CH06pt3 - Must postulate general base catalysis to increase...

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8 •Must postulate general base catalysis to increase nucleophilicity of Ser-195 without discrete anion formation. •Good candidate for general base is imidazole of his-57 •X-ray data reveals close proximity of His-57, Ser-195, and Asp- 102. Suggests extensive charge-relay system in active site: p. 208 6-20 Microscopic ionization behavior of charge-relay system : •Velocity is maximal at pH 8, decreasing on either side •K cat increases with pH (decreases with increasing [H + ] - due to ionization of His-57) Proposed mechanism (pp. 208-209): Tetrahedral acyl-enzyme intermediate
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9 Catalytic efficiency 1. provided by charge-relay system through avoidance of unstable species with charge separation and through facile transfer of protons from enzymatic residues to and from substrates and departing products = general acid-base catalysis 2. Acyl-enzyme covalent intermediate is transiently formed by attack of nucleophilic hydroxyl of serine on electrophilic acyl group = covalent catalysis
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CH06pt3 - Must postulate general base catalysis to increase...

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