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要旨

Methanogenic archaea use a [NiFe]-hydrogenase, Frh, for oxidation/reduction of F₄₂₀, an important hydride carrier in the methanogenesis pathway from H₂ and CO₂. Frh accounts for about 1% of the cytoplasmic protein and forms a huge complex consisting of FrhABG heterotrimers with each a [NiFe] center, four Fe-S clusters and an FAD. Here, we report the structure determined by near-atomic resolution cryo-EM of Frh with and without bound substrate F₄₂₀. The polypeptide chains of FrhB, for which there was no homolog, was traced de novo from the EM map. The 1.2-MDa complex contains 12 copies of the heterotrimer, which unexpectedly form a spherical protein shell with a hollow core. The cryo-EM map reveals strong electron density of the chains of metal clusters running parallel to the protein shell, and the F₄₂₀-binding site is located at the end of the chain near the outside of the spherical structure.