CO 2 fixation in acetogenic bacteria: Variations on a theme

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An increasing number of strict anaerobic bacteria are being found which use an alternative pathway to the ubiquitous Calvin cycle for CO 2 fixation into cell compounds and the ubiquitous Krebs cycle for acetyl CoA oxidation to CO 2. The principles of this non-cyclic pathway, the acetyl CoA pathway, have long been studied in acetogenic bacteria. These bacteria can catalyze the exergonic reduction of 2 CO 2 with 8 reducing equivalents to acetate. In this pathway, CO 2 reduction is part of a catabolic redox process which functions to accept reducing equivalents from a variety of dehydrogenated substrates. This process yields net ATP generated by electron transport phosphorylation. Acetyl CoA is an intermediate, formed from one CO 2 via a tetrahydropteridine-bound 1-carbon unit (methyl group of acetate), and from another CO 2 via a bound carbon monoxide (carboxyl group of acetate). The most characteristic and complex enzyme involved in acetyl CoA synthesis is carbon monoxide dehydrogenase (‘acetyl CoA synthase’). The enzymes of this acetyl CoA pathway not only participate in (1) acetate synthesis in energy metabolism of acetogenic bacteria, but also mediate (2) acetyl CoA oxidation in sulfate-reducing bacteria and possibly other anaerobes; (3) acetate disproportionation to CO 2 and CH 4 in the energy metabolism of many methanogenic bacteria; (4) autotrophic CO 2 fixation in autotrophic acetogenic, methanogenic, and most autotrophic sulfate-reducing bacteria; (5) assimilation and/or dissimilation of 1-carbon compounds in many anaerobes; (6) CO oxidation to CO 2 in anaerobes. A specialized group of anaerobes performs acetate synthesis from CO 2 or from C 1 units via a different pathway, the glycine synthase/glycine reductase pathway. Glycine is an intermediate which is formed from 2 C 1 compounds, and is then reduced to acetate. The principal features of the two pathways and some open questions are discussed in this review. Emphasis is placed upon the acetyl CoA pathway in acetogenic bacteria, but important advances in the study of other strict anaerobes are also considered.