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 stru ctures of lipases show a large variation in the shapes of their active site environments that may explain the large variation in substrate s pecificity of these enzymes. We have determined the three-dimensional structure of Candida antarctica lipase B (CALB) cocrystallized with th e detergent Tween 80. In another crystal form, the structure of the en zyme in complex with a covalently bound phosphonate inhibitor has been determined. In both structures, the active site is exposed to the ext ernal solvent. The potential lid-forming helix alpha 5 in CALB is well -ordered in the Tween 80 structure and disordered in the inhibitor com plex. The tetrahedral intermediates of two chiral substrates have been modeled on the basis of available structural and biochemical informat ion. The results of this study provide a structural explanation for th e high stereoselectivity of CALB toward many secondary alcohols.