Herter Sylvia, Busch Andreas, Fuchs Georg
Mikrobiologie, Institut für Biologie II, Albert-Ludwigs-Universität Freiburg, Germany.
J Bacteriol. 2002 Nov;184(21):5999-6006. doi: 10.1128/JB.184.21.5999-6006.2002.
The 3-hydroxypropionate cycle is a bicyclic autotrophic CO(2) fixation pathway in the phototrophic Chloroflexus aurantiacus (Bacteria), and a similar pathway is operating in autotrophic members of the Sulfolobaceae (Archaea). The proposed pathway involves in a first cycle the conversion of acetyl-coenzyme A (acetyl-CoA) and two bicarbonates to L-malyl-CoA via 3-hydroxypropionate and propionyl-CoA; L-malyl-CoA is cleaved by L-malyl-CoA lyase into acetyl-CoA and glyoxylate. In a second cycle, glyoxylate and another molecule of propionyl-CoA (derived from acetyl-CoA and bicarbonate) are condensed by a putative beta-methylmalyl-CoA lyase to beta-methylmalyl-CoA, which is converted to acetyl-CoA and pyruvate. The putative L-malyl-CoA lyase gene of C. aurantiacus was cloned and expressed in Escherichia coli, and the recombinant enzyme was purified and studied. Beta-methylmalyl-CoA lyase was purified from cell extracts of C. aurantiacus and characterized. We show that these two enzymes are identical and that both enzymatic reactions are catalyzed by one single bifunctional enzyme, L-malyl-CoA lyase/beta-methylmalyl-CoA lyase. Interestingly, this enzyme works with two different substrates in two different directions: in the first cycle of CO(2) fixation, it cleaves L-malyl-CoA into acetyl-CoA and glyoxylate (lyase reaction), and in the second cycle it condenses glyoxylate with propionyl-CoA to beta-methylmalyl-CoA (condensation reaction). The combination of forward and reverse directions of a reversible enzymatic reaction, using two different substrates, is rather uncommon and reduces the number of enzymes required in the pathway. In summary, L-malyl-CoA lyase/beta-methylmalyl-CoA lyase catalyzes the interconversion of L-malyl-CoA plus propionyl-CoA to beta-methylmalyl-CoA plus acetyl-CoA.
3-羟基丙酸循环是光合细菌嗜热栖热放线菌中一种双环自养型二氧化碳固定途径，在硫化叶菌科（古菌）的自养成员中也存在类似途径。该途径在第一个循环中，通过3-羟基丙酸和丙酰辅酶A将乙酰辅酶A和两个碳酸氢盐转化为L-苹果酰辅酶A；L-苹果酰辅酶A被L-苹果酰辅酶A裂解酶裂解为乙酰辅酶A和乙醛酸。在第二个循环中，乙醛酸与另一个丙酰辅酶A分子（由乙酰辅酶A和碳酸氢盐衍生而来）通过一种假定的β-甲基苹果酰辅酶A裂解酶缩合为β-甲基苹果酰辅酶A，后者再转化为乙酰辅酶A和丙酮酸。嗜热栖热放线菌假定的L-苹果酰辅酶A裂解酶基因被克隆并在大肠杆菌中表达，重组酶被纯化并进行了研究。β-甲基苹果酰辅酶A裂解酶从嗜热栖热放线菌的细胞提取物中纯化并进行了特性鉴定。我们发现这两种酶是相同的，并且这两个酶促反应均由一种单一的双功能酶L-苹果酰辅酶A裂解酶/β-甲基苹果酰辅酶A裂解酶催化。有趣的是，这种酶在两个不同方向上作用于两种不同的底物：在二氧化碳固定的第一个循环中，它将L-苹果酰辅酶A裂解为乙酰辅酶A和乙醛酸（裂解反应），而在第二个循环中，它将乙醛酸与丙酰辅酶A缩合为β-甲基苹果酰辅酶A（缩合反应）。使用两种不同底物的可逆酶促反应的正向和反向方向的组合相当罕见，并且减少了该途径所需的酶的数量。总之，L-苹果酰辅酶A裂解酶/β-甲基苹果酰辅酶A裂解酶催化L-苹果酰辅酶A与丙酰辅酶A向β-甲基苹果酰辅酶A与乙酰辅酶A的相互转化。
