MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 101

1984) involves initial hydrogen abstraction from the alkane by (Por • FeIV-O) followed by
the alkyl radical rebound on the ferryl-hydroxo intermediate with formation alcohol.
Another possible reaction of the ferryl-oxene with the hydrocarbon is the oxenoid
insertion. The “mixed” mechanism, involving the elementary reaction, starts with the
partial abstraction of the hydrogen atom from the C-H group by the ferryl-oxene and
ends with the insertion of an oxynen atom across the C-H bond (Shilov, 1997).
3. The masked radical rebound mechanism (Likhtenshtein, 1979 a, 1988 a) involves
the reaction of a superoxide-like radical structure formed in the heme coordination
sphere with the hydrocarbon followed by fast radical recombination:

The reaction I is to a certain extent analogous to the reaction of hydrogen atom
abstraction by the peroxide ot hydroperoxide radicals from the hydrocarbons:

The heat effect of such a reaction in a non-polar solvent for linear hydrocarbons (e.g.
decane) is equal to about 4 kcal/moles. The radical activity of superoxide is dampened
by the molecule stabilization on account of the conjugation of unpaired electrons with
the second oxygen electron pair. This activity can be strengthened by a competitive
involving of the sulfur ligand orbitals in the interaction resulting in weakening the
stabilization, and therefore, increasing the reactivity of superoxide in the heme
coordination sphere. The reaction II is essentially exothermic with kcal/mole

and should run very fast.

Recent developments
One of the most intriguing reactions in the chytochrome P450 catalysis is the transfer of
second electron and dioxygen activation, which appears to be a key step of the entire
process. The chemical nature of reactive oxidizing species appears in the coordination
sphere of heme iron and the mechanism of hydroxylation of organic compounds,
saturated hydrocarbons in particular, is a much debated question in the field of the
cytochrome P450 catalysis. To solve this problem, an entire arsenal of modern
experimental and theoretical methods are employed. The catalytic pathway of
cytochrome P450cam from Pseudomonas putida obtained on the basis of X-ray analysis
at atomic resolution is presented in Fig. 3.10.