Many bacteria and other organisms carry out fermentations forming acetate. These fermentations have broad importance to foods, agriculture, and industry. They also are important to bacteria themselves because they often generate ATP. Here we found a biochemical pathway for forming acetate and synthesizing ATP that was unknown in fermentative bacteria. We found the bacterium Cutibacterium granulosum formed acetate during fermentation of glucose. It did not use phosphotransacetylase or acetate kinase, enzymes found in nearly all acetate-forming bacteria. Instead, it used a pathway involving two different enzymes. The first enzyme, succinyl-CoA:acetate CoA-transferase (SCACT), forms acetate from acetyl-CoA. The second enzyme, succinyl-CoA synthetase (SCS), synthesizes ATP. We identified the genes encoding these enzymes, and they were homologs of SCACT and SCS genes found in other bacteria. The pathway resembles one described in eukaryotes, but it uses bacterial, not eukaryotic, gene homologs. To find other instances of the pathway, we analyzed sequences of all biochemically-characterized homologs of SCACT and SCS (103 enzymes from 64 publications). Homologs with similar enzymatic activity had similar sequences, enabling a large-scale search for them in genomes. We searched nearly 600 genomes of bacteria known to form acetate, and we found 6% encoded homologs with SCACT and SCS activity. This included >30 species belonging to 5 different phyla, showing a diverse range of bacteria encode the SCACT/SCS pathway. This work suggests the SCACT/SCS pathway is important to forming acetate in many branches of the tree of life.
Importance
Pathways for forming acetate during fermentation have been studied for over 80 years. In that time, several pathways have been described in a range of organisms, from bacteria to animals. However, one pathway (involving succinyl-CoA:acetate CoA-transferase and succinyl-CoA synthetase) has not been reported in prokaryotes. Here we discovered enzymes for this pathway in the fermentative bacterium Cutibacterium granulosum. We also found >30 other fermentative bacteria that encode this pathway, demonstrating it could be common. This pathway represents a new way for bacteria to form acetate from acetyl-CoA and synthesize ATP via substrate-level phosphorylation. It could be a target for controlling yield of acetate during fermentation, with relevance to foods, agriculture, and industry.