E. coli HPII catalase interaction with high spin ligands: formate and fluoride as active site probes

E. coli catalase (HPII) wild type and mutant enzymes (heme dcis-containing) were examined (i) to study the role of a distal haem cavity residue, asparagine-201, in high spin ligand binding and (ii) to compare the differences in this binding between heme d and protoheme enzymes such as that from beef liver (BLC). High spin fluoride complexes were formed by all three HPII catalases examined, wild type (201 asn) and 201gln and 201asp mutants, but with a lower fluoride affinity than that of BLC. The binding of fluoride was pH-dependent, indicating that a proton is bound as well as a fluoride anion. HPII 201glu and 201 asp mutants showed lower affinities for fluoride than did wild type, unlike their reactions with cyanide which are essentially independent of the nature of residue 201. The equilibria and rates of fluoride and formate binding to BLC were reexamined. The rates of reaction with formate were similar to those reported previously. Dissociation rates for fluoride-catalase are higher than for formate suggesting that the latter may be bound differently. High spin complexes between formate and all three HPII forms showed a substantially higher affinity than that of BLC for HPII wild type and progressively lower affinities for the two mutants. As with fluoride the reactions were pH-dependent, indicating that a proton is bound together with the formate anion (or that undissociated formic acid is the ligand). The known structures of the heme groups and heme pockets involved are discussed. Formate may be bound by secondary H-bounds within the heme pocket in both heme dcis and protoheme enzymes. The nature of the heme pocket and the heme access channel may be more important than the chemical nature of the prosthetic group in controlling both high spin ligand interactions and reactions with the substrate hydrogen peroxide.