Strategies for Improving the Lipase-Catalyzed Preparation of Chiral Compounds

Lipases (triacylglycerol-hydrolases, EC 3.1.1.3) occur in plants, mammalians and microorganisms, where their physiological role is believed to be digestive. Common natural substrates are triglycerides and derivatives thereof such as di-and monoglycerides. In contrast to carboxyl esterases (EC 3.1.1.1), they act efficiently on water-insoluble substrates by binding to the water-organic interface, whereas the organic interface usually consists of a triglyceride. This binding not only places the lipase close to the substrate, but also increases the catalytic efficiency drastically by a phenomenon called interfacial activation. This observation was made long time ago by measuring lipase activity at different substrate concentrations. However, only recently this phenomenon could be explained at a molecular level by X-ray crystallographic analysis of lipase crystals containing bound transition state analogs. It was found that in most lipases, a helical lid covers the active site thus blocking access of substrates. Upon binding to a hydrophobic interface such as a lipid droplet, the lid opens enabling substrates access to the active site thus increasing the catalytic activity of lipases. Structures of 11 lipases have been elucidated and it turned out that all of them share a similar motif, called the a/s-hydrolase fold. This consists of a core of eight (mostly parallel) I3-sheets surrounded on both sides by a-helices. Lipases are serine esterases and the catalytic machinery is similar to that of serine proteases [1, 2, 3, 4, 5, 6, 7].