Abstract
The reaction of almond β-glucosidase with p -nitrophenyl-β- D -glucoside has been investigated over the temperature range +25° to −45° using 50% aqueous dimethyl sulfoxide (DMSO) as solvent. At temperatures below those at which turnover occurs a “burst” of p -nitrophenol proportional to the enzyme concentration is observed. Such a “burst” suggests the existence of a glucosyl-enzyme intermediate whose breakdown is rate-limiting, and provides a method for measuring the active-site normality. At pH 5.9, 25°, the presence of 50% DMSO causes an increase in K m from 1.7×10 −3M (0%) to 1.7×10 −2M, whereas V max is unchanged. The DMSO thus apparently acts as a competitive inhibitor with K i = 0.7M. The Arrhenius plot for turnover is linear over the accessible temperature range with E a = 23.0 ± 2.0 kcal/mole.
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