BIOINORGANIC CHEMISTRY A Short Course Second Edition

366 IRON-CONTAINING PROTEINS AND ENZYMES

and other P450 reactions. The so - called “ compound I, ” a π - cation radical inter-
mediate that has an oxidation state two electrons above the Fe III resting state,
is thought to be analogous to that established for most peroxidases and cata-
lases.^39 Several researchers have observed absorption spectra resembling that
ofC. fumago chloroperoxidase compound I for a transient species formed
when substrate - free cytochrome P450 was mixed with m - chloroperoxybenzoic
acid ( m - CPBA).^40 Other workers have attempted to obtain M ö ssbauer and
EPR spectra by freeze - quenching substrate - free cytochrome P450 in reactions
with peracetic acid as the oxygen donor. These researchers observed a tyrosine
radical and an Fe IV – heme derivative that was concluded to be a P450 com-
pound II moiety. The so - called “ compound II ” species has been identifi ed,
again in the catalytic cycles of peroxidases and catalases, as an oxoiron(IV)
porphyrin species, one electron above the Fe III resting state of the enzymes. 56a
This intermediate does not appear in Figure 7.14 but will be discussed further
in Section 7.4.5. The question then arises: Do the peroxoferric, [ III( )( 22 −+)]Fe, Por O
(5a ) or hydroperoxoferric, [Fe III (Por)(O 2 H − )] 2+ , ( 5b ) intermediates function as
alternate catalysts to compound I? This possibility is discussed in the literature
as the “ two - oxidant ” or “ multiple - oxidant ” hypothesis, and again discussion
will be deferred to Section 7.4.5. However, note that the peroxoferric ( 5a )
intermediate with its negatively charged peroxo group might be a nucleophilic
reagent, whereas all the other proposed intermediates function as electrophilic
reagents. (See Figure 7.15 .) In addition, Section 7.4.5 will discuss the possibility
of side - on peroxide coordination as in the research of J. S. Valentine and
co - workers. 60 – 62
Most published cytochrome P450 catalytic cycles will show the end - on
peroxo coordination as shown in Figure 7.14 ( 4, 5a, 5b ). Shaik and co - workers
have used density functional theory methods to propose a “ two - state ” theory

Figure 7.15 Intermediates in the cytochrome P450 reaction cycle. (Adapted with
permission from Scheme 1 of reference 39. Copyright 2004, Society of Biological Inor-
ganic Chemistry.)

N

NN

N

FeIII

O

O

S

Cys

2 -

N

NN

N

FeIII

O

OH

S

Cys

S

Cys

N

NN

N

FeIV

O

Electrophilic

Epoxidation

Sulfoxidation?

Hydroxylation

(Inserts OH+)

Electrophilic

Epoxidation

Sulfoxidation

Hydroxylation

(Inserts O)

Nucleophilic

Deformylation

H+ H+ O

S

Cys

N

NN

N

FeIV

[FeIV(Por+.)(O)]+

oxoiron(IV)(Por+.)

ferryl oxoiron

low spin compound I

[FeIV(Por)(O.)]+

oxoiron(IV)(Por)

ferryl oxoiron

high spin compound I

H 2 O

[FeIII(Por)(O 2 2-)]+ [FeIII(Por)(O 2 H-)]2+

peroxoferric hydroperoxoferric

Reactions

Electrophilic

Hydroxylation

(abstracts H)

spin

inversion