CYTOCHROME C PEROXIDASE
(this page is under construction)
Cytochrome c peroxidase (CcP)
is one member of a huge family of heme peroxidases. These enzymes
link reduction of hydroperoxides (i.e. ROOH to ROH + HOH) to
an exogenous electron donor.
The reaction with peroxide is quite similar for all of the heme
peroxidases, and the
heme binding pocket is highly conserved
among heme peroxidases.
electron donor is
characteristic for each
peroxidase, and considerable variability is seen in the
nature of the donor.
the diverse ways
link the oxidizing
power of peroxide to various
does not have a well defined metabolic role, but
seems to be designed
to detoxify hydrogen peroxide formed in the intermembrane
space of mitochrondria during aerobic metabolism.
It does so using ferro-cytochrome c in the intermembrane space as the electron donor.
Thus, CcP holds little interest from a metabolic perspective, but its
ability to promote rapid electron transfer from cytochrome c makes it
a useful structural model for the more complex electron transfer systems
that participate in aerobic metabolism.
Moreover, its reaction with peroxide is quite similar to other known peroxidases, and it is widely viewed as an archetype for this reaction.
The catalytic cycle of heme peroxidases begins with the
coordination of peroxide to the ferric heme.
This reaction is
near (but below) the diffusion limit.
Bound peroxide undergoes
rapid heterolytic cleavage, producing a molecule of water
and the semi-stable intermediate
referred to (for historical reasons) as Compound I.
involves transfer of a proton from peroxide O1 to
O2, followed by O-O bond breaking.
Departure of O2 as water leaves
O1 coordinated to the heme with only six electrons.
It completes its octet
by abstracting the two most readily
available electrons from the enzyme.
One electron is removed from
the iron, creating an oxy-ferryl (Fe=O) center.
For most peroxidases,
the second electron
is removed from the porphyrin ring, creating
a porphyrin pi-cation radical.
For CcP, however,
the second electron is removed
from a Trp residue (Trp 191) that is in van der Waals contact with the heme.
Oxidation of Trp 191
produces an indolyl cation radical
stable in solution
for several hours at room temperature. Click
for a picture of how the reaction sequence might look.
The second stage of the peroxidase
catalytic cycle involves
reduction of Compound I
by an electron
this occurs by the sequential
Compound I with two molecules of ferro-cytochrome c.
Reaction with the first molecule of
cytochrome c produces
Compound II and a molecule of ferri-cytochrome c.
Subsequent reaction of Compound II with ferro-cytochrome c
returns CcP to
the resting ferric state, producing a second molecule
of ferri-cytochrome c and releasing
O1 of peroxide as water.
involve reversible formation of a CcP:Cc complex, followed by
STRUCTURE:FUNCTION RELATIONSHIPS FOR CCP
aspects of the mechanism of CcP are extremely interesting from
the standpoint of
I have chosen to focus on
three specific areas, and my research has benefitted
from collaborations with many talented individual in several other groups.
Perhaps the most provocative issue to address is how electrons can
move rapidly (approximately 10^6 s-1) from ferro-cytochrome c to
the oxidized form of CcP. It is crucial to understand how both the
of the reaction are attained.
A second important issue is how the enzyme manages to create
an indolyl radical in solution that has has a half life of more
than 4 hr at room temperature. This alone seems remarkable, but
it is even more remarkable when one considers the fact that this
stable radical exists in van der Waals contact with a porphyrin ring,
close proximity to several Tyr residues, which would be rapidly
oxidized by a normal Trp radical. That is,
how does the local enzyme structure stabilize the Trp 191 radical?
Another important issue is how the
ferric protoporphyrin IX prosthetic group
of heme peroxidases
reacts so rapidly
while in the globins, the same peroxidase is approximately
5-orders of magnitude slower. That is, what factors determine
the influence of local protein environment upon heme reactivity?