Acetoin and Phenylacetylcarbinol Formation by the Pyruvate Decarboxylases ofZymomonas MobilisandSaccharomyces CarlsbergensisStephanie Bringer-Meyer

Abstract

Cell suspensions of Zymomonas mobilis and Saccharomyces carlsbergensis and the pyruvate decarboxylases from the two organisms were compared with respect to their efficiencies of acyloin formation. Although Z. mobilis contained five times more pyruvate decarboxylase activity than yeast, sugar-fermenting suspensions of Z. mobilis produced, in the presence of benzaldehyde, 4–5 times less phenylacetylcarbinol than the yeast. The pyruvate decarboxylases of both organisms catalyzed acetoin and phenylacetylcarbinol synthesis from pyruvate and acetaldehyde or benzaldehyde, but the affinity of the Z. mobilis pyruvate decarboxylase towards the aldehyde reactants was lower than that of the yeast enzyme. Because of the limited solubility of benzaldehyde, neither enzyme could be saturated with this substrate for phenyl-acetylcarbinol synthesis. Studies with 2-toluidinonaphthalene-6-sulfonate and substrate analogues showed that the catalytic sites of pyruvate decarboxylase from Z. mobilis were less lipophilic than those of the enzyme from yeast. This difference could explain the lower affinity for benzaldehyde of the Z. mobilis enzyme.