Klebsiella pneumoniae nitrogenase. Mechanism of acetylene reduction and its inhibition by carbon monoxide.

The electron flux through the MoFe-protein of nitrogenase from Klebsiella pneumoniae determines the absolute and relative rates of 2H+ reduction to H2 and acetylene (C2H2) reduction to ethylene (C2H4) at saturating levels of reductant (Na2S2O4) and MgATP. High electron flux, induced by a high Fe-protein (Kp2)/MoFe protein (Kp1) ratio, favours C2H2 reduction. These data can be explained if ethylene, the two-electron reduction product of C2H2, is not released until three electrons have been transferred from Kp2 to Kp1. This explanation is also consistent with a pre-steady-state lag phase for C2H4 formation of 250 ms observed when functioning enzyme is quenched with acid. Electron flux through nitrogenase is inhibited by C2H2 at high protein concentrations. This is because the association rate between Kp1 and oxidized Kp2 is enhanced by C2H2, leading to an increased steady-state concentration of the inhibitory complex Kp2oxKp1C2H2. This effect is not relieved by CO. Thus CO and C2H2 (or C2H4) must be bound at the same time to distinct sites, presumably at Mo or Fe centres, on the enzyme.