Studies on the energy metabolism during the respiratory process by baker's yeast

Microcalorimetry, in combination with conventional methods for determining metabolic activity, opens the possibility to study the efficiency with which ATP, produced as a result of metabolic activity, is utilized by the cell for energy-requiring synthetic reactions. Using commercial baker's yeast as a test organism and glucose, ethanol, acetic and lactic acids as substrates, the fate of the ATP produced by the respiratory process was studied by measuring oxygen consumption (using the Warburg technique) and the corresponding heat development (using the LKB Flow-Microcalorimeter). From these data heat development per mm3 oxygen consumed was calculated. Values obtained should fall within a heat production range that can be calculated from the combustion heat of the process (maximum heat development) and maximum energy conservation, assuming full participation of ATP in energy-requiring synthetic reactions (minimum heat development). It was found that during the respiratory process of "resting" cells of baker's yeast, regardless of the substrate used, heat development was close to the maximum value inherent with substrate oxidation. Consequently, practically all ATP, produced as a result of the respiratory process, is de-phosphorylated under heat development and thus is not (or very inefficiently) utilized for energy-requiring synthetic reactions. In accordance with this conclusion it was found that addition of 2-4-DNP, a powerful uncoupler of phosphorylation from the respiratory process, did not result in an appriciable increase in heat development. Even in the presence of an assimilable N source, allowing unrestricted growth, initially only a very small percentage of the ATP produced is utilized for synthetic processes. A gradual improvement of this poor economic ATP utilization was observed during the prelogarithmic growth phase. As a possible explanation of this wasteful aerobic metabolism of baker's yeast and its restricted ability to utilize ATP for synthetic processes was mentioned the exceptional low content of messenger RNA, typical for a baker's yeast subjected to a ripening process before harvesting.