X-RAY STRUCTURES OF A HYDROLYTIC ANTIBODY AND OF COMPLEXES ELUCIDATE CATALYTIC PATHWAY FROM SUBSTRATE-BINDING AND TRANSITION-STATE STABILIZATION THROUGH WATER ATTACK AND PRODUCT RELEASE

The x-ray structures of the unliganded esterase-like catalytic antibod y D2.3 and its complexes with a substrate analogue and with one of the reaction products are analyzed, Together with the structure of the ph osphonate transition state analogue hapten complex, these crystal stru ctures provide a complete description of the reaction pathway, At alka line pH, D2.3 acts by preferential stabilization of the negatively cha rged oxyanion intermediate of the reaction that results from hydroxide attack on the substrate. A tyrosine residue plays a crucial role in c atalysis: it activates the ester substrate and, together with an aspar agine, it stabilizes the oxyanion intermediate, A canal allows facile diffusion of water molecules to the reaction center that is deeply bur ied in the structure, Residues bordering this canal provide targets fo r mutagenesis to introduce a general base in the vicinity of the react ion center.