerals in the blood. This effect is especially important with micronutrients such as copper, chromium and zinc. Since these solutes are normally present in small quantities, chelation of small numbers of ions may lead to deficiency diseases, immune system impairment and even insulin resistance, a component of type II diabetes (Higdon).

"The medical profession thinks fructose is better for diabetics than sugar," says Meira Field, Ph.D., a research chemist at the USDA, in the Fall 2001 issue of the quarterly magazine of the Weston A. Price Foundation, "but every cell in the body can metabolize glucose. However, all fructose must be metabolized in the liver. The livers of the rats on the high fructose diet looked like the livers of alcoholics, plugged with fat and cirrhotic." This is not entirely true as certain other tissues do use fructose directly, notably the cells of the intestine, and sperm cells (for which fructose is the main energy source).

Fructose is a reducing sugar, as are all monosaccharides. The spontaneous addition of single sugar molecules to proteins, known as glycation, is a significant cause of damage in diabetics. Fructose appears to be as dangerous as glucose in this regard and so does not seem to be the answer for diabetes (McPherson et al, 1988) This may be an important contribution to senescence and many age-related chronic diseases (Levi & Werman 1998).

Fructose is used as a substitute for sucrose (common sugar) because it is less expensive and has little effect on measured blood glucose levels. Often Fructose is consumed as high fructose corn syrup which is corn syrup (glucose) which has been enzymatically treated, by the enzyme glucose isomerase, to convert a portion of the glucose into fructose thus making it sweeter. This is done to such a degree to yield corn syrup with an equivalent sweetness as sucrose by weight. While most carbohydrates have around the same amount of calories, fructose is sweeter, so manufacturers may use less fructose to get the same sweetness. The free fructose present in fruits, their juice, and honey is responsible for the greater sweetness of these natural sugar sources.

Sucrose

Sucrose (common name: table sugar, also called saccharose) is a disaccharide (glucose + fructose) with the molecular formula C12H22O11. Its systematic name is α-D-glucopyranosyl-(1→2)-β-D-fructofuranose. It is best known for its role in human nutrition.

Physical and chemical properties

Pure sucrose is most often prepared as a fine, colorless, odorless crystalline powder with a pleasing, sweet taste. Large crystals are sometimes precipitated from water solutions of sucrose onto a string (or other nucleation surface) to form rock candy, a confection.

Like other carbohydrates, sucrose has a hydrogen to oxygen ratio of 2:1. It consists of two monosaccharides, α-glucose and fructose, joined by a glycosidic bond between carbon atom 1 of the glucose unit and carbon atom 2 of the fructose unit. What is notable about sucrose is that unlike most polysaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the nonreducing end of the other. The effect of this inhibits further bonding to other saccharide units.

Sucrose melts and decomposes at 186 °C to form caramel, and when combusted produces carbon, carbon dioxide, and water. Water breaks down sucrose by hydrolysis, however the process is so gradual that it could sit in solution for years with negligible change. If the enzyme sucrase is added however, the reaction will proceed rapidly.

Reacting sucrose with sulfuric acid dehydrates the sucrose and forms elemental carbon, as demonstrated in the following equation:

C12~H22O11 + H2SO4 catalyst → 12 C + 11 H2O

Commercial production and use

Sucrose is the most common food sweetener in the industrialized world, although it has been replaced in industrial food production by other sweeteners such as fructose syrups or combinations of functional ingredients and high intensity sweeteners.

Sucrose is the most important sugar in plants, and can be found in the phloem sap. It is generally extracted from sugar cane or sugar beet and then purified and crystallized. Other (minor) commercial sources are sweet sorghum and sugar maples.

Sucrose is ubiquitous in food preparations due to both its sweetness and its functional properties; it is important to the structure of many foods including biscuits and cookies, ice cream and sorbets, and also assists in the preservation of foods. As such it is common in many processed and so-called “junk foods.”

Sugar as a macronutrient

Human beings, and in fact most other mammals—except members of the cat family, who lack the ability to taste sweetness—will typically accept food sweetened with sucrose even if they are not hungry (see Dessert).

In mammals, sucrose is very readily digested in the stomach into its component sugars, by acidic hydrolysis. The glucose and fructose are then rapidly absorbed into the bloodstream in the small intestine. Undigested sucrose passing into the intestine is also broken down by sucrase or isomaltase enzymes, which are located in the membrane of the microvilli lining the duodenum. These products are also transferred rapidly into the bloodstream.

Sucrose is digested by the enzyme invertase in bacteria and some animals.

Acidic hydrolysis can also be used in laboratories.

In human nutrition

Sucrose is an easily assimilated macronutrient that provides a quick source of energy to the body, provoking a rapid rise in blood glucose upon ingestion. However, pure sucrose is not normally part of a human diet balanced for good nutrition, although it may be included sparingly to make certain foods more palatable.

Overconsumption of sucrose has been linked with some adverse health effects. The most common is dental caries or tooth decay, in which oral bacteria convert sugars (including sucrose) from food into acids that attack tooth enamel. Sucrose, as a pure carbohydrate, has a high food energy content (4 kilocalories per gram or 17 kilojoules per gram), and thus can make a diet hypercaloric even in small amounts, contributing to obesity.

The rapidity with which sucrose raises blood glucose can cause problems for people suffering from defects in glucose metabolism, such as persons with hypoglycemia or diabetes mellitus. Sucrose can contribute to development of the metabolic syndrome. An experiment with rats that were fed a diet one-third of which was sucrose may serve as a model for the development of the metabolic syndrome. The sucrose first elevated blood levels of triglycerides, which induced visceral fat and ultimately resulted in insulin resistance.

Disaccharides

Disaccharides are the simplest polysaccharides. They are composed of two monosaccharide units bound together by a covalent glycosidic bond formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other, so the formula of unmodified disaccharides is C₁₂H₂₂O₁₁. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.

Sucrose is the most abundant disaccharide and the main form in which carbohydrates are transported in plants. It is composed of one glucose molecule and one fructose molecule. The systematic name for sucrose, O-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things:

• Its monosaccharides: glucose and fructose
• Their ring types: glucose is a pyranose, and fructose is a furanose
• How they're linked together: the oxygen on the number 1 carbon (C1) of α-glucose is linked to the C2 of fructose.
• The -oside suffix indicates that the anomeric carbon of both monosacchaides participates in the glycosidic bond.

Lactose, a disaccharide composed of one galactose molecule and one glucose molecule, occurs naturally only in milk. The systematic name for lactose is O-β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include maltose (two glucoses linked α-1,4) and cellobiose (two glucoses linked β-1,4).

Chemistry

The two monosaccharides are bonded via a condensation reaction that leads to the loss of a molecule of water. The glycosidic bond can be formed between any hydroxyl group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties.

Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting. And it is well known as one of the three types of carbohydrates.

Common disaccharides

• Sucrose (known as table sugar, cane sugar, saccharose, or beet sugar) is composed of glucose + fructose.
• Lactose (milk sugar) is glucose + galactose.
• Maltose is produced during the malting of barley. It is a glucose + glucose disaccharide, where its glucose monomers are connected with a α(1→4) bond.
• Trehalose is present in fungi and insects. It is also a glucose + glucose disaccharide, where its glucose monomers are connected with a α(1→1)α bond. Trehalose has been successfully produced at an industial scale by enzymatic treatment of starch for use as a food ingredient.
• Cellobiose is another of the glucose + glucose disaccharides, where its glucose monomers are connected with a β(1→4) bond. Maltose and cellobiose are hydrolysis products of the polysaccharides, starch and cellulose, respectively.

Oligosaccharides

An oligosaccharide is a saccharide polymer containing a small number (typically three to nine) of component monosaccharide units bound together by glycosidic bonds. They are generally found either O- or N-linked to compatible amino acid side chains in proteins or to lipid moieties.

Oligosaccharides are often found as a component of glycoproteins or glycolipids and as such are often used as chemical markers, often for cell recognition. An example is ABO blood type specificity. A and B blood types have two different oligosaccharide glycolipids embedded in the cell membranes of the red blood cells, AB-type blood has both, while O blood type has neither.

Mannan-oligosaccharides (MOS) are widely used in animal feed to encourage gastrointestinal health and performance. They are normally obtained from the yeast cell walls of Saccharomyces cerevisiae. Some brand names are: Bio-Mos, SAF-Mannan, Y-MOS and Celmanax.

Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the disaccharides mentioned above, the trisaccharide raffinose and the tetrasaccharide stachyose.

Oligosaccharides are found as a common form of protein post-translational modification. Such post-translational modifications include the Lewis oligosaccharides responsible for blood group incompatibilities, the alpha-Gal epitope responsible for hyperacute rejection in xenotransplanation, and O-GlcNAc modifications.

Therapeutic effects

When oligosaccharides are consumed, the undigested portion serves as food for “friendly” bacteria, such as Bifidobacteria and Lactobacillus species.

Clinical studies have shown that administering FOS, GOS, or inulin can increase the number of these friendly bacteria in the colon while simultaneously reducing the population of harmful bacteria.[citation needed]

Other benefits noted with FOS, GOS, or inulin supplementation include increased production of beneficial short-chain fatty acids such as butyrate, increased absorption of calcium and magnesium, and improved elimination of toxic compounds.[citation needed]

Because FOS, GOS, and inulin improve colon function and increase the number of friendly bacteria, one might expect these compounds would help relieve the symptoms of irritable bowel syndrome. However, a double-blind trial found no clear benefit with FOS supplementation (2 grams three times daily) in patients with this condition.[citation needed]

Experimental studies with FOS in animals suggest a possible benefit in lowering blood sugar levels in people with diabetes and in reducing elevated blood cholesterol and triglyceride levels.

In a double-blind trial of middle-aged men and women with elevated cholesterol and triglyceride levels, supplementation with inulin (10 grams per day for eight weeks) significantly reduced insulin concentrations, suggesting an improvement in blood-glucose control, and significantly lowered triglyceride levels.[citation needed]

In a preliminary trial, administration of FOS (8 grams per day for two weeks) significantly lowered fasting blood-sugar levels and serum total-cholesterol levels in patients with type 2 (non-insulin-dependent) diabetes.[citation needed]

However, in another trial, people with type 2 diabetes supplementing with FOS (15 grams per day) for 20 days found no effect on blood-glucose or lipid levels[citation needed]. Because of these conflicting results, more research is needed to determine the effect of FOS and inulin on diabetes and lipid levels.

Several double-blind trials[citation needed] have looked at the ability of FOS or inulin to lower blood cholesterol and triglyceride levels. These trials have shown that in people with elevated total cholesterol or triglyceride levels, including people with type 2 (adult onset) diabetes, FOS or inulin (in amounts ranging from 8 to 20 grams daily) produced significant reductions in triglyceride levels. However, the effect on cholesterol levels was inconsistent. In people with normal or low cholesterol or triglyceride levels, FOS or inulin produced little effect.

FOS and inulin are found naturally in Jerusalem artichoke, burdock, chicory, leeks, onions, and asparagus. FOS products derived from chicory root contain significant quantities of inulin, a fiber widely distributed in fruits, vegetables and plants. Inulin is a significant part of the daily diet of most of the world’s population. FOS can also be synthesized by enzymes of the fungus Aspergillus niger acting on sucrose. GOS is naturally found in soybeans and can be synthesized from lactose (milk sugar). FOS, GOS, and inulin are available as nutritional supplements in capsules, tablets, and as a powder.

Not all natural oligosaccharides occur as components of glycoproteins or glycolipids. Some, such as the raffinose series, occur as storage or transport carbohydrates in plants. Others, such as maltodextrins or cellodextrins, result from the microbial breakdown of larger polysaccharides such as starch or cellulose.

Polysaccharides

Polysaccharides (sometimes called glycans) are relatively complex carbohydrates.

They are polymers made up of many monosaccharides joined together by glycosidic links. They are therefore very large, often branched, molecules. They tend to be amorphous, insoluble in water, and have no sweet taste.

When all the constituent monosaccharides are of the same type they are termed homopolysaccharides; when more than one type of monosaccharide is present they are termed heteropolysaccharides.

Polysaccharides have a general formula of Cn(H2O)n-1 where n is usually a large number between 200 and 2500. The general formula can also be represented as (C6H10O5)n where n=100-3000.

Polysaccharides represent an important class of biological polymer. Examples include starch, cellulose, chitin, glycogen, callose, laminarin, xylan, and galactomannan.

Starch

Starches are glucose polymers in which glucopyranose units are bonded by alpha-linkages. It is made up of a mixture of Amylose and Amylopectin. Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units. Starches are insoluble in water. They can be digested by hydrolysis catalyzed by enzymes called amylases, which can break the alpha-linkages. Humans and other animals have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet.

Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the other being amylopectin.

Tests

Starch solution is used to test for elemental iodine. A blue/black color indicates the presence of iodine in starch solution. The details of this reaction are not yet fully known, but it is thought that the iodine (I3- and I5- ions) fits inside the coils of amylose, the charge transfers between the iodine and the starch, and the energy level spacings in the resulting complex correspond to the absorption spectrum in the visible light region. A 0.4% w/w solution is the standard concentration for a dilute starch indicator solution. It is made by adding 4 grams of soluble starch to 1 litre of heated water; the solution is cooled before use (starch-iodine complex becomes unstable at temperatures above 35°C). This complex is often used in redox titrations: in presence of an oxidizing agent the solution turns blue, in presence of reducing agent blue color disappears because I5- ions break up into iodine and iodide.

Under the microscope, starch grains show a distinctive Maltese cross effect (also known as 'extinction cross' and birefringence) under polarized light.

Starch derivatives

Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two. The extent of conversion is typically quantified by dextrose equivalent (DE), which is roughly the fraction of the glycoside bonds in starch that have been broken. Food products made in this way include

• Maltodextrin, a lightly hydrolyzed (DE 10–20) starch product used as a bland-tasting filler and thickener.
• Dextrose (DE 100), commercial glucose, prepared by the complete hydrolysis of starch.
• High fructose syrup, made by treating dextrose solutions to the enzyme glucose isomerase, until a substantial fraction of the glucose has been converted to fructose. In the United States, high fructose corn syrup is the principal sweetener used in sweetened beverages because fructose tastes sweeter than glucose, and less sweetener may be used.

Glycogen

Glycogen (commonly known as animal starch although this name is inaccurate) is a polysaccharide that is the principal storage form of glucose (Glc) in animal and human cells. Glycogen is found in the form of granules in the cytosol in many cell types. Hepatocytes (liver cells) have the highest concentration of it - up to 8% of the fresh weight in well fed state, or 100–120 g in an adult. In the muscles, glycogen is found in a much lower concentration (1% of the muscle mass), but the total amount exceeds that in liver. Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain glial cells in the brain and white blood cells. Glycogen plays an important role in the glucose cycle.

Structure and biochemistry

Glycogen is a highly branched polymer that is better described as a dendrimer of about 60,000 glucose residues and has a molecular weight between 106 and 107 daltons (4.8 million approx.). Most of Glc units are linked by α-1,4 glycosidic bonds, approximately 1 in 12 Glc residues also makes -1,6 glycosidic bond with a second Glc, which results in the creation of a branch. Glycogen does not possess a reducing end: the 'reducing end' glucose residue is not free but is covalently bound to a protein termed glycogenin as a beta-linkage to a surface tyrosine residue. Glycogenin is a glycosyltransferase and occurs as a dimer in the core of glycogen. The glycogen granules contain both glycogen and the enzymes of glycogen synthesis (glycogenesis) and degradation (glycogenolysis). The enzymes are nested between the outer branches of the glycogen molecules and act on the non-reducing ends. Therefore, the many non-reducing end-branches of glycogen facilitate its rapid synthesis and catabolism.

Function and regulation of liver glycogen

As a carbohydrate meal is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin. Glucose from the portal vein enters the liver cells (hepatocytes). Insulin acts on the hepatocytes to stimulate the action of several enzymes, including glycogen synthase. Glucose molecules are added to the chains of glycogen as long as both insulin and glucose remain plentiful. In this postprandial or "fed" state, the liver takes in more glucose from the blood than it releases.

After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. About four hours after a meal, glycogen begins to be broken down to be converted again to glucose. Glycogen phosphorylase is the primary enzyme of glycogen breakdown. For the next 8–12 hours, glucose derived from liver glycogen will be the primary source of blood glucose to be used by the rest of the body for fuel.

Glucagon is another hormone produced by the pancreas, which in many respects serves as a counter-signal to insulin. When the blood sugar begins to fall below normal, glucagon is secreted in increasing amounts. It stimulates glycogen breakdown into glucose even when insulin levels are abnormally high.

Glycogen in muscle and other cells

Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well. Muscle cells lack the ability to pass glucose into the blood, so the glycogen they store internally is destined for internal use and is not shared with other cells, unlike liver cells.

Glycogen and marathon running

Due to the body's ability to hold no more than around 2,000 kcal of glycogen, marathon runners commonly experience a phenomenon referred to as "hitting the wall" around the 20 mile (32 km) point of a marathon. (Approximately 100 kcal are utilized per mile, depending on the size of the runner and the race course.) When experiencing glycogen debt, runners often experience fatigue.

Disorders of glycogen metabolism

The most common disease in which glycogen metabolism becomes abnormal is diabetes, in which, because of abnormal amounts of insulin, liver glycogen can be abnormally accumulated or depleted. Restoration of normal glucose metabolism usually normalizes glycogen metabolism as well.

In hypoglycemia caused by excessive insulin, liver glycogen levels are high, but the high insulin level prevents the glycogenolysis necessary to maintain normal blood sugar levels. Glucagon is a common treatment for this type of hypoglycemia.

Various inborn errors of metabolism are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown. These are collectively referred to as glycogen storage diseases.

Glycogen Breakdown

Glycogen undergoes a 3-step process to become G6P, which can then participate in glycolysis or the pentose phosphate pathway.

The overall reaction for the 1st step is:

Glycogen (n residues) + Pi <-----> Glycogen (n-1 residues)+ G1P

Here, glycogen phosphorylase cleaves the bond at the 1 position by substitution of a phosphoryl group.

The 2nd step involves the debranching enzyme that moves the remaining glucose units to another non-reducing end. This results in more glucose units available to glycogen phosphorylase (step 1)

The 3rd and last stage converts G1P to G6P through the enzyme phosphoglucomutase.

Glycogen Synthesis

Glycogen synthesis consists of a 3 step reaction that differs from that of glycogen breakdown. It is endergonic, meaning that glycogen is not synthesized without the input of energy. This is contrary to glycogen breakdown, which is exergonic. Hence, these 2 processes occur in separate pathways.

Step 1: G1P + UTP ---> UDPG + 2 pi.

Overall reaction is exergonic due to the free energy released from ppi hydrolysis. This coupling is necessary to overcome the unfavourable glycogen synthesis.

Step 2: UDPG + glycogen(n residues) ----> UDP + glycogen (n+1 residues)

In this step, glycogen synthase aids in the transfer of glucose units from UDPG to glycogen according to the equation

The activity of the enyzme is under allosteric control; hence the binding of ligand at 1 site in a protein affects the binding of other ligands at other sites in the same protein.

Step 3: Glycogen synthase only generates alpha (1-4) linked glucose polymer. Hence, the glycogen branching enzyme transfers blocks of 7 glucose chain to a new branching point alpha (1-6).

Cellulose

Cellulose (C₆H₁₀O₅)n is a long-chain polymeric polysaccharide carbohydrate, of beta-glucose. It forms the primary structural component of green plants. The primary cell wall of green plants is made of cellulose; the secondary wall contains cellulose with variable amounts of lignin. Lignin and cellulose, considered together, are termed lignocellulose, which (as wood) is argued to be one of the most common biopolymers on Earth (chrysolaminarin is often argued to be the other). Only one group of animals, the tunicates, has the ability to create and use cellulose. Some acetic acid bacteria are also known to synthesize cellulose.

Cellulose is a common material in plant cell walls and was first noted as such in 1533. It occurs naturally in almost pure form in cotton fiber. In combination with lignin and hemicellulose, it is found in all plant material. Cellulose is the most abundant form of living terrestrial biomass [1] with an estimated annual production of 1.5x1012 Tonnes [3].

Some animals, particularly ruminants and termites, can digest cellulose with the help of symbiotic micro-organisms - see methanogen. Cellulose is not digestible by humans, and is often referred to as 'dietary fiber' or 'roughage', acting as a hydrophilic bulking agent for faeces.

Cellulose is the major constituent of paper; further processing can be performed to make cellophane and rayon, and more recently Modal, a textile derived from beechwood cellulose. Cellulose is used within the laboratory as a solid-state substrate for thin layer chromatography, and cotton linters, is used in the manufacture of nitrocellulose, historically used in smokeless gunpowder.

Chemistry

Cellulose monomers (β-glucose) are linked together through ß1→4 glycosidic bonds by condensation. This is in contrast to the α 1→4 glycosidic bonds present in other carbohydrates like starch. Cellulose is a straight chain polymer: unlike starch, no coiling occurs, and the molecule adopts an extended rod-like conformation. In microfibrils, the multiple hydroxyl groups on the glucose residues hydrogen bond with each other, holding the chains firmly together and contributing to their high tensile strength. This strength is important in cell walls, where they are meshed into a carbohydrate matrix, helping keep plant cells rigid.

In contrast to starch, cellulose is also much more crystalline. Whereas starch has an crystalline to amorphous transition at 60 -70 °C in water as in cooking, it takes 320°C and 25 MPa for cellulose to become amorphous in water.

Given a cellulose material, the portion that does not dissolve in a 17.5% solution of sodium hydroxide at 20 °C is α cellulose, which is true cellulose; the portion that dissolves and then precipitates upon acidification is β cellulose; and the proportion that dissolves but does not precipitate is γ cellulose.

Cellulose can be assayed using a method described by Updegraff in 1969, where the fiber is dissolved in acetic and nitric acid, and allowed to react with anthrone in sulfuric acid. The resulting coloured compound is assayed spectrophotometrically at a wavelength of approximately 635 nm.

Biosynthesis

Cellulose is synthesized in higher plants by enzyme complexes localized at the cell membrane called cellulose synthase. Cellulose synthase utilizes UDP-D-glucose precursors to generate microcrystalline cellulose. The enzyme complex contains three different subunits encoded by CesA genes in an unknown stoichiometry. Separate sets of CesA genes are involved in primary and secondary cell wall biosynthesis. Cellulose synthesis requires chain initiation and elongation, and the two processes are separate. CesA glucosyltransferase initiates cellulose polymerization using a steroid primer, 'sitosterol-beta-glucoside' and UDP-glucose.[4] A cellulase may function to cleave the primer from the mature chain.

Breakdown

The ability to breakdown cellulose is not possessed by mammals. Typically, this ability is possessed only by certain bacteria (which have specific enzymes) like Cellulomonas etc, and which are often the flora on the gut walls of ruminants like cows and sheep, or by fungi, which in nature are responsible for cycling of nutrients. The enzymes utilized to cleave the glycosidic linkage in cellulose are glycoside hydrolases including endo-acting cellulases and exo-acting glucosidases. Such enzymes are usually secreted as part of multienzyme complexes that may include dockerins and cellulose binding modules.

Derivatives

The hydroxyl groups of cellulose can be partially or fully reacted with various chemicals to provide derivates with useful properties. Cellulose esters and cellulose ethers are the most important commercial materials. In principle, though not always in current industrial practice, cellulosic polymers are renewable resources.

Among the esters are cellulose acetate and cellulose triacetate, which are film- and fiber-forming materials that find a variety of uses. The inorganic ester nitrocellulose was initially used as an explosive and was an early film forming material.

Ether derivatives include

• Ethylcellulose, a water-insoluble commercial thermoplastic used in coatings, inks, binders, and controlled-release drug tablets;
• Hydroxypropyl cellulose;
• Carboxymethyl cellulose;
• Hydroxypropyl methyl cellulose, E464, used as a viscosity modifier, gelling agent, foaming agent and binding agent;
• Hydroxyethyl methyl cellulose, used in production of cellulose films.

Many cellulolytic bacteria break down cellulose into shorter linked chains known as cellodextrins

Chitin

wp chitin

Chitin is a hard, semitransparent material that is found in many places in the natural world. For example, chitin is the main component of the shells of crustaceans, such as the crab, lobster, and shrimp. Many insects, such as ants and beetles, have a covering made from chitin. Chitin is even found in the cell walls of some fungi, molds and yeast. It has several medical and industrial uses.

Detailed Description

Chitin is one of the main components in the cell walls of fungi, the exoskeletons of insects and other arthropods, and in some other animals. It is a polysaccharide; it is constructed from units of N-acetylglucosamine (more completely, N-acetyl-D-glucos-2-amine). These are linked together in β-1,4 fashion (in a similar manner to the glucose units which form cellulose). In effect chitin may be described as cellulose with one hydroxyl group on each monomer replaced by an acetylamine group. This allows for increased hydrogen bonding between adjacent polymers, giving the polymer increased strength.

In its unmodified form, chitin is translucent, pliable and resilient, and quite tough. In arthropods, however, it is frequently modified, by being embedded in a hardened proteinaceous matrix, which forms much of the exoskeleton. The difference between unmodified and modified chitinous exoskeleton can be seen by comparing the body wall of a caterpillar to a beetle, for example.

Chitin is an unusual substance as it is a naturally occurring polymer. Its breakdown is conducted by bacteria which have receptors to simple sugars from the decomposition of chitin. If chitin is detected they then produce enzymes to digest the chitin by reducing it to simple sugars and ammonia.

Chitin is closely related to chitosan (a more water-soluble derivative of chitin).

Chitin is also closely related chemically to cellulose, in that it is a long unbranched chain of glucose derivatives. Both materials contribute structure and strength, protecting the organism.

Medicial Uses

Chitin's properties as a tough, and strong material make it favourable as surgical thread. Its biodegradibility also means it wears away with time as the wound heals.

Chitin also has some unusual properties in that it accelerates healing in wounds in humans. Therefore, chitin is used as a wound-healing agent.

Acidic polysaccharides

Acidic polysaccharides are polysaccharides that contain carboxyl groups, phosphate groups and/or sulfuric ester groups.

Bacterial Capsule Polysaccharides

Pathogenic bacteria commonly produce a thick, mucous-like, layer of polysaccharide. This "capsule" cloaks antigenic proteins on the bacterial surface that would otherwise provoke an immune response and thereby lead to the destruction of the bacteria. Capsular polysaccharides are water soluble, commonly acidic, and have molecular weights on the order of 100-1000 kDa. They are linear and consist of regularly repeating subunits of one ~ six monosaccharides. There is enormous structural diversity; nearly two hundred different polysaccharides are produced by E. coli alone. Mixtures of capsular polysaccharides, either conjugated or native are used as vaccines.

Bacteria and many other microbes, including fungi and algae, often secrete polysaccharides as an evolutionary adaptation to help them adhere to surfaces and to prevent them from drying out. Humans have developed some of these polysaccharides into useful products, including xanthan gum, dextran, gellan gum, and pullulan.

Metabolism

Carbohydrates require less water to digest than proteins or fats and are the most common source of energy. Proteins and fat are vital building components for body tissue and cells, and thus it could be considered advisable not to deplete such resources by necessitating their use in energy production. However, carbohydrates are not essential nutrients. It is possible for the body to obtain all its energy from protein and fats. The brain cannot burn fat and needs glucose for energy, but the body can make this glucose from protein. Carbohydrates, like proteins, contain 4 kilocalories per gram while fats contain 9 kilocalories and alcohol contains 7 kilocalories per gram.

Based on evidence for risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% of dietary energy from carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates. [reference needed]

Catabolism

There are two major metabolic pathways of monosaccharide catabolism:

1. Glycolysis 2. Citric acid cycle

Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called Glycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism.

Anabolism

Complex carbohydrates are assembled from sugar nucleotides by the action of glycosyltransferases