Enzymatic and Physiological Properties of the Yeast Glutamate-α-Ketoadipate Transaminase

Abstract

In a study of differences between the two glutamate-α-ketoadipate transaminases of Saccharomyces, it was observed that glutamate-α-ketoadipate transaminase I exhibited lower substrate specificity, functioning with aspartate in addition to glutamate as amino donor but not with alanine. Glutamate-α-ketoadipate transaminase II functioned with glutamate but not with aspartate or alanine. Glutamate-α-ketoadipate transaminase II readily catalyzed the reverse reaction (toward α-ketoadipate), whereas glutamate-α-ketoadipate transaminase I exhibited only weak activity in the reverse direction. A kinetic study of the forward reaction revealed that α-ketoglutarate, a product, behaved as a strong noncompetitive inhibitor of glutamate-α-ketoadipate transaminase I but not of glutamate-α-ketoadipate transaminase II. On the other hand, glutamate-α-ketoadipate transaminase II formation was derepressed by growth on α-aminoadipate, and to a lesser extent by lysine, and was repressed by glucose, while glutamate-α-ketoadipate transaminase I was scarcely affected by the growth conditions. These mechanisms may play a role in the regulation of lysine biosynthesis in yeast.