Aspects on the catalysis of lipase from porcine pancreas (type VI-s) in aqueous media: development of ion-pairs

this article reports a first contribution for the elucidation of catalytic mechanism of lipase from porcine pancreas, type vi-s (ppl), in hydrolyzing an ester substrate in aqueous media. the conclusions were based on the ph-profiles of michaelis-menten parameters kcat/km, kcat and km, as well as on the absolute temperature profile of kcat/km, obtained during the hydrolysis of p-nitrophenyl laurate by ppl. it was found that (a) ppl performs catalysis by means of ion pairs formed either as ser152-ο-/his263-im +h and/or carbonyl-ο-/his263-im +h, (b) the parameter kcat/km equals to k1 and thus es is formed and destroyed in the course of a series of consecutive reactions governed by the dynamic constant ks = k2/k1, and (c) the hydrolysis of substrate is assisted by a hydrogen bond developed between deprotonated asp176 and the positively charged imidazole of his263 across a pka-value 3.85, necessary for efficient catalysis.