Regulation of the C₁ Metabolism of Acetogens: Metabolic By-Passes and Ecological Implications

Abstract

The central energy-conserving process for acetogenic bacteria involves the use of CO₂ as a terminal electron accepter. The end product of CO₂ fixation is acetate. The overall process by which CO₂ is reduced to acetate by acetogens is termed the acetyl-CoA “Wood/Ljungdahl” pathway and involves the reduction of two molecules of CO₂ to either the CH₃-level via formate dehydrogenase and tetrahydrofolate-mediated reactions, or the CO-level via acetyl-CoA synthase (i.e., CO-dehydrogenase) (Wood, Ljungdahl 1991; Ragsdale 1994; Diekert, Wohlfarth 1994; Drake 1994). This 8 electron reduction of two molecules of CO₂ culminates in the synthesis of acetyl-CoA via acetyl-CoA synthase, acetyl-CoA being subsequently converted to acetate via phosphotransacetylase and acetate kinase. Energy conservation occurs by both substrate phosphorylation and ion translocation/ATPases (Ljungdahl 1994; Müller, Gottschalk 1994). For acetogens, the acetyl-CoA pathway is used for autotrophic carbon assimilation as well as energy conservation (Drake 1994), as illustrated in the following reaction:

CO₂ + [8 reducing equivalents] ⇢ [acetyl-CoA] ⇢ acetate + biomass