Heterodisulfide reductase from Methanothermobacter marburgensis contains an active-site [4Fe-4S] cluster that is directly involved in mediating heterodisulfide reduction

Heterodisulfide reductases (HDRs) from methanogenic archaea are iron-sulfur flavoproteins or hemoproteins that catalyze the reversible reduction of the heterodisulfide (CoM-SS-CoB) of the methanogenic thiol coenzymes, coenzyme M (CoM-SH) and coenzyme B (CoB-SH). In this work, the ground and excited-state electronic properties of the paramagnetic Fe-S clusters in Methanothermohacter marburgensis HDR have been characterized using the combination of electron paramagnetic resonance and variable-temperature magnetic circular dichroism spectroscopies. The results confirm multiple S = 1/2 [4Fe-4S](+) clusters in dithionite-reduced HDR and reveal spectroscopically distinct S=112 [4Fe-4S](3+) clusters in oxidized HDR samples treated separately with the CoM-SH and CoB-SH cosubstrates, The active site of HDR is therefore shown to contain a [4Fe-4S] cluster that is directly involved in mediating heterodisulfide reduction. The catalytic mechanism of HDR is discussed in light of the crystallographic and spectroscopic studies of the related chloroplast ferredoxin:thioredoxin reductase class of disulfide reductases. (C) 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.