Chapter 1. Introduction

Collagens are a superfamily of glycoproteins, which are found in the extracellular matrix of almost all tissues of the body. They play an important role in maintaining the structures of various tissues and organs, and in regulating cellular behavior.

The biosynthesis of collagen is a multi-step process that involves a large number of post-translational modifications, including the hydroxylation of lysyl residues and glycosylation of certain hydroxylysyl residues to galactosylhydroxylysyl and glucosylgalactosylhydroxylysyl residues. The enzymes catalyzing these reactions are lysyl hydroxylase (EC 1.14.11.4, LH), hydroxylysyl galactosyltransferase (EC 2.4.1.50, GT), and galactosylhydroxylysyl glucosyltransferase (EC 2.4.1.66, GGT). The reactions take place within the endoplasmic reticulum.

The number of free or glycosylated hydroxylysyl residues varies greatly among different collagen types and within the same collagen type in different tissues and even within the same tissue in different physiological and pathological states. Three isoforms of lysyl hydroxylase (LH1, LH2, LH3) have been identified so far from human and mouse tissues. The expression patterns of the isoforms in different tissues differ from each other. GT and GGT have been purified from chicken embryos and their catalytic and molecular properties have been characterized as well. However, the genes or cDNAs have not yet been cloned.

In this work, the mRNA expression levels of the LH isoforms together with the major collagen types I, III, IV, V and the α subunit of prolyl 4-hydroxylase were studied in different human cells and the correlation coefficients between them were analyzed. Both the human LH isoform, LH3, and the C. elegans LH were found to be multifunctional, possessing also GT and GGT activities. These enzymes were characterized and the amino acids critical for GT and GGT activities associated with the human LH3 were identified.