Lipases from different sources vary widely in dependence of catalytic activity on water activity

We have measured the rates of esterification in hexane catalysed by suspended immobilised lipases (triacylglycerol acylhydrolase, EC 3.1.1.3), with pre-equilibration to known thermodynamic water activity (a(w)). There were important differences between the enzymes from five different microbes in their retention of activity at low a(w). That from Rhizomucor miehei showed over 40% maximal activity at an a(w) of 0.12, and that from Rhizopus niveus was also fairly active at low a(w). Lipases from other sources required higher a(w) values to show good activity, increasing in the sequence Humicola sp., Candida rugosa and Pseudomonas cepacia. The behaviour was generally similar to two very different support materials, anion-exchange resin and macroporous polypropylene. Comparison of the sequences of the homologous enzymes from Rh. miehei, Rh. niveus and Humicola sp. suggests that changes in charged residues in the 'hinge and lid' region of the structure may be significant in low a(w) tolerance.