How [Fe]‐Hydrogenase from Methanothermobacter is Protected Against Light and Oxidative Stress

Abstract

Abstract[Fe]‐hydrogenase (Hmd) catalyzes the reversible hydrogenation of methenyltetrahydromethanopterin (methenyl‐H4MPT+) with H2. Hmd contains the iron–guanylylpyridinol (FeGP) cofactor, which is sensitive to light and oxidative stress. A natural protection mechanism is reported for Hmd based on structural and biophysical data. Hmd from Methanothermobacter marburgensis (mHmd) was found in a hexameric state, where an expanded oligomerization loop is detached from the dimer core and intrudes into the active site of a neighboring dimer. An aspartic acid residue from the loop ligates to FeII of the FeGP cofactor and thus blocks the postulated H2‐binding site. In solution, this enzyme is in a hexamer‐to‐dimer equilibrium. Lower enzyme concentrations, and the presence of methenyl‐H4MPT+, shift the equilibrium toward the active dimer side. At higher enzyme concentrations—as present in the cell—the enzyme is predominantly in the inactive hexameric state and is thereby protected against light and oxidative stress.