Structural Biochemistry/Cell Organelles/Plant Cell/Cellulose

General Information

Cellulose is a structural glucose polysaccharide component of the plant cell wall, along with hemicellulose and pectins. It has β-linkages that allows it to form fibers with higher tensile strength than what α-linkages present in other glucose polysaccharides such as glycogen and starch allows. Whereas the α-linkages creates an open helix form that allows easy access of the sugar, β-linkages form straight, parallel chains connected to each other through hydrogen bonds, which makes it a lot more difficult to access.

Energy

Plants transform solar energy and carbon dioxide (CO₂) into organic carbons as a primary source of energy through the process of photosynthesis and carbon fixation. In particular, the carbons in cellulose are created from left over triose phosphates that were not used up during the reaction and have condensed.

These fixated organic carbons in the cellulose structure provide valuable energy that researches today hope to tap into as a renewable fuel source. However, breaking down the cellulose to get the glucose is difficult and requires an extensive enzyme system.

Cellulosome

The plant cell wall contains cellulose, hemicellulose, and pectins. The chemical and physical complexity restrict enzymes from attacking it. In order to break this down, the enzymes cellulase, hemicellulase, and pectinase need to work in conjunction with one another. In anaerobic bacteria, research has shown that there is a multienzyme complex called cellulosome which contains all three enzymes, and which breaks down the plant cell wall most effectively.

Bacteria assemble cellulosomes using Cohesin-Dockerin interactions. These protein interactions are essential for the cellulosomes to successfully destruct the cell wall. Dockerin is a protein found in catalytic components while cohesin is found on scaffoldins. The two proteins interact by hydrophobic forces.

The cellulosome is considered nature's most detailed nanomachine. It is a cell-bound complex consisting of many enzymes. It is located on the cell surface and deconstructs the plant cell wall via carbohydrate binding molecules (CBM's). There are three types of CBMs: Type A, Type B, and Type C. Type A is responsible for interacting with cellulose itself. Type B binds to the inner regions of single glycan chains. Type C recognizes the small saccharides.

Bacterial cellulosomes are the most prominent type of cellulosomes. They are categorized into two groups: those that are assembled with many types of scaffoldin and those that are assembled with only a single primary scaffoldin.

Fungal cellulosomes are found in the gastrointestinal tract of herbivores. They are not as developed as bacterial cellulosomes.

Biological Functions of Cellulosome

Cellulosomes known as the proficient nanomachine in nature are cell bound multi-enzyme complexes that break down cellulose and hemicelluloses. They are very important in the process of carbon turnover. The cellulosome complexes require highly ordered proteins (the interactions between cohesions and dockerins) to assembly cellulases and hemicellulases into a scaffold structure. The protein interactions between cohesion and dockerin play an important role in cellulosome assembly and the attachment of cellulosome to the surface of cells while remaining flexible to provide a stable catalytic synergy.

One function of cellulosomes is breaking down plant’s structural polysaccharides. It is hypothesize that the constraints of the cellulosomes system created by bacteria and fungi caused the deconstruction of plant cell wall to become more and more efficient. The splicing of the plant-cell wall involves the addition of enzymes on to a macromolecule complex will increase the effectiveness of the synergistic interactions between the cellulosome complexes. The cellulosomes are then amplified by the enzyme-substrate targeting of the scaffolding-borne CBM.

The rate at which cellulose are hydrolyzed depends on the source of the polysaccharide, the period of the assay, and the deconstruction process. Although most bacteria integrate themselves into cellulosomes, Anaerocellum thermophilum, an efficient cellulose-degrading bacterium does not. It produces large amounts of enzymes which improves the synergistic interactions. The decrease the rate of the hydrolysis of cellulose due to the insertion of transposon into the CipA gene suggests that the hydrolysis reaction of cellulose depends on the addition of cellulases to cellulosome. Another function of the cellulosome is maintaining the enzyme system on the surface of the bacteria. This will impact the bacteria’s ability to use mono-and oligosaccharides from the cell wall. According to Lynd and colleagues, when the cellulosome is on the outside of the outer membrane of the bacteria, the hydrolysis process of cellulose increase immensely compared to when the cellulosome is inside the bacteria cell. A possible explanation of this high rate is cellulose’s activities were inhibited due to the release of glucose and cellobiose. The hydrolysis process drives the interaction between the multi-enzyme complex and the glycoside hydrolases on the bacteria’s surface. The function of cellulosome remains unknown in many organisms because there is no evidence proving that the cellulosome gets attached to the membrane of organisms.

How Cellulosomes Bind to the Plant Cell Wall

The degrading process of the plant cell wall requires the participation of the noncatalytic carbohydrate-binding modules (CBMs). The three types of CBMs are type A CBMs, type B CBMs, and type C CBMs. Type A CBMs collaborate with cellulose (crystalline polysaccharides), type B CBMs attaches itself to the glycan chains, and type C CBMs identifies the ends of the polysaccharides. The binding of the cellulosome to the cell wall of plants is facilitated by CBM3 (type A CBM) which can be found in the scaffoldins. CBM3 attached itself to the surface of the cellulose. When CBM3 gets attached to cellulose, supramolecular targeting is needed in order for the catalyze the reaction of enzyme complexes binding to their corresponding substates. The reason for this occurrence is because the cell wall of plants is made up of the many interacting polysaccharides.

Resources

Fontes, C., Gilbert, H., (2010, April 7). Cellulosomes: Highly Efficient Nanomachines Designed to Deconstruct Plant Cell Wall Complex Carbohydrates. Annual Review of Biochemistry, 1-2, 5, 9, 15, 19, 79, 655-681. Retrieved October 22, 2010 from www.annualreviews.org.

Berg, Jeremy M. "Biochemistry", Fifth Edition, W.H. Freeman and Company, N.Y., 2002