Substrate-Determined Diastereoselectivity in an Enzymatic Carboligation

Thiamine diphosphate‐dependent enzymes catalyze the formation of C−C bonds, thereby generating chiral secondary or tertiary alcohols. By the use of vibrational circular dichroism (VCD) spectroscopy we studied the stereoselectivity of carboligations catalyzed by YerE, a carbohydrate‐modifying enzyme from Yersinia pseudotuberculosis. Conversion of the non‐physiological substrate (R)‐3‐methylcyclohexanone led to an R,R‐configured tertiary alcohol (diastereomeric ratio (dr) >99:1), whereas the corresponding reaction with the S enantiomer gave the S,S‐configured product (dr>99:1). This suggests that YerE‐catalyzed carboligations can undergo either an R‐ or an S‐specific pathway. We show that, in this case, the high stereoselectivity of the YerE‐catalyzed reaction depends on the substrate's preference to acquire a low‐energy conformation. Breaking the rules: Diastereoselectivity is achieved through stereochemical induction by a pre‐existing stereocenter, but for enzymatic reactions this rule usually does not apply. Transformation of both enantiomers of a chiral substrate by YerE leads to enantiomeric instead of the expected diastereomeric products, thus suggesting that substrate‐induced diastereoselectivity is also possible in enzymatic reactions.