MOLECULAR REASONS FOR LIPASE-SENSITIVITY AGAINST ACETALDEHYDE

The molecular reasons for the sensitivity of microbial lipases towards acetaldehyde, emerging as unavoidable by-product from acyl transfer r eactions employing vinyl esters as acyl donors, were shown to be assoc iated with specific properties of lysine residues. Since the mechanism of deactivation involves the formation of Schiff bases at the lysine epsilon-amino groups, the relative reactivity (i.e, nucleophilicity) o f each residue was estimated by using an electronic (pK(a) value) and a steric parameter (accessible surface area of the side chain). Sensit ive lipases, as from Candida rugosa and Geotrichum candidum, possess s everal lysine residues that have high pK(a) values(> 12) and are highl y exposed to the solvent (surface areas of 210-220 Angstrom). In contr ast, the lysine groups of stable lipases like from Rhizomucor miehei, Candida antarctica B and Pseudomonas glumae have moderate pK(a) values (up to 11.6) and are rather buried (surface areas of 130-150 Angstrom (2)). A close investigation of Candida rugosa lipase revealed that the most exposed lysine residues are located in the lid region (Lys75 and Lys85). The data suggest that Lys75, which is involved in fixing the lid in its open conformation, is presumably the prime target for deact ivation by acetaldehyde.