Alber Birgit E, Spanheimer Regina, Ebenau-Jehle Christa, Fuchs Georg
Mikrobiologie, Fakultät für Biologie, Albert-Ludwigs-Universität Freiburg, Germany.
Mol Microbiol. 2006 Jul;61(2):297-309. doi: 10.1111/j.1365-2958.2006.05238.x.
Organisms, which grow on organic substrates that are metabolized via acetyl-CoA, are faced with the problem to form all cell constituents from this C(2)-unit. The problem was solved by the seminal work of Kornberg and is known as the glyoxylate cycle. However, many bacteria are known to not contain isocitrate lyase, the key enzyme of this pathway. This problem was addressed in acetate-grown Rhodobacter sphaeroides. An acetate-minus mutant identified by transposon mutagenesis was affected in the gene for beta-ketothiolase forming acetoacetyl-CoA from two molecules of acetyl-CoA. This enzyme activity was missing in this mutant, which grew on acetoacetate and on acetate plus glyoxylate. A second acetate/acetoacetate-minus mutant was affected in the gene for a putative mesaconyl-CoA hydratase, an enzyme which catalyses the hydration of mesaconyl-CoA to beta-methylmalyl-CoA. Beta-methylmalyl-CoA is further cleaved into glyoxylate and propionyl-CoA. These results as well as identification of acetate-upregulated proteins by two-dimensional gel electrophoresis lead to the proposal of a new pathway for acetate assimilation. In a first part, affected by the mutations, two molecules of acetyl-CoA and one molecule CO(2) are converted via acetoacetyl-CoA and mesaconyl-CoA to glyoxylate and propionyl-CoA. In a second part glyoxylate and propionyl-CoA are converted with another molecule of acetyl-CoA and CO(2) to l-malyl-CoA and succinyl-CoA.
依靠通过乙酰辅酶A代谢的有机底物生长的生物体面临着从这个二碳单位形成所有细胞成分的问题。这个问题通过科恩伯格的开创性工作得以解决，这一过程被称为乙醛酸循环。然而，已知许多细菌不含异柠檬酸裂解酶，而该酶是此途径的关键酶。在以乙酸盐为生长底物的球形红细菌中解决了这个问题。通过转座子诱变鉴定出的一个乙酸盐缺陷型突变体，其β-酮硫解酶基因受到影响，该酶能从两分子乙酰辅酶A形成乙酰乙酰辅酶A。在这个突变体中这种酶活性缺失，它能在乙酰乙酸以及乙酸盐加乙醛酸的培养基上生长。第二个乙酸盐/乙酰乙酸缺陷型突变体的一个假定的甲基巴豆酰辅酶A水合酶基因受到影响，该酶催化甲基巴豆酰辅酶A水合形成β-甲基苹果酰辅酶A。β-甲基苹果酰辅酶A进一步裂解为乙醛酸和丙酰辅酶A。这些结果以及通过二维凝胶电泳对乙酸盐上调蛋白的鉴定，导致了一个新的乙酸盐同化途径的提出。在第一部分，受突变影响，两分子乙酰辅酶A和一分子二氧化碳通过乙酰乙酰辅酶A和甲基巴豆酰辅酶A转化为乙醛酸和丙酰辅酶A。在第二部分，乙醛酸和丙酰辅酶A与另一分子乙酰辅酶A和二氧化碳一起转化为L-苹果酰辅酶A和琥珀酰辅酶A。
