Horseradish peroxidase. XLI. Complex formation with nitrate and its effect upon compound I formation

Ferric horseradish peroxidase reacts with nitrate and acetate in acidic solution to form weakly bound complexes. Competitive binding experiments with cyanide show that the nitrate binding site is not at the sixth coordination position of the heme iron. The nitrate inhibits compound I formation apparently by binding inside the heme pocket. One physical manifestation of this binding is to increase the apparent pK_a value of the conjugate acid of a catalytic distal group.