The Reaction of the Iron‐Sulfur Protein Hydrogenase with Carbon Monoxide

The iron‐sulfur protein hydroenase from Clostridium pasteurianum was shown to be specifically inhibited by carbon monoxide. Cyanide (10 mM), azide (10 mM), and fluoride (10 mM) were without effect. The inhibition of hydrogenase by carbon monoxide was studied under pre‐steady‐state and under steady‐state conditions. The data obtained from both studies indicate a 1:1 stoichiometry of the reversible reaction between an independent active site of hydrogenase and carbon monoxide. The second‐order rate constant of the formation ( k +1 ) and the first‐order rate constant of the dissociation ( k −1 ) of the hydrogenase · carbon‐monoxide complex at pH values between 6.5 and 8.5 and at 25°C were found to be 5.7 s −1 mM −1 and 0.013 s −1 , respectively. The equilibrium constant K was calculated from pre‐steady‐state data to be 440 mM −1 and from steady‐state data to be 420 mM −1. The carbon monoxide inhibition of hydrogenase could be reversed bylight. The light sensitivity indicates that in the active site iron is being attacked by carbon monoxide.