Cytochrome P450 (Section 12-4D) catalyzes a reaction in which two electrons supplied by NADPH reduce the heme Fe atom so that it can then reduce O2 preparatory to the hydroxylation of a substrate molecule. Substrate binding to the enzyme displaces a water molecule that forms a ligand to the heme iron atom. This changes the reduction potential of the Fe from -0.300 V to -0.170 V. Why is this change necessary for efficient catalysis?