| Abstract: | We used substrate mapping to develop a rule that predicts which enantiomer of chiral carboxylic acid esters reacts faster in hydrolyses catalyzed by lipase from Candida rugosa (CRL, triacylglycerol hydrolase, E. C. 3.1.1.3). This rule, based on the size of the substituents at the stereocenter, is not reliable for crude CRL. It predicts the favoured enantiomer for only 23 out of 34 examples, 68% reliability. However, this rule is completely reliable for purified CRL; it predicts the favoured enantiomer for all 16 examples correctly. The examples include arylpropanoicacids, aryloxypropanoic acids, α-halophenylacetic acids, mandelic acid and O-methylmandelic acid. Further, purified CRL did not catalyse the hydrolysis of N-CBZ-phenylalanine methyl ester and N-CBZ-norleucine methyl ester. These two substrates were exceptions to the rule with crude CRL as the catalyst. Besides eliminating several exceptions, purification also raised the enantioselectivity of CRL toward carboxylic acid esters. To provide a structural basis for this proposed rule we examined the x-ray crystal structure of CRL containing transition state analogs of ester hydrolysis. We suggest that the large substituent of chiral carboxylic acids binds in a tunnel that normally binds the alkyl chain of a fatty acid. The phenyl rings of Phe 345 and Phe 415 lie close to the stereocenter, thereby fixing the orientation of the medium substituent. The three-dimensional orientation of these proposed binding sites is consistent with the rule derived from substrate mapping. |
