Evidence for a specific function for histidine residues in transaldolase

Abstract

Experiments carried out with crystalline transaldolase from Candida utilis suggest that histidine residues play a specific role in initiating the cleavage reaction. The dihydroxyacetone complex, formed from fructose 6-phosphate and enzyme in the presence of tritium-labeled water, was found to contain tritium, approaching one equivalent per mole of enzyme. This tritium exchanged very slowly with water. It was quantitatively recovered in the C-3 position of β-glyceryllysine when the complex was reduced with sodium borohydride. It was also recovered in the C-3 position of dihydroxyacetone when this was released by heating the complex to 75 °. 3 On the other hand, in the complex itself, the proton appeared to be associated with an essential histidine residue. The following evidence for such a functional histidine residue was obtained: (1) Loss of enzyme activity during photooxidation in the presence of rose Bengal was correlated with the loss of one of the two histidine residues in the protein. (2) In contrast to the free enzyme, the Schiff base intermediate was not sensitive to photooxidation at pH 7.9, indicating that in the complex the histidine residue is protonated. It is proposed that the essential histidine residue acts as a base catalyst which accepts the proton from the C-4 hydroxyl group during the dealdolization reaction.