Assimilation of carbon dioxide by yeasts

Abstract

The mechanism of carbon dioxide fixation by Saccharomyces cerevisiae (baker''s yeast) and S. carlsbergensis was studied. Baker''s yeast assimilates carbon dioxide during the oxidation of acetate, acetaldehyde, pyruvate and glucose, and also during glucose fermentation, but the oxidation of endogenous substrates is far less effective. After incorporation of radioactive carbon dioxide, C14 labels the intermediates of the Krebs cycle, a number of amino acids and compounds related to glycolysis. Aspartic, glutamic, malic and the tri-carboxylic acids are the main reservoirs of the C14-fixed. The distribution patterns vary somewhat according to the substrate oxidized. Wide variations in pH of the medium do not significantly affect the fixation and distribution of radioactive carbon dioxide. Oxaloacetate is believed to be the primary product of fixation and its formation to take place by carboxylation of phosphoenolpyruvate. The patterns of C14 distribution and the location of C14 in aspartic, glutamic and succinic acid are, as a rule, consistent with the Krebs-cycle model. In the presence of glucose, carbon dioxide fixation is inhibited by 2,4-dinitrophenol, sodium azide, iodoacetate and fluoride, and increased by antimycin A. The fixation dependent on endogenous substrate oxidation is inhibited by 2,4-dinitrophenol, sodium azide, iodoacetate and antimycin A, and increased by fluoride. With glucose as substrate, aerobic and anaerobic patterns of fixation differ. In the absence of oxygen (or inhibition of oxidizing mechanisms with antimycin A) more C14 enters into malic, fumaric, succinic and aspartic acid. In S. carlsbergensis, the distribution of C14 belongs to the anaerobic type. Carbon dioxide fixation is an essential reaction for yeast metabolism as it supplies carbon for biosynthetic processes and promotes aerobic and anaerobic oxidations. Baker''s yeast consumes exogenous pyruvate mainly by oxidation.