Fig. 5: Catalytic cycle of Ni–Fe CODHs.

a, In the C_red2 state (PDB: 9FPH), the C cluster carries an unprotonated carbonite ligand. b,c, Hydrogen bonds from His93 and Lys563, together with the covalency of the Ni–C bond, maintain negative charges on the O atoms and foster the protonation of the Fe-bound O of CO₂ (b), destabilizing the C–O bond and splitting the COOH ligand (c). d, The C_red1–CO state is likely to be short-lived because Ni(II) is an atypical CO binder⁵¹. Under oxidizing conditions, CO will diffuse out of the active site and generate the C_red1 state (PDB: 9FPG). e,f, A two-electron reduction will generate the C_int,o state (PDB: 9FPF) (e), whose protonation gives the C_int,i state (PDB: 9FPN) (f). g, The C_int,i state is ready to activate CO₂, giving the C_int,i–CO₂ state, and regenerate the C_red2 state in a, which requires two internal electron transfers to CO₂, one from Ni(I) and the second from B_red via the C cluster, marked by a hash (#). h, Under strong reducing conditions, the B cluster (B_red) may be reduced so that Ni(II)–CO (C_red1–CO) can be reduced to Ni(I)–CO (C_int,o–CO state; PDB: 9FPI, 9FPJ and 9FPO). The electron transfer is probably facilitated by a shift of the Ni(I)/Ni(II) reduction potential caused by the π-acidic CO ligand. After reduction, Ni(I) moves out of the S plane, which aligns CO with the hydrophobic gas tunnel. This state can react after a one-electron reduction, either by reacting with CO₂, which would support the dissociation of CO, or CO can dissociate first, giving the C_int,o state. The potentially slower pathway along C_int,o–CO may be prevented in the presence of CO₂ by the more negative redox potential of the B cluster¹². When the external CO concentration is high under reducing conditions, the formation of the C_int,o–CO state may be responsible for the observed inhibitory effect of CO (ref. ²⁵). We propose the reverse cycle including C_red1 for the CO oxidation reaction. The asterisks indicate that B_ox can be reduced to the B_red state under reducing conditions.