The carboxyl group in the active center of transketolase.

Transketolase from baker's yeast is rapidly inactivated in the presence of 1-ethyl-3 (3'-dimethylaminopropyl)-carbodiimide or Woodward's reagent K. In both cases the kinetics of inactivation is biphasic, which agrees with the presence of two active centers in the enzyme molecule differing in their sensitivity to the inhibitors. There is some evidence that inactivation of transketolase is due to modification of carboxyl groups of enzyme. Complete inactivation is achieved by modification of one carboxyl per active site of the enzyme. The experimental results suggest that the carboxyl group is essential for the enzymatic activity of transketolase.