CO as a substrate and inhibitor of H+ reduction for the Mo-, V-, and Fe-nitrogenase isozymes.

Three known nitrogenase isozymes, Mo-, V-, and Fe-, catalyze biological reduction of dinitrogen (N2) to ammonia (NH3). All three utilize the same reductive elimination mechanism: an intermediate with two metal-bound hydrides reductively-eliminates hydrogen gas (H2) in a reaction coupled to binding and activation of N2. Nonetheless, the three isozymes show dramatically different relative rates of H2 formation and N2 reduction, revealing important differences in reactivity with substrates. Carbon monoxide (CO) has been characterized as both an inhibitor and substrate for Mo- and V‑nitrogenases, but not for the Fe‑nitrogenase. Here, we present a comparative study of the reactivity of the three isozymes with CO, examining CO both as a substrate and as an inhibitor of proton (H+) reduction under steady-state conditions. For Mo‑nitrogenase, there is neither detectable reduction of CO nor inhibition of H+ reduction. Fe- and V‑nitrogenase show CO reduction and inhibition of H+ reduction that depends on the CO partial pressure. For V‑nitrogenase, ethylene (C2H4) is the major reduction product with a maximum specific activity of ~7.5 nmol C2H4/nmol VFe protein/min at 1 atm CO. The major product of CO reduction for Fe‑nitrogenase is methane (CH4) with a maximum specific activity of ~4.8 nmol CH4/nmol FeFe protein/min at 0.05 atm CO. The rate of CH4 production by Fe‑nitrogenase progressively increases to a maximum at 0.05 atm CO and then declines by ~90% with increasing CO partial pressure up to 1 atm. CO does not inhibit proton reduction in Mo‑nitrogenase but shows 16% inhibition for V‑nitrogenase and 35% for Fe‑nitrogenase.