Abstract
The reaction of β-galactosidase ( E. coli K12) with o -nitrophenyl-β- D -galactoside has been investigated over the temperature range +25° to −30° using 50% aqueous dimethyl sulfoxide as solvent. At temperatures below −10° turnover becomes very slow and a burst of o -nitrophenol is observed. Such a burst indicates the existence of a galactosyl-enzyme intermediate whose breakdown is rate-limiting and provides a means of determining the active site normality. The Arrhenius plot for turnover is linear in the −25 to +25° range with E a = 26 ± 3 kcal/mole. The presence of the 50% DMSO had no effect on K m but caused a small decrease in K cat.
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