Energy metabolism of Saccharomyces cerevisiae discrepancy between ATP balance and known metabolic functions.

The contribution of metabolic pathways to the catabolism of glucose, galactose and ethanol by Saccharomyces cerevisiae in aerobiosis has been studied. The results suggest that: 1. Of the total ATP formed in catabolism yeast obtain as much as 60% from ethylic fermentation during logarithmic growth on glucose. However, about 80% of ATP is formed in oxidation of galactose. Oxidation seems to be the only important catabolic pathway of ethanol. 2. The ratios between growth yield and ATP formed in catabolism were approx. 9, 7 and 3 g dry yeast/mol ATP in glucose, galactose and ethanol cultures, respectively. 3. The balance between ATP produced in catabolism of substrates and the requirements of ATP for the biosynthesis of cellular material indicates that as much as 60% of ATP is spent in functions other than net biosynthesis. 4. The rate of ATP expenditure in non net-biosynthetic functions during growth was approx. 20 mmol/g dry yeast per h. 5. In conditions in which no growth occurred but cell viability was maintained, that is, in the absence of exogenous carbon and nitrogen source, the ATP production rate was approx. 1 mmol ATP/g dry yeast per h. 6. These results indicate that the ATP required for maintaining the yeast alive, what would be considered maintenance energy "sensu stricto", is only a minor proportion of the ATP spent in non net-biosynthetic functions during growth. The identification of the processes related to growth which spend more energy than that required for net biosynthesis could lead to important insights in cell biology.