Aerobic, glucose-limited chemostat of CBS 8066 cometabolized propionate when this compound was added to the reservoir medium. Co-metabolism of propionate led to an increase of the biomass and protein yields. Attempts to grow on propionate as a sole source of carbon and energy were not successful. Activities of propionyl-CoA synthetase in cell-free extracts were sufficient to account for the rates of propionate consumption observed in the chemostat cultures. Activities of propionyl-CoA carboxylase, a key enzyme of the methylmalonyl-CoA pathway of propionate metabolism, were negligible. In contrast, activities of 2-methylcitrate synthase, a key enzyme activity of the 2-methylcitrate pathway of propionate metabolism, increased substantially with increasing propionateto-glucose ratios in the reservoir media, and were sufficient to account for the propionate consumption rates observed in the chemostat cultures. This suggested that the 2-methylcitrate pathway is the major pathway of propionate metabolism in In the literature, labelling patterns observed after incubation of this yeast with [3-C]propionate have been interpreted as evidence for channelling of tricarboxylic acid (TCA) cycle intermediates, possibly as a consequence of the organization of TCA cycle enzymes in a metabolon. However, this interpretation of C-labelling patterns rested on the assumption that propionate metabolism in occurs via the methylmalonyl-CoA pathway. Since the distribution of C in alanine reported in the literature is fully compatible with a major role of the 2-methylcitrate pathway in propionate metabolism, it cannot be interpreted as evidence for the existence of a TCA cycle metabolon in


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