Advanced electron paramagnetic resonance on the catalytic iron-sulfur cluster bound to the CCG domain of heterodisulfide reductase and succinate: quinone reductase.

Heterodisulfide reductase (Hdr) is a key enzyme in the energy metabolism of methanogenic archaea. The enzyme catalyzes the reversible reduction of the heterodisulfide (CoM-S-S-CoB) to the thiol coenzymes M (CoM-SH) and B (CoB-SH). Cleavage of CoM-S-S-CoB at an unusual FeS cluster reveals unique substrate chemistry. The cluster is fixed by cysteines of two cysteine-rich CCG domain sequence motifs (CX31-39CCX35-36CXXC) of subunit HdrB of the Methanothermobacter marburgensis HdrABC complex. We report on Q-band (34 GHz) (57)Fe electron-nuclear double resonance (ENDOR) spectroscopic measurements on the oxidized form of the cluster found in HdrABC and in two other CCG-domain-containing proteins, recombinant HdrB of Hdr from M. marburgensis and recombinant SdhE of succinate: quinone reductase from Sulfolobus solfataricus P2. The spectra at 34 GHz show clearly improved resolution arising from the absence of proton resonances and polarization effects. Systematic spectral simulations of 34 GHz data combined with previous 9 GHz data allowed the unambiguous assignment of four (57)Fe hyperfine couplings to the cluster in all three proteins. (13)C Mims ENDOR spectra of labelled CoM-SH were consistent with the attachment of the substrate to the cluster in HdrABC, whereas in the other two proteins no substrate is present. (57)Fe resonances in all three systems revealed unusually large (57)Fe ENDOR hyperfine splitting as compared to known systems. The results infer that the cluster's unique magnetic properties arise from the CCG binding motif.