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Lysyl hydroxylase (LH), with three isoenzymes in vertebrates, catalyzes the formation of hydroxylysine by acting on -X-Lys-Gly- triplets in the collagenous domains of proteins of the collagen superfamily and also in -X-Lys-Ala- or -X-Lys-Ser- sequences in the telopeptides located at the ends of the polypeptide chains in some fibril-forming collagens. The hydroxylysine residues are essential for the stability of collagen crosslinks and act as carbohydrate attachment sites. The extent of lysine hydroxylation varies between collagen types, between tissues in the same collagen type and in certain diseases, suggesting that the LH isoenzymes may have different substrate specificities. We studied here the hydroxylation of synthetic peptides representing various hydroxylation sites in type I and IV collagens by purified recombinant LHs in vitro and of a recombinant full-length type I procollagen chain coexpressed with each LH in insect cells. All three LHs hydroxylated peptides representing collagenous sequences of type I and IV collagens, although with different K(m) and V(max) values. Furthermore, all three hydroxylated the collagenous domain of the coexpressed type I procollagen chain to a similar extent. None of the isoenzymes hydroxylated peptides representing the N and C telopeptides of type I collagen, but LH2, unlike the other two isoenzymes, hydroxylated the N telopeptide in the coexpressed procollagen chain. Hydroxylation of the telopeptide lysines by LH2 thus occurs only in the context of a long peptide. These data provide the first direct evidence that LH2 is a specific telopeptide hydroxylase, while all three LHs act on collagenous sequences.

Bruck syndrome is a rare autosomal recessive connective tissue disorder characterized by fragile bones, joint contractures, scoliosis, and osteoporosis. The telopeptides of bone collagen I are underhydroxylated in these patients, leading to abnormal collagen cross-linking. Three point mutations in lysyl hydroxylase (LH) 2, the enzyme responsible for the hydroxylation of collagen telopeptides, have been identified in Bruck syndrome. As none of them affects the residues known to be critical for LH activity, we studied their consequences at the molecular level by analyzing the folding and catalytic properties of the corresponding mutant recombinant polypeptides. Folding and oligomerization of the R594H and G597V mutants were abnormal, and their activity was reduced by95% relative to the wild type. The T604I mutation did not affect the folding properties, although the mutant retained only approximately 8% activity under standard assay conditions. As the reduced activity was caused by a 10-fold increase in the K(m) for 2-oxoglutarate, the mutation interferes with binding of this cosubstrate. In the presence of a saturating 2-oxoglutarate concentration, the activity of the T604I mutant was approximately 30% of that of the wild type. However, the T604I mutant did not generate detectable amounts of hydroxylysine in the N-terminal telopeptide of a recombinant procollagen I chain when coexpressed in insect cells. The low activity of the mutant LH2 polypeptides is in accordance with the markedly reduced extent of collagen telopeptide hydroxylation in Bruck syndrome, with consequent changes in the cross-linking of collagen fibrils and severe abnormalities in the skeletal structures.