The ester hydrolytic and synthetic activities of X-prolyl dipeptidyl peptidase from Streptococcus thermophilus : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Biochemistry at Massey University, New Zealand
X-prolyl dipeptidyl peptidase (EC 18.104.22.168), or PepX, is a dipeptidase found in most dairy lactic acid bacteria that hydrolyses N-terminal dipeptides from larger peptides where proline is the residue penultimate to the scissile bond. It has recently been found that PepX will also catalyse the hydrolysis of some chromogenic esters and synthesise esters via an acyltransferase mechanism that uses ethanol as the acceptor molecule and tributyrin as the donor molecule. In this study, the pepX gene from Streptococcus thermophilus strain B2513 was cloned and sequenced. This sequence was found to differ in several positions from the recently published pepX sequence of S.thermophilus strain ACA-DC4. None of the observed substitutions occurred in the catalytic domain of the enzyme, all being localised to the C-terminal β-sheet domain. An activity assay using a chromogenic peptide substrate with tributyrin as an was used to prove that PepX binds peptide substrates and acylglycerides at the same binding site, implying that the same catalytic machinery carries out both peptide hydrolysis and activities involving acylglycerides. PepX was found to form esters only from the acylglyceride tributyrin, and was not active on any of the larger triglycerides tested. The chemical mechanism for this ester formation is proposed to involve the direct transfer of an acyl group from the donor to an acceptor, rather than acyl hydrolysis followed by the separate transfer of a carboxylic acid product onto an acceptor, as the enzyme does not form esters when provided with butyric acid and ethanol. PepX was found to be incapable of hydrolysing milkfat and tributyrin in aqueous solution. This contrasts with the ability of PepX to hydrolyse the synthetic ester p-nitrophenyl butyrate, which probably is a reflection of the lability of the ester bond in this substrate. The results of this study show that PepX is a peptidase that has a secondary acyltransferase activity, with no hydrolase activity on natural acylglyceride substrates.