Lipase and other unusual reactions

One group of enzymes which will tolerate solvents is the lipases. These catalyse the hydrolysis of fatty acid esters. Their substrates range from simple acetate esters of ethanol and glycerol to the esters of pectic acids and of steroid alcohols. Many of these esters are insoluble in water, but are soluble in organic solvents, and some of the lipases reflect this by showing their greatest activity in solvents containing only very small amounts (1-4% v/v) of water. 

The catalytic action of the lipase may not be the same in the wet solvent as it is in water. The enzyme from Candida cylindrocarpon catalyses the hydrolysis of a different range of esters in the solvent, and it also catalyses ester transfer. This transesterification reaction has some potential for the upgrading of food oils, whose value is dependent on the nature and the position of the fatty acids which esterify the glycerol. Some cocoa butter equivalents are now manufactured from palm oil by exchanging a proportion of its esterified 

There is some justification for supposing that the lipases are not unique in their behaviour towards solvents. Much cellular biochemistry catalyses the reactions of substrates which are only slightly soluble in water. This is particularly true of those enzymes which carry out their reactions in cellular membranes. These reactions often prove very intractable when they are studied out of their cellular environment, and to some extent this may originate in our difficulties in handling enzymes in solvents other than water. It may be that the need to push enzymatic catalysis into these other solvents will also extend our knowledge of catalysis in the cellular membrane. 

Examples of this sort are not unusual, although most biochemists would accept the high specificity of enzyme catalysis as axiomatic. However, this does not imply that there is a one-to-one relationship between the enzyme and the specific reaction. A good case can be made for suggesting that the enzymes which are responsible for some metabolic pathways are multifunctional (section 6.3.2). This feature of their catalysis, coupled with our poor understanding of their behaviour outside of a wholly aqueous environment, suggests that enzyme catalysis may have more to offer the organic chemist than we presently realize. 

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