High-resolution proton NMR spectra are reported for the paramagnetic ferric native and cyano complexes of the five major horseradish root peroxidase (HRP) isoenzymes (A1, A2, A3, B, and C). Axial imidazole resonances are observed in the native and cyano-complex spectra of all the isoenzymes, thus indicating the presence of a common axial histidine ligand. Proton NMR spectra outside the usual diamagnetic region are identical for sets of A1 and A2 isoenzymes and for the B and C isoenzyme set. Variation in heme residue chemical shift positions may be controlled in part by porphyrin vinyl side chain-protein interactions. Diverse upfield spectra among the isoenzymes reflect amino acid substitutions and/or conformational differences near the prosthetic group, as signals in this region must result from amino acid residues in proximity to the heme center. Acid-base dependence studies reveal an "alkaline" transition that converts the native high-spin iron (III) porphyrin to the low-spin state. The transition occurs at pH 9.3, 9.4, 9.8, and 10.9 for respective HRP A1, A2, A3, and C isoenzymes, respectively. Significantly, this ordering also reflects specific activities for the isoenzymes in the order A1 = A2 greater than A3 greater than B = C. Identical proton NMR spectra for A1/A2 and B/C isoenzyme sets parallel equivalent specific activities for members of a particular set. Proton NMR spectra thus appear to be highly sensitive to protein modifications that affect catalytic activity.