Evolution of a Pattern of Terminal Respiration in Bacteria

Abstract

The tricarboxylic acid cycle is now generally recognized as constituting the major mechanism for oxidation of acetyl groups in those bacteria which have been studied in detail. To date, Escherichia coli is the best representative of this group. Evidence for other pathways have largely been disproven. The alternate mechanisms for acetate oxidation proposed by Umbreit and co-workers and Barron and co-workers require further elucidation. The only aerobic organism in which absence of the Krebs cycle has been reasonably well established in Acetobacter suboxydans, although the cycle does function in Acetobacter pasteurianum and in Acetobacter peroxydans. Alternate pathways for the breakdown of Krebs cycle intermediates not involving the traditional Krebs cycle reactions continue to accumulate both in bacteria and other forms of life. New enzymes, such as citritase, isocitritase, and malate synthetase, are continuously being discovered. With the discovery of these enzymes additional cyclic reactions for acetate oxidation become apparent. One such possibility has been explored in this review. Further advances in this active field of research are anticipated.