Clustering for methane

The team of researchers found that ATP is necessary for the MCR methane production and that it also affects the binding of subunits among the MCR activation complex. Indeed, the high-resolution cryo-EM structures confirmed that the presence of ATP molecules stabilizes the A2 subunit within the MCR complex. The A2 component binds asymmetrically to only one of the MCR heterotrimers, while a latch-like structure within McrA regulates access to the F₄₃₀ cavity. Interestingly, this feature comes into play and allows the exposure of the F₄₃₀ complex in the proximal side of the complex, so that the contained Ni centre can be reduced to Ni(I), with a nascent coordination with the thiol in CoM–SH.

Further structural and electronic investigations of the potential electron pathways towards F₄₃₀ also allowed the identification of three electron densities coordinated by McrC, Mmp7 and Mmp17. Each of these is constituted by two [4Fe-3S] clusters bridged by belt sulfur atoms, in a fashion reminiscing of the L-cluster topology in the [8Fe-9S-C] precursor of the M-cluster involved in nitrogenase enzymes. This feature led the researchers to understand which enzyme family – MCR or nitrogenase – first developed this type of clusters: through phylogenetic studies, the team confirmed that these clusters were first used by MCR in methanogenesis and later incorporated into nitrogenase enzymatic scaffolds.