Prolyl 4-hydroxylase and its role in collagen synthesis

Excessive accumulation of collagen in the extracellular matrix has a crucial role in fibrosis. Thus pharmacological inhibition of collagen deposition is likely to be beneficial for patients suffering from fibrotic disorders such as liver cirrhosis. Prolyl 4-hydroxylase catalyzes the formation of 4-hydroxyproline in collagens and other proteins with collagen-like amino acid sequences by the hydroxylation of proline residues in -X-Pro-Glysequences. The reaction products, 4-hydroxyproline residues, serve to stabilize the collagen triple helices under physiological conditions. Conversely, collagen chains that contain no 4-hydroxyproline cannot fold into triple helical molecules that are stable at body temperature. The prolyl 4-hydroxylase reaction therefore seems to be a particularly suitable target for the pharmological regulation of excessive collagen formation. The reaction catalyzed by prolyl 4-hydroxylase requires Fe2+, 2-oxoglutarate, O2 and ascorbate and involves an oxidative decarboxylation of 2-oxoglutarate. The active enzyme is an α2β2 tetramer that consists of two types of inactive monomer and has two catalytic sites. Some parts of the catalytic sites may be built up cooperatively of both the α and β subunits, but the α subunit appears to contribute the major part. The β subunit contains the carboxyl-terminal tetrapeptide sequence-Lys-Asp-Glu-Leu which is essential for the retention of a polypeptide within the lumen of the endoplasmic reticulum. Since the α subunit lacks the carboxyl-terminal retention signal, one function of the β subunit in the prolyl 4-hydroxylase tetramer may be to retain the enzyme within the endoplasmic reticulum. The β subunit has proved to be a multifunctional protein as it is identical to protein disulphide isomerase, a major cellular thyroid hormone binding protein and one of the two subunits of the microsomal triglyceride transfer protein, and highly similar to the glycosylation site binding component of oligosaccharyl transferase.