Department of Microbiology, National Science Foundation Integrative Graduate Education Research Traineeship Program in Astrobiology, University of Washington, Seattle, WA 98195, USA.
Proc Natl Acad Sci U S A. 2010 Jun 15;107(24):11050-5. doi: 10.1073/pnas.1003653107. Epub 2010 Jun 1.
In methanogenic Archaea, the final step of methanogenesis generates methane and a heterodisulfide of coenzyme M and coenzyme B (CoM-S-S-CoB). Reduction of this heterodisulfide by heterodisulfide reductase to regenerate HS-CoM and HS-CoB is an exergonic process. Thauer et al. [Thauer, et al. 2008 Nat Rev Microbiol 6:579-591] recently suggested that in hydrogenotrophic methanogens the energy of heterodisulfide reduction powers the most endergonic reaction in the pathway, catalyzed by the formylmethanofuran dehydrogenase, via flavin-based electron bifurcation. Here we present evidence that these two steps in methanogenesis are physically linked. We identify a protein complex from the hydrogenotrophic methanogen, Methanococcus maripaludis, that contains heterodisulfide reductase, formylmethanofuran dehydrogenase, F(420)-nonreducing hydrogenase, and formate dehydrogenase. In addition to establishing a physical basis for the electron-bifurcation model of energy conservation, the composition of the complex also suggests that either H(2) or formate (two alternative electron donors for methanogenesis) can donate electrons to the heterodisulfide-H(2) via F(420)-nonreducing hydrogenase or formate via formate dehydrogenase. Electron flow from formate to the heterodisulfide rather than the use of H(2) as an intermediate represents a previously unknown path of electron flow in methanogenesis. We further tested whether this path occurs by constructing a mutant lacking F(420)-nonreducing hydrogenase. The mutant displayed growth equal to wild-type with formate but markedly slower growth with hydrogen. The results support the model of electron bifurcation and suggest that formate, like H(2), is closely integrated into the methanogenic pathway.
在产甲烷古菌中，产甲烷的最后一步生成甲烷和辅酶 M 与辅酶 B 的异双硫（CoM-S-S-CoB）。异双硫还原酶将这种异双硫还原为 HS-CoM 和 HS-CoB 是一个放能过程。Thauer 等人[Thauer 等人，2008 年《自然评论微生物学》579-591]最近提出，在氢营养型产甲烷菌中，异双硫还原的能量通过黄素基电子分叉为途径中最吸能的反应提供动力，该反应由甲酰甲烷呋喃脱氢酶催化。在这里，我们提供了证据表明这两个产甲烷步骤在物理上是相关的。我们从氢营养型产甲烷菌 Methanococcus maripaludis 中鉴定出一个包含异双硫还原酶、甲酰甲烷呋喃脱氢酶、F（420）-非还原型氢化酶和甲酸脱氢酶的蛋白质复合物。除了为能量守恒的电子分叉模型建立物理基础外，该复合物的组成还表明，H2 或甲酸（产甲烷的两种替代电子供体）都可以通过 F（420）-非还原型氢化酶或甲酸脱氢酶将电子传递给异双硫-H2。与使用 H2 作为中间体相比，电子从甲酸流向异双硫代表了产甲烷中以前未知的电子流途径。我们进一步通过构建缺乏 F（420）-非还原型氢化酶的突变体来测试这条途径是否存在。该突变体在甲酸中与野生型的生长相当，但在氢气中生长明显较慢。结果支持电子分叉模型，并表明甲酸与 H2 一样，紧密地整合到产甲烷途径中。
