Crystallographic and molecular-modeling studies of lipase B from Candida antarctica reveal a stereospecificity pocket for secondary alcohols.

Many lipases are potent catalysts of stereoselective reactions and are therefore of interest for use in chemical synthesis. The crystal structures of lipases show a large variation in the shapes of their active site environments that may explain the large variation in substrate specificity of these enzymes. We have determined the three-dimensional structure of Candida antarctica lipase B (CALB) cocrystallized with the detergent Tween 80. In another crystal form, the structure of the enzyme in complex with a covalently bound phosphonate inhibitor has been determined. In both structures, the active site is exposed to the external solvent. The potential lid-forming helix alpha 5 in CALB is well-ordered in the Tween 80 structure and disordered in the inhibitor complex. The tetrahedral intermediates of two chiral substrates have been modeled on the basis of available structural and biochemical information. The results of this study provide a structural explanation for the high stereoselectivity of CALB toward many secondary alcohols.

Stereoselectivity of Baker's yeast reduction of 2-propanones: influence of substituents.

The stereoselectivity of Baker's yeast reduction of prochiral alpha-oxygenated 2-propanones has been studied by varying the substrate structure. The 1-hydroxy-3-methoxy-3-propanone 1a was reduced to the corresponding alcohol (R)-2a with 88% enantiomeric excess. Replacing the hydroxy group in 1a with phenoxy or benzyloxy (1b and 1c) gave the alcohols (S)-2b and (S)-2c with 53 and 32% ee, respectively. Reduction of the methyl ketone 1d gave the alcohol (S)-2d with 91% ee. Attempts to improve the enantioselectivity of the reduction of 1c by lowering the substrate concentration or addition of selective reductase inhibitors had only small effect on the enantioselectivity.

Homochiral glycerol derivatives by lipase-catalysed racemate resolution.

Various aspects of enzyme-catalysed racemate resolution are discussed by using examples from hydrolysis of butanoates of glycerol derivatives. Primary esters of various 1,2-ketals gave low enantioselectivity (E). The results with secondary esters varied. The highest E value (> 100) was obtained when the primary ether groups were methyl and phenylethyl. For the corresponding methyl, phenyl diether E was observed to increase as the hydrolysis proceeded. The stereochemistry of the products agree with a proposed model for lipases.