Proteins Based on Shape and Solubility Quiz

Fibrous Proteins

• Long, rod-shaped, tough, and insoluble in water
• Consist mostly of a single secondary structure
• Examples: keratins, which have structural and protective roles
• Silk (fibroin) mainly contains ß-sheets; wool (keratin) is mostly alpha-helix

Collagen

• Most abundant vertebrate protein
• Major structural protein in the extracellular matrix
• Typical collagen: Long, inelastic, stiff, triple-stranded helical structure
• ~ 3000 Å long and 15 Å in diameter
• Consists of three polypeptide chains called alpha-chains
• Each alpha-chain is over 1000 residues long, and they may be identical or different
• Alpha-chains are left-handed polypeptide helices with 3.3 amino acid residues per turn
• Three alpha-chains wind around each other in a characteristic right-handed triple helix
• Vertebrates have about 25 types of alpha-chains, coded by different genes, forming at least 27 types of collagen molecules

Collagen Types

• Types I, II, and III collagen make up 90% of all collagens
• Type I collagen: predominant in bones, tendons, and skin
• Type II collagen: found in cartilage
• Type III collagen: found in blood vessels

Amino Acid Sequence

• Amino acid sequence in alpha-chain often Gly-X-Y
• X is often proline and Y is often 3- or 4-hydroxyproline or 5-hydroxylysine
• Glycine makes up about one-third of the amino acid residues, provides flexibility
• Proline and hydroxyproline provide rigidity and together comprise up to 30% of collagen

Collagen Structure

• Hydrogen bonds are absent within a strand of collagen
• The helix is stabilized by steric repulsion of the pyrrolidine rings of proline and hydroxyproline residues
• The pyrrolidine rings of proline and hydroxyproline residues prevent interference with each other in the helical conformation
• The collagen helix forms a superhelical cable with three strands winding around each other
• The stability of the collagen superhelix is maintained by hydrogen bonds between the peptide NH groups of glycine residues and the CO groups of residues on the other chains
• The hydroxyl groups of hydroxyproline residues also participate in hydrogen bonding within collagen

Hydroxylation

• Hydroxylation by enzymes: prolyl-4-hydroxylase and lysyl-hydroxylase
• Ascorbate (vitamin C) acts as a cofactor
• Hydroxylation stabilizes collagen
• Vitamin C deficiency causes scurvy

Collagen Related Disease

• Scurvy disease: results from vitamin C deficiency, which leads to inactive hydroxylase enzyme, no hydroxylation of proline and glycine, and no triple helix formation
• One of the oldest neurotoxic diseases known to Man, results from excessive consumption of the chickling pea, Lathyrus sativus, and certain related species