Investigation of CO Binding and Release from Mo-Nitrogenase during Catalytic Turnover

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Abstract
During enzymatic turnover in the presence of CO, Mo-nitrogenase has been shown to generate two different EPR signals termed lo-CO (PCO = 0.08 atm) and hi-CO (PCO = 0.5 atm). When the formation of hi-CO is monitored under the conditions of very low electron flux, a 2 min lag is observed prior to the initial detection of the signal followed by a near-linear rate of formation during which the S = 3/2 cofactor signal exhibits similar decay kinetics. Increasing the electron flux produces a significant increase in the rate of both the formation of hi-CO and the decay of the S = 3/2 cofactor. These results are interpreted in terms of a state of the enzyme (redox or structural) generated only during turnover which is needed to initially bind CO to the cofactor. Under high electron flux conditions, new EPR inflections are observed at g = 5.78, 5.15 and g = 1.95, 1.81 and tentatively assigned to S = 3/2 and 1/2 states of the CO-bound cofactor and 1 equiv of oxidized P cluster, respectively. Sudden removal of CO from the environment results in the slow decay (>10 min) of both the hi-CO signal and CO inhibition of acetylene reduction activity. The use of ethylene glycol to quench enzymatic activity strongly inhibits the decay of hi-CO (in the presence of CO) and the subsequent decay of lo-CO (after removal of CO) but does not prevent the reversible interconversion hi-CO left and right arrow lo-CO + CO.