Structural Biochemistry/Proteins/Fibrous Proteins

Introduction

A Fibrous protein is a protein with an elongated shape. Fibrous proteins provide structural support for cells and tissues. There are special types of helices present in two fibrous proteins α-keratin and collagen. These proteins form long fibers that serve a structural role in the human body. Fibrous proteins are distinguished from globular proteins by their filamentous, elongated form. Also, fibrous proteins have low solubility in water compared with high solubility in water of globular proteins. Most of them play structural roles in animal cells and tissues, holding things together. Fibrous proteins have amino acid sequences that favour a particular kind of secondary structure which, in turn, confer particular mechanical properties on the proteins.

Examples

Collagen is a triple helix formed by three extended proteins that wrap around one another. Many rodlike collagen molecules are cross-linked together in the extracellular space to form collagen fibrils that have the tensile strength of steel. The striping on the collagen fibril is caused by regular repeating arrangement of the collagen molecules within the fibril.

Elastin polypeptide chains are cross-linked together to form rubberlike, elastic fibers. Each elastin molecule uncoils into a more extended conformation when the fiber is stretched and will recoil spontaneously as soon as the stretching force is relaxed.

	alpha helix	beta pleated sheet	triple helix
Hydrogen bonding	Peptide -C=O----HN-, Intrachain between, and n+4 residues Parallel to helix axis	Peptide -C=O-----HN- , Interchain, Perpendicular to chain axis	Peptide, -C-----HN- and -C=O-----HO- (hydroxyl from side chain of Hyp), Interchain
Residues	Many types, Small or uncharged residues, such as Ala, Leu, and Phe, most common; Pro never found	Mostly Gly, Ala, and Ser	Many types, Gly every third residue; Pro and Hyp common
Covalent cross-linking	Interchain disulfide cross-link	None	Interchain lysine-derived cross-links
Chain direction and aggregation	Four parallel right-handed alpha helices form a left-handed supercoil.	Antiparallel chains	Three parallel left-handed helices form a right-handed supercoil.