An alternative to the glyoxylate shunt.

A cycle remains a cycle only as long as the spokes of the wheel are not stolen. To keep the citric acid cycle going requires anaplerotic reactions such as the glyoxylate shunt to restore the cycle intermediates that are withdrawn for the biosynthesis of cell constituents, e.g. amino acids and haemin precursors. The article by Erb et al. in this issue of Molecular Microbiology documents an alternative path that replenishes four-carbon intermediates during growth on acetate in the absence of the glyoxylate shunt. The reaction sequence forms malate and succinyl-CoA from three acetyl-CoA, one CO(2) and one HCO(3) in a linear pathway. This new pathway was discovered in phototrophic anoxygenic bacteria and in few aerobic bacteria, but it is probably widespread among many metabolic groups of bacteria.