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The crystal structure of horseradish peroxidase isozyme C (HRPC) has been solved
to 2.15 A resolution. An important feature unique to the class III peroxidases
is a long insertion, 34 residues in HRPC, between helices F and G. This region,
which defines part of the substrate access channel, is not present in the core
conserved fold typical of peroxidases from classes I and II. Comparison of HRPC
and peanut peroxidase (PNP), the only other class III (higher plant) peroxidase
for which an X-ray structure has been completed, reveals that the structure in
this region is highly variable even within class III. For peroxidases of the
HRPC type, characterized by a larger FG insertion (seven residues relative to
PNP) and a shorter F' helix, we have identified the key residue involved in
direct interactions with aromatic donor molecules. HRPC is unique in having a
ring of three peripheral Phe residues, 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.
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