Lysine Hydroxylation and Crosslinking of Collagen

Collagen is a large family of structurally related proteins (1) and is the most abundant molecule in vertebrates. Type I collagen, the predominant genetic type in the collagen family, is a heterotrimeric molecule composed of two α1 chains and one α2 chain and is the major fibrillar component in most connective tissues. This molecule consists of three domains: amino-terminal nontriple helical (N-telopeptide), triple helical (helical), and carboxy-terminal nontriple helical (C-telopeptide) domains (Fig. 1). The central uninterrupted helical domain of each chain contains more than 300 repeats of (Gly-X-Y) sequence and represents more than 95% of the polypeptide. One of the characteristic features of collagen is its extensive posttranslational modifications, most of which are unique to collagen protein. Such modifications include hydroxylation of proline (Pro) and lysine (Lys) residues, glycosylation of specific hydroxylysine (Hyl) residues, oxidative deamination of the e-amino groups of Lys/Hyl in the telopeptide domains of the molecule, and subsequent intra/intermolecular crosslinking (2) (Fig. 1). The hydroxylation of Pro is catalyzed by prolyl 4-hydroxylase and prolyl 3-hydroxylase. The former reacts on Pro with the minimum sequence X-Pro-Gly and the latter appears to require a Pro-4-Hyp-Gly sequence (3). The presence of 4-Hyp (predominant form) is critical in stabilizing the triple helical conformation of collagen providing hydrogen bonds and water bridges (4). The content of Pro/Hyp in type I collagen is relatively high, representing 22–23 of the total amino acids, and 40–45% of Pro is hydroxylated (mostly 4-Hyp).