The Heme Pocket of CcP
The local environment of the heme is highly conserved among peroxidases.
A global view of the active site of CcP is shown above, and
a close-up is shown at left.
The fifth coordination site of the heme iron is occupied by a
a histidine side chain (for CcP, this residue is His 175).
In all known peroxidases, a carboxylate side chain interacts with
ND of the His ligand, thus increasing the electron density on
the iron (for CcP, this residue is Asp 235).
The figure at the left shows
the location of these residues in relation to
Trp
191, site of the stable Compound I radical of CcP.
The
distal heme face has the following conserved features:
a conserved His residue that can interact readily with
peroxide or other bound ligands,
an Arg residue that can interact either with a water molecule near the
heme carboxylates or with a bound ligand,
and an aromatic residue
(Trp or Phe). For CcP, these residues are His 52, Arg 48, and
Trp 51.
A clue to how these residues work to promote peroxide heterolysis
is provided by the
complex of dioxygen with ferro-peroxidase.
Mutagenesis has been used to evaluate the role of these residues
in the reaction of the enzyme with peroxide, and in the subsequent
electron transfer reaction with cytochrome c.
Click here for abstracts pertaining to the role of
distal His 52 ,
Arg 48, and
Asp 235, in the
kinetics of the peroxide reaction.
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