Abstract
Heterodisulfide reductase (Hdr) from methanogenic Archaea catalyzes the reversible reduction of the heterodisulfide (CoM-S–S-CoB) of the methanogenic thiol coenzymes, coenzyme M (CoM-SH) and coenzyme B (CoB-SH). Upon reaction of the oxidized enzyme with CoM-SH a unique paramagnetic species is formed, which has been shown to be due to a novel type of [4Fe–4S]3+ cluster. In this work, it was addressed whether CoM-SH is directly attached to this [4Fe–4S] cluster using CoM-33SH as substrate and purified Hdr from Methanothermobacter marburgensis and Methanosarcina barkeri. With both enzymes treatment with CoM-33SH in the presence of duroquinone as an oxidant resulted in a significant broadening of the electron paramagnetic resonance spectrum as compared to CoM-SH as substrate. The signal broadening resulted from an unresolved anisotropic hyperfine coupling between the 33S nucleus and the paramagnetic center. The results provide compelling evidence for a direct binding of CoM-SH to the [4Fe–4S] cluster in the active site of the enzyme.
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