Characterization of proton-transfer catalysis by serum albumins

Two independent investigations of catalysis by BSA and other serum albumins are combined to provide detailed insight into the mechanism of a classical proton-transfer reaction taking place on the surface of a protein. The Kem p, elimination involves the general base-catalyzed removal of a proton from carbon and is known to be highly sensitive to medium effects. The serum al bumins bind and catalyze the eliminative fragmentation of 5-nitrobenzisoxaz ole, with remarkable efficiency and "accidental specificity:" A binding sit e and a likely catalytic lysine are identified, and factors contributing to the efficiency of catalysis analyzed. A key factor appears to be a differe ntiated microenvironment, which allows delocalized negative charge to devel op in a stabilizing hydrophobic pocket next to a polar region where the dev eloping ammonium cation is not disfavored. Catalytic efficiency is discusse d in terms of effective molarities fur the reaction catalyzed by serum albu mins and by catalytic antibodies and compared with a selection of published EMs for enzyme-catalyzed reactions.