Yeast 3-Hydroxy-3-methylglutaryl-CoA Reductase: an Enzyme Committed to Catabolite Derepression Before Exhaustion of Fermentable Substrate

Saccharomyces cerevisiae grown on glucose (1 %, w/v) medium exhibited diauxic growth because of the transition from catabolite-repressed fermentative metabolism to derepressed oxidative metabolism. 3-Hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase; EC 1.1.1.34) was object to catabolite repression and required cytoplasmic protein synthesis for derepression. The addition of glucose (2 %, w/v) at up to 4 h prior to glucose exhaustion had no effect on subsequent derepression of HMG-CoA reductase, suggesting that the enzyme was irreversibly committed to derepression at this stage. However, other derepressible enzyme activities such as NAD-dependent glutamate dehydrogenase (EC 1.4.1.2) and malate dehydrogenase (EC 1.1.1.37) did not show a commitment to derepression at this time. In contrast, NADP-dependent glutamate dehydrogenase (EC 1.4.1.4) was induced by the addition of glucose.