The crystal structure of fluoride-inhibited cytochrome c peroxidase

The three-dimensional crystal structure of yeast cytochrome c peroxidase complexed with fluoride (F- or HF) has been determined by difference Fourier techniques and partially refined at 2.5-A resolution. Fluoride binding induces significant perturbations of the enzyme structure of the distal side of the heme. The major effect occurs at the active-site arginine residue (Arg-48) which moves about 2 A in order to optimize hydrogen-bonded interactions with the fluorine atom. A small readjustment of the distal histidine (His-52), about 0.5 A, is also seen upon fluoride binding. Additionally, a hydrogen-bonded network of 4 water molecules at the active site is reorganized. No significant movements are detectable in either the heme itself or in the proximal histidine ligand. These observations imply that movement of the Arg-48 side chain may play a key role in the enzymic mechanism of cytochrome c peroxidase. Although we cannot unequivocally determine whether fluoride is bound as HF or F-, the hydrogen-bonding pattern around the ligand points to the protonated form. Structural comparison suggests that there is a difference between the tautomeric state of the imidazole side chain of the distal histidine in cytochrome c peroxidase and of the similarly positioned distal histidine in the globins. This difference accounts for the observation that cytochrome c peroxidase preferentially binds the protonated form of ligands, whereas the globins bind the anionic form. The tautomer indicated by the peroxidase structure is the one required for acid base catalysis (Poulos, T.L., and Finzel, B. C. (1984) in Peptide and Protein Reviews (Decker, M., ed) in press).