His103 in yeast transketolase is required for substrate recognition and catalysis

Crystallographic studies of thiamin-diphosphate-dependent transketolase from Saccharomyces cerevisiae suggested the invariant active-site residue H103 as a possible enzymic group binding the C1 hydroxyl group of the donor substrate and stabilizing the reaction intermediate. To test this hypothesis, H103 was replaced by alanine, asparagine and phenylalanine using site-directed mutagenesis. The crystallographic analysis of the mutant transketolases verified that no structural changes occurred as a consequence of the side-chain replacements. The residual catalytic activities of the mutant enzymes were 4.3% for the H103A, 2.4% for the H103N and 0.1% for the H103F mutant transketolase. Further kinetic analysis of the H103A and H103N mutant enzymes showed that the Km values for the coenzyme were increased by about eightfold. The Km values for the acceptor substrate ribose 5-phosphate were similar to the Km value for wild-type transketolase. However, the Km value for the donor substrate, xylulose 5-phosphate is increased more than tenfold in these two mutants. Circular dichroism spectra of the mutant enzymes also indicated a weaker binding of the donor substrate and/or a less stable reaction intermediate. These observations provide further evidence in support of the proposed role for this invariant residue in recognition of the donor substrate by forming a hydrogen bond between the side chain of H103 and the C1 hydroxyl group of the sugar phosphate. The significant decrease in catalytic activity suggests that this residue also facilitates catalysis, possibly by maintaining the optimal orientation of the donor substrate and reaction intermediates.