The tricarboxylic acid (TCA) cycle is central to energy production and biosynthetic precursor synthesis in aerobic organisms. There exist few known variations of a complete TCA cycle, with the common notion being that the enzymes involved have already evolved towards optimal performance. Here, we present evidence that an alternative TCA cycle, in which acetate:succinate CoA-transferase (ASCT) replaces the enzymatic step typically performed by succinyl-CoA synthetase (SCS), has arisen in diverse bacterial groups, including microbial symbionts of animals such as humans and insects.Keywords citric acid cycle; Krebs cycle; acetate; gut microbiota; Snodgrassella alvi
Main textThe tricarboxylic acid (TCA) cycle arose during early evolution and represents a core process in aerobic respiration and production of carbon-based precursor molecules needed for the biosynthesis of amino acids, nucleotides and cofactors. The enzymes that complete the cycle show homology across Bacteria, Archaea, and Eukaryota, and are highly efficient 1,2 ; consequently, there are relatively few known variations of a complete TCA cycle. Many bacteria have incomplete cycles, while some autotrophs use a reverse (reductive) cycle to fix carbon from CO 2 3 . Alternative oxidative cycles have been proposed to operate in Cyanobacteria 4 , Mycobacterium 5 , and Helicobacter 6 . Another variant cycle was identified in Acetobacter aceti, whereby succinyl-CoA is converted to succinate by an acetate:succinate CoA-transferase (ASCT) instead of the typical succinyl-CoA synthetase Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: