The Formate Dehydrogenase Involved in Electron Transport from Formate to Fumarate in Vibrio succinogenes

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1. The formate dehydrogenase of Vibrio succinogenes, which is involved in electron transport with fumarate as terminal acceptor, was solubilized with Triton X-100 and purified some 200-fold by means of chromatography on hydroxyapatite, sucrose-density-gradient centrifugation and chromatography on DEAE-Sephadex. Gel filtration failed to increase the specific acitivity of the enzyme while gel electrophoresis in the presence of dodecylsulfate revealed that 73% of the protein of the preparation consisted of a polypeptide of Mr 110 000. The Mr of the functional enzyme was found to be 263 000 on the basis of the Stokes radius (5.8 nm) and the sedimentation coefficient (11.3 S). 2. The preparation contained 9 micronmol molybdenum/g protein and about 170 mumol iron-sulfur/g protein. The contents of b and c cytochromes varied and were lower than that of molybdenum. The low-potential cytochrome b [Kröger, A. and Innerhofer, A. (1976) Eur. J. Biochem. 69, 497-506] present in the preparation was reduced by formate. 3. The preparation catalyzed the reduction of a variety of dyes by formate, but not of NAD, FMN, ferredoxin or oxygen. The reduction of CO2 or bicarbonate by reduced methyl viologen was not catalyzed. The reaction with benzyl viologen obeyed the rate law consistent with a ping-pong mechanism. The Km for formate was 1.5 mM at infinite concentration of benzyl viologen while that for benzyl viologen was 0.53 mM at infinite formate concentration. Enzymic activity was inhibited by azide, KCN and HgCl2, but not by 4-chloromercuriphenylsulfonate or 2-(n-nonyl)-4-hydroxyquinoline-N-oxide, both of which inhibit overall electron transport. The inhibition by azide was competitive with formate; the Ki was 45 micron. 4. The midpoint potential of the low-potential cytochrome b of the membrane fraction was shifted -40 mV by the presence of 2-(n-nonyl)-4-hydroxyquinoline-N-oxide. 5. It is concluded that the formate dehydrogenase of V. succinogenes is isolated as a dimer consisting of two identical subunits of Mr 110,000, each of which carries one atom of molybdenum and iron-sulfur groups. The low-potential cytochrome b is the direct acceptor for the electrons of formate dehydrogenase in the electron transport of formate-fumarate reduction of V. succinogenes. Inhibition of electron transport of the membrane fraction between formate dehydrogenase and menaquinone by 2-(n-nonyl)-4-hydroxyquinoline-N-oxide [Kröger, A. and Innerhofer, A. (1976) Eur. J. Biochem. 69, 487-495] is caused by the inhibitor binding to the low-potential cytochrome b.