Collagen triple-helical regions

The triplet Cly-X-Y is constantly repeated in the sequence of the triple-helical regions— i. e., every third amino acid in such sequences is aglycine. Proline (Pro) is frequently found in positions X or Y the Y position is often occupied by 4-hydroxyproline (4Hyp), although 3-hydroxyproline (3Hyp) and 5-hydroxylysine (5Hyl) also occur. These hydroxylated amino acids are characteristic components of collagen. They are only produced after protein biosynthesis by hydroxylation of the amino acids in the peptide chain (see p. 62). 

Clostridium histolyticum collagenase. Clostridiopeptidase A. Digestion of native collagen in the triple helical region at Xaa-I-Gly... 

Fig. J Supramolecular organizations of some nonfibrillar collagens. The steps leading to the formation of supramolecular structures from monomeric molecules. Nand C A-terminus and C-terminus of the molecules closed circles and thick black lines indicate noncollagenous and triple-helical domains, respectively GAG glycosaminoglycan 7S is the triple-helical region of the Type IV collagen molecules involved in the formation of tetramers NCI is the carboxyl-terminal noncollagenous domain of Type IV collagen molecules involved in the formation of dimers.

Myers, J.C., Yang, H, D Ippolito, Presente, A., Miller, M.K and Dion, A.S. (1994) The triple-helical region of human type XIX collagen consists of multiple collagenous subdomains and exhibts limited sequence homology to al(X 4)./. Biol. Chem., 269, 18549-18557. 

One striking example of the complexity of biomineralization is the fact that bone is largely mineralized by hydroxyapatite entrained in collagen fibrils, where it is believed to nucleate at the acidic terminal regions of collagen triple helices. Many synthetic studies have produced the mineral on collagen fibrils but none has produced mineral in the fibrils. We are missing some key aspect of the process. 

Kilchherr, E., Hofmann, H., Steigemann, W., and Engel, J. (1985). Structural model of the collagen-like region of Clq comprising the kink region and the fibre-like packing of the six triple helices. / Mol. Biol. 186, 403-415. 

Fig. 5. Schematic representation of the collagen IV molecule, which consists of two al(IV) chains and one a2(IV) chain. The non-triple-helical interruptions of the triple helix are indicated by black bars. The cysteine residues (C) and lysine or hydroxylysine (K) residues putatively involved in intra- or intermolecular bonds are shown. CHO designates a N-giycosidically bound oligosaccharide chain. The subscript numerals indicate the number of residues in a distinct region, summarized for all three a-chains. P designates a main pepsin cleavage site. In interruption 13, the a2(IV) chain forms a 21-residue-long loop, stabilized by an interchain disulfide bridge. NCI, Noncollagenous domain 1 TH, triple-helical domain 7 S, carboxyl-terminal domain.

Fig. 8. The network structure of type IV collagen as deduced from electron microscopy of fragments solubilized with proteases (Timpl et at., 1981). The top left micrograph shows the short form of the 7 S region. In the top right, a more limited enzyme digestion allows a portion of the helical domain of each molecule to remain linked to the 7 S domain. The bottom left micrograph shows isolated dimeric NCI fragments and, at the lower right, dimeric triple helical material connected in the center by the NCI domain.

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