Mikrobiologie, Fakultät für Biologie, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany.
Proc Natl Acad Sci U S A. 2009 Dec 15;106(50):21317-22. doi: 10.1073/pnas.0908356106. Epub 2009 Dec 2.
The phototrophic bacterium Chloroflexus aurantiacus uses a yet unsolved 3-hydroxypropionate cycle for autotrophic CO(2) fixation. It starts from acetyl-CoA, with acetyl-CoA and propionyl-CoA carboxylases acting as carboxylating enzymes. In a first cycle, (S)-malyl-CoA is formed from acetyl-CoA and 2 molecules of bicarbonate. (S)-Malyl-CoA cleavage releases the CO(2) fixation product glyoxylate and regenerates the starting molecule acetyl-CoA. Here we complete the missing steps devoted to glyoxylate assimilation. In a second cycle, glyoxylate is combined with propionyl-CoA, an intermediate of the first cycle, to form beta-methylmalyl-CoA. This condensation is followed by dehydration to mesaconyl-C1-CoA. An unprecedented CoA transferase catalyzes the intramolecular transfer of the CoA moiety to the C4 carboxyl group of mesaconate. Mesaconyl-C4-CoA then is hydrated by an enoyl-CoA hydratase to (S)-citramalyl-CoA. (S)-Citramalyl-CoA is cleaved into acetyl-CoA and pyruvate by a tri-functional lyase, which previously cleaved (S)-malyl-CoA and formed beta-methylmalyl-CoA. Thus, the enigmatic disproportionation of glyoxylate and propionyl-CoA into acetyl-CoA and pyruvate is solved in an elegant and economic way requiring only 3 additional enzymes. The whole bicyclic pathway results in pyruvate formation from 3 molecules of bicarbonate and involves 19 steps but only 13 enzymes. Elements of the 3-hydroxypropionate cycle may be used for the assimilation of small organic molecules. The 3-hydroxypropionate cycle is compared with the Calvin-Benson-Bassham cycle and other autotrophic pathways.
光养细菌绿屈挠菌利用一个尚未解决的 3-羟基丙酸盐循环进行自养 CO2 固定。它从乙酰辅酶 A 开始，乙酰辅酶 A 和丙酰辅酶 A 羧化酶作为羧化酶。在第一个循环中，（S）-苹果酰辅酶 A 由乙酰辅酶 A 和 2 个分子的碳酸氢盐形成。（S）-苹果酰辅酶 A 裂解释放 CO2 固定产物乙醛酸，并再生起始分子乙酰辅酶 A。在这里，我们完成了缺失的步骤，致力于乙醛酸同化。在第二个循环中，乙醛酸与丙酰辅酶 A（第一循环的中间体）结合形成β-甲基丙二酰辅酶 A。该缩合后脱水形成中康酸 C1-辅酶 A。一种前所未有的辅酶转移酶催化辅酶部分的分子内转移到中康酸的 C4 羧基上。然后，中康酰基 C4-辅酶 A 被烯酰辅酶 A 水合酶水合生成（S）-柠檬酸基辅酶 A。（S）-柠檬酸基辅酶 A 被一种三功能裂解酶裂解为乙酰辅酶 A 和丙酮酸，该裂解酶先前裂解（S）-苹果酰辅酶 A 并形成β-甲基丙二酰辅酶 A。因此，乙醛酸和丙酰辅酶 A 神秘地歧化为乙酰辅酶 A 和丙酮酸，以一种优雅而经济的方式解决，只需要 3 个额外的酶。整个双环途径导致从 3 个分子的碳酸氢盐形成丙酮酸，涉及 19 个步骤，但只有 13 个酶。3-羟基丙酸盐循环的元素可用于小分子的同化。3-羟基丙酸盐循环与卡尔文-本森-巴斯汉姆循环和其他自养途径进行了比较。
