Schink Bernhard
Microbial Ecology, Department of Biology, University of Konstanz, Konstanz, Germany.
Mol Microbiol. 2009 Sep;73(6):975-7. doi: 10.1111/j.1365-2958.2009.06835.x. Epub 2009 Aug 4.
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cycle going requires anaplerotic reactions such as the glyoxylate shunt to restore the cycle intermediates that are withdrawn for the biosynthesis of cell constituents, e.g. amino acids and haemin precursors.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.
只要车轮的辐条不被偷走，一个循环才能保持为一个循环。要维持柠檬酸循环的运转，需要回补反应，比如乙醛酸循环，以恢复那些被用于细胞成分（如氨基酸和血红素前体）生物合成而消耗的循环中间产物。Erb等人在本期《分子微生物学》上发表的文章记录了一条替代途径，该途径在没有乙醛酸循环的情况下，在以乙酸盐为碳源生长期间补充四碳中间产物。该反应序列通过一条线性途径，由三个乙酰辅酶A、一个CO₂和一个HCO₃⁻形成苹果酸和琥珀酰辅酶A。这条新途径是在光合不产氧细菌和少数需氧细菌中发现的，但它可能在许多细菌代谢类群中广泛存在。
