372 IRON-CONTAINING PROTEINS AND ENZYMES
intermediates that they have described explain how the conserved residues
asp251 and thr252 become part of a hydrogen - bonding network (with water
molecules) that could deliver protons to the iron - bound dioxygen. Specifi cally
for PDB: 1DZ8, the P450 CAM – O 2 complex, they believe the carbonyl oxygen
of asp251 fl ips so that it can interact with the amide and amino groups of
asn255 upon dioxygen binding. At the same time the amide nitrogen of thr252
rotates toward the active site, providing an additional hydrogen bond to sta-
bilize H 2 O 901. The thr252 rotation also places its hydroxyl in position to interact
with the bound dioxygen molecule as well as both H 2 O 901 and H 2 O 902 (see
Figure 7.13 ). In conclusion the reference 35 authors hope that the structures
they presented will provide reference points for future modeling of the P450
reaction mechanism, either computationally or by design and synthesis of
model compounds. Computational analyses of cytochrome P450 structures has
been discussed previously in this section. This research, by Shaik and co -
workers, postulated the “ two states ” theory — namely that high - and low - spin
states of the compound I intermediate were responsible for differences in
cytochrome P450 reactivity. 46 – 48
7.4.5 Cytochrome P450 Model Compounds,
7.4.5.1 Introduction. The chemistry and biology of cytochrome P450 model
compounds has a rich history. Many intermediates have been identifi ed or
proposed to exist in the catalytic cycles proposed by researchers studying
cytochrome P450 biomimetic chemistry. These include (1) ferric – peroxo
(Fe III O()−^22 −) complexes, (2) oxoiron(IV) porphyrin π - cation radicals (fi rst
identifi ed in peroxidase and catalase catalytic cycles and commonly called
“ compound I ” ),^53 (3) oxidant - iron(III) porphyrin adducts,^54 (4) oxoiron(V)
porphyrins (isoelectronic with oxoiron(IV) porphyrinπ - cation radicals),^55 and
(5) oxoiron(IV) porphyrins^56 (known as “ compounds II ” in catalytic cycles of
peroxidases and catalases). (See also Figure 7.14 .) Note that formation of
oxoiron(IV) porphyrinπ - cation radicals requires a two - electron oxidation
from the Fe(III) porphyrin resting state, whereas the oxoiron(IV) porphyrins
require a one electron oxidation. Some of these intermediates are subjects of
intense discussion among researchers in the fi eld. For instance, oxoiron(IV)
porphyrinπ - cation radicals have been proposed as the sole oxidant in P450
chemistry, whereas other researchers have proposed multiple oxidizing species
in these oxygen transfer reactions.^57 Evidence in support of the various inter-
mediates proposed for cytochrome P450 model compounds will be presented
in Section 7.4.5.3.
7.4.5.2 A Cytochrome P450 Model Compound: Structural. The crystal
structure of a ferric H93G (his93gly) myoglobin, Mb, cavity mutant at a resolu-
tion of 1.7 Å has been published recently (PDB: 1EVP).^58 The H93G mutant
protein includes a myoglobin (Mb) heme featuring a high - spin fi ve - coordinate