Direct interaction of coenzyme M with the active‐site Fe–S cluster of heterodisulfide reductase

Heterodisulfide reductase (HDR) catalyzes the formation of coenzyme M (CoM–SH) and coenzyme B (CoB–SH) by the reversible reduction of the heterodisulfide, CoM–S–S–CoB. This reaction recycles the two thiol coenzymes involved in the final step of microbial methanogenesis. Electron paramagnetic resonance (EPR) and variable‐temperature magnetic circular dichroism spectroscopic experiments on oxidized HDR incubated with CoM–SH revealed a S = 1/2 [4Fe–4S] 3+ cluster, the EPR spectrum of which is broadened in the presence of CoM– 33 SH [Duin, E.C., Madadi‐Kahkesh, S., Hedderich, R., Clay, M.D. and Johnson, M.K. (2002) Heterodisulfide reductase from Methanothermobacter marburgensis contains an active‐site [4Fe–4S] cluster that is directly involved in mediating heterodisulfide reduction. FEBS Lett. 512, 263–268; Duin, E.C., Bauer, C., Jaun, B. and Hedderich, R. (2003) Coenzyme M binds to a [4Fe–4S] cluster in the active site of heterodisulfide reductase as deduced from EPR studies with the [ 33 S]coenzyme M‐treated enzyme. FEBS Lett. 538, 81–84]. These results provide indirect evidence that the disulfide binds to the iron–sulfur cluster during reduction. We report here direct structural evidence for this interaction from Se X‐ray absorption spectroscopic investigation of HDR treated with the selenium analog of coenzyme M (CoM–SeH). Se K edge extended X‐ray absorption fine structure confirms a direct interaction of the Se in CoM–SeH‐treated HDR with an Fe atom of the Fe–S cluster at an Fe–Se distance of 2.4 Å.