Collagen three-chain

Atomic Parameters for the Collagen Three-Chain Helix, y, IS, AND p IN A... 

Diagrammatic representation of the collagen three-chain helix. 

Calculated form factor and square of form factor for equatorial reflections of the collagen three-chain helix, calculated for cylindrical symmetry. 

Figure 14.2 Models of a collagen-like peptide with a mutation Gly to Ala in the middle of the peptide (orange). Each polypeptide chain is folded into a polyproline type II helix and three chains form a superhelix similar to part of the collagen molecule. The alanine side chain is accommodated inside the superhelix causing a slight change in the twist of the individual chains, (a) Space-filling model, (b) Ribbon diagram. Compare with Figure 14.1c for the change caused by the alanine substitution. (Adapted from J. Bella et al.. Science 266 75-81, 1994.)...

Collagen forms a triple helix, where three chains of connected amino acids form weak hydrogen bonds between the double-bonded oxygen atoms and the hydrogen atoms attached to the adjacent chain s nitrogens. The three chains then twist together like three cords in a rope. 

Collagen-like triple helices also occur within other proteins. One of these is protein Clq, a component of the complement system of blood (Chapter 31). This protein interacts with antibodies to trigger a major aspect of the immune response. Clq has six subunits, each made up of three different polypeptide chains of about 200 residues apiece. Beginning a few residues from the N termini, there are over 80 residues in each chain with collagen-like sequences. The three chains apparently form a triple helix within each subunit. However, the C-terminal portions are globular in nature.200 Collagen-like tails also are present on some forms of the enzyme acetylcholinesterase (see Chapter 12C,10). Tire extensins of plant cell walls contain 4-hydroxyproline and evidently have a structure... 

Within the extracellular space two procollagen peptidases act to cleave a 35-kDa peptide from the C terminus631 and a 20-kDa peptide from the N-terminal end of each of the three chains of the secreted procollagen. The amino acid composition of the peptides removed is quite unlike that of the remaining collagen monomer (also called tropocollagen) which contains one-third glycine and much proline. 

The classic example of a Type B repeat is that presented by the o-chains in collagen. Three such chains aggregate to form a triple-helical collagen molecule, but they can only do so if glycine is positioned in every third residue of the sequence (Hulmes, 1992). This is because glycine is located internally and, due to its size, is the only residue that can fit stereochemi-cally into the space available. In the Type I collagen Q-chain, the repeat occurs 338 times contiguously. 

---