CRYSTAL-STRUCTURE OF HORSERADISH-PEROXIDASE-C AT 2.15 ANGSTROM RESOLUTION

The crystal structure of horseradish peroxidase isozyme C (HRPC) has b een solved to 2.15 Angstrom resolution. An important feature unique to the class III peroxidases is a long insertion, 34 residues in HRPC, b etween helices F and G. This region, which defines part of the substra te access channel, is not present in the core conserved fold typical o f peroxidases from classes I and II. Comparison of HRPC and peanut per oxidase (PNP), the only other class III (higher plant) peroxidase for which an X-ray structure has been completed, reveals that the structur e in this region is highly variable even within class III. For peroxid ases of the HRPC type, characterized by a larger FG insertion (seven r esidues relative to PNP) and a shorter F' helix, we have identified th e key residue involved in direct interactions with aromatic donor mole cules. HRPC is unique in having a ring of three peripheral Phe residue s, 142, 68 and 179. These guard the entrance to the exposed haem edge. We predict that this aromatic region is important for the ability of HRPC to bind aromatic substrates.