Electron Bifurcation Involved in the Energy Metabolism of the Acetogenic Bacterium Moorella thermoacetica Growing on Glucose or H 2 plus CO 2

Abstract

Moorella thermoacetica ferments glucose to three acetic acids. In the oxidative part of the fermentation, the hexose is converted to 2 acetic acids and 2 CO ₂ molecules with the formation of 2 NADH and 2 reduced ferredoxin (Fd _red ²⁻ ) molecules. In the reductive part, 2 CO ₂ molecules are reduced to acetic acid, consuming the 8 reducing equivalents generated in the oxidative part. An open question is how the two parts are electronically connected, since two of the four oxidoreductases involved in acetogenesis from CO ₂ are NADP specific rather than NAD specific. We report here that the 2 NADPH molecules required for CO ₂ reduction to acetic acid are generated by the reduction of 2 NADP ⁺ molecules with 1 NADH and 1 Fd _red ²⁻ catalyzed by the electron-bifurcating NADH-dependent reduced ferredoxin:NADP ⁺ oxidoreductase (NfnAB). The cytoplasmic iron-sulfur flavoprotein was heterologously produced in Escherichia coli , purified, and characterized. The purified enzyme was composed of 30-kDa (NfnA) and 50-kDa (NfnB) subunits in a 1-to-1 stoichiometry. NfnA harbors a [2Fe2S] cluster and flavin adenine dinucleotide (FAD), and NfnB harbors two [4Fe4S] clusters and FAD. M. thermoacetica contains a second electron-bifurcating enzyme. Cell extracts catalyzed the coupled reduction of NAD ⁺ and Fd with 2 H ₂ molecules. The specific activity of this cytoplasmic enzyme was 3-fold higher in H ₂ -CO ₂ -grown cells than in glucose-grown cells. The function of this electron-bifurcating hydrogenase is not yet clear, since H ₂ -CO ₂ -grown cells additionally contain high specific activities of an NADP ⁺ -dependent hydrogenase that catalyzes the reduction of NADP ⁺ with H ₂ . This activity is hardly detectable in glucose-grown cells.