CHANGES IN YEAST AMINO-ACID POOL WITH RESPIRATORY VERSUS FERMENTATIVE METABOLISM

The precursors of the amino acid yeast pool are intermediates of either the glycolytic or the tricarboxilic acid pathway (TCA). Accordingly, the influence of the metabolism (fermentative or respiratory) on the internal amino acid pool of the yeast Saccharomyces cerevisiae was established by measuring the intracellular amino acid concentration of the "grande" strain IFI256 and its "petite" mutant either in steady-state or when shifting from fermentative to respiratory conditions. Under steady-state conditions, when the cells only respire, there is a decrease in nearly all the amino acids whose precursors are intermediates of the glycolytic pathway. When the metabolism is exclusively fermentative, the opposite change takes place. This effect is not observed in those amino acids whose precursors come from the TCA cycle. However, in continuous culture and at dilution rates lower than 0.06 h-1, there is an enormous increase in the concentration of all the amino acids in both strains, whatever their precursor, whereas, in batch cultures, both strains undergo variations in the concentration of most amino acids, when entering stationary growth phase. Results therefore indicate that, the fact that the cells ferment or respire effectively affect their amino acid pool according to their precursors coming from the glycolytic or the TCA pathway, but other parameters, such as growth rate or exponential versus stationary phase, have a much stronger effect on yeast amino acid concentration.