Relationship between water activity and catalytic activity of lipases in organic media. Effects of supports, loading and enzyme preparation.

The rates of synthesis of dodecyl decanoate in hexane have been measured as a function of water activity (aw), for various immobilised preparations of the lipases from Rhizomucor miehei and Candida rugosa. Only very large changes in the amount of enzyme adsorbed to the support affect the shape of the rate/aw profile; at the highest loadings the profiles tend to become somewhat flatter. A similar levelling can be obtained by pre-adsorbing an inert protein. The effect is probably due to adjacent protein molecules effectively replacing water; it does not simply reflect mass transfer or interfacial area limitation. The activity/aw profile was essentially the same with most supports tested: polypropylene, anion-exchange resin, celite, anion-exchange modified silica. A hydrophobic porous glass support reduced the rate somewhat at intermediate aw values with both enzymes; a polyamide material had this effect only with the lipase from Rh. miehei. The shape of the activity/aw profile was not affected by large differences in purity of the lipase preparation, but did differ between forms that probably differ in glycosylation. Overall, relatively few manipulations of the system can significantly affect the shape of the rate/aw profiles, which seem to be mainly an intrinsic property of the enzyme molecules used.