Kinetic Mechanism of Serine Transacetylase from Salmonella typhimurium

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Abstract
Serine transacetylase from Salmonella typhimurium was purified to near homogeneity. A complete initial velocity study in both reaction directions suggests that the enzyme catalyzes the conversion of acetyl CoA and L-serine to O-acetyl-L-serine (OAS) and coenzyme A (CoASH) by a ping pong bi bi kinetic mechanism. Initial velocity patterns in the absence of added inhibitors in both directions are best described by a series of parallel lines. Product inhibition by OAS is competitive with respect to acetyl CoA and noncompetitive with respect to L-serine, while product inhibition by L-serine is competitive against CoASH and noncompetitive against OAS. Glycine and S-methyl-L-cysteine (SMC) were used as dead-end analogs of L-serine and OAS, respectively. Glycine is competitive against L-serine, and uncompetitive against acetyl CoA, while SMC is competitive against OAS and uncompetitive against CoASH. All of the above inhibition patterns are consistent with those predicted for a single site ping pong bi bi kinetic mechanism. The equilibrium constant for the transacetylase reaction is 15 in the direction of serine acetylation. The constant was measured by monitoring the change in CoASH concentration obtained for reactions in which the ratio of acetyl CoA/CoASH was constant and the ratio of OAS/serine was varied. The Keq calculated from the Haldane relationship is in good agreement with the value obtained by direct measurement.

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