Cross-linking in collagen and elastin

Abstract

The fibrous proteins, collagen and elastin, are cross-linked by a unique mechanism based on aldehyde formation from lysine or hydroxylysine side chains. The only enzyme required, lysyl oxidase, serves both collagen elastin. Over the years a variety of cross-linking amino acids have been identified in both proteins. Desmosine and isodesmosine, the first discovered, are the main cross-linking residues of mature elastin. The main cross-linking residues of mature collagen, however, have proved elusive despite research over fifteen years that has revealed an assortment of borohydride-reducible intermediates that disappear as collagen fibrils mature. Lately, research has centered on the mature, nonreducible cross-links. Two pathways of cross-linking can be defined in the fibrillar collagens, one based on allysine, the lysine-derived aldehyde, the other on hydroxyallysine, the hydroxylysine-derived aldehyde. There is now strong evidence that trifunctional, 3-hydroxypyridinium residues are the adult cross-links on the hydroxyallysine route, which predominates in most connective tissues except skin. Two forms of 3-hydroxypyridinium cross-link have been identified. The major one, hydroxylysyl pyridinoline (HP), embodies three hydroxylysines, and a less abundant form, lysyl pyridinoline (LP) embodies two hydroxylysines and one lysine. Sensitive methods have been developed for quantifying these naturally fluorescent amino acids in hydrolysates of whole connective tissue. The mature cross-linking residues on the allysine pathway are still unknown, although complex structures capable of linking three or more molecules are suspected. Histidine is probably a component on this pathway, although there is not yet general agreement on this point. Four homologous loci of cross-linking are evident in molecules of types I, II, and III collagens. Two are aldehyde sites, one in each telopeptide region. The other two sites are hydroxylysines symmetrically placed at about 90 residues from each end of the molecule. When collagen molecules pack into fibrils, these latter sites in the helical region align and react with telopeptide aldehydes in adjacent molecules staggered axially by 4D periods (4 x 67 nm). Characteristic amino acid sequences are conserved about the helical cross-linking sites in α chains of types I, II, and III collagens. Type IV collagen of basement membranes is also believed to be cross-linked by the aldehyde-mediated mechanism, but the mature cross-linking structures are unknown. Little is known, also, of aldehyde-mediated cross-linking in type V or other minor collagens. Knowledge of the chemistry of elastin cross-linking has changed little in the last five years. The main advances are in determining 90% of the amino acid sequence of the protein, including flanking sequences for several of the desmosine and isodesmosine residues. Sequences around lysinonorleucine cross-links are not yet known. Inhibited cross-linking is part of the underlying pathology in several inborn diseases of the collagen molecule. At least three subtypes of Ehlers-Danlos Syndrome, cases of Marfan's Syndrome, and an X-linked form of cutis laxa exhibit collaen cross-linking defects secondary to the primary lesion. This article concentrates on recent research progress. Several comprehensive reviews of the field are available.