Inspection of an acyl-enzyme intermediate in a lipase reaction by gas chromatography-mass spectrometry and modelling of the reaction mechanism

An acyl-enzyme intermediate proposed in the reaction mechanism of lipase was inspected by the exchange of oxygen between substrate (oleic acid) and solvent (¹⁸O-labelled water). Gas chromatography-mass spectrometry analysis supported the formation of an acyl-enzyme intermediate in the reaction mechanism through the observed incorporation of ¹⁸O into oleic acid. The incorporation did not occur in the absence of the lipase. When Ser residues were modified with diisopropylfluorophosphate, the activity of lipase OF 360 was markedly decreased. Photooxidation of His residues also resulted in a decrease in the activity of the lipase. Chemical modification studies suggested the existence of a charge relay system (Ser-His-Asp/Glu) in the active site. Based on these results, a model of the active site and reaction mechanism of the lipase are presented.