The Sodium Ion Cycle in Acetogenic and Methanogenic Bacteria: Generation and Utilization of a Primary Electrochemical Sodium Ion Gradient

Abstract

Acetogenic bacteria are strictly anaerobic bacteria which use a wide variety of organic substrates for growth and acetate formation. Glucose, for example, is metabolized via the Embden-Meyerhoff pathway to pyruvate which is then split in the phosphoroclastic reaction to CO2 and acetyl CoA; the latter is converted to acetate via acetyl phosphate. The term “acetogen” is used for a variety of different organisms which produce acetate as a major fermentation end product. Homoacetogenic bacteria (which are referred to in the following as “acetogens”) differ from other organisms in that the CO2 formed is not an end product; the reducing equivalents obtained during glycolysis are used to reduce the 2 mol of CO2 produced in the phosphoroclastic reaction to acetate via the acetyl-CoA pathway. Therefore, homoacetogenic bacteria convert 1 mol of glucose to 3 mol of acetate via glycolysis and acetyl-CoA pathway. As the pathway is well established, one can calculate a net formation of 4 mol of ATP per mole of glucose fermented by the mechanism of substrate-level phosphorylation (SLP) (Fuchs, 1986; Ljungdahl, 1986; Wood et al., 1986), which is sufficient to ensure the energy supply of the cells during heterotrophic acetogenesis.