Paramagnetic centers in the nickel-containing, deazaflavin-reducing hydrogenase from Methanobacterium thermoautotrophicum

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Two hydrogenases from the methanogenic bacterium Methanobacterium thermoautotrophicum strain ΔH have been purified and contain tightly bound nickel as well as the anticipated iron/sulfur atoms with a fixed ratio of 15-20 iron atoms per nickel. One hydrogenase reduces the 8-hydroxy-5-deazaflavin coenzyme factor 420 (F420), whereas the other has been purified as a methyl viologen-reducing hydrogenase. Both enzymes possess an EPR signal attributed to paramagnetic nickel as demonstrated by hyperfine coupling in 61Ni-containing hydrogenases. Comparison to model compounds suggests a nickel(III) oxidation state in the inactive forms of these aerobically purified enzymes. Loss of the nickel(III) signal accompanies reductive activation but is not kinetically correlated with regain of high specific activity. On replacement of H2 by argon in the gas phase over reduced, active, F420-reducing enzyme, several EPR signals appear, including a signal at g = 2.004 that is probably enzyme-bound FADH semiquinone, two signals at g = 2.140 and 2.196 that reflect a new form of paramagnetic nickel(III), and also a signal at g = 2.036 that may be an iron signal. The F420-reducing hydrogenase in the second paramagnetic nickel form is either itself active or in facile equilibrium with active enzyme. The size of the signal at g = 2.036 may correlate with the degree of activation of the enzyme. In contrast to the hydrogenase of Clostridium pasteurianum [Erbes, D. L., Burris, R. H. & Orme-Johnson, W. H. (1975) Proc. Natl. Acad. Sci. USA 72, 4795-4799], which appears to use only iron/sulfur prosthetic groups and which reacts with one-electron-transfer agents, this methanogen hydrogenase seems to utilize iron, nickel, and flavin redox sites and to reduce obligate one-electron (viologen) and two-electron (deazaflavin) oxidants.

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