Page 116 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions
P. 116
92 5 Multi-Enzyme Systems and Cascade Reactions Involving Cytochrome P450 Monooxygenases
Cascade type 1 Cascade type 3
P450 1 P450 1 P450 1 P450
S I 1 I 2 P S P
NADPH NADP +
Coproduct Cosubstrate
Cascade type 2 Enzyme
P 1
P450 /P450 2 P450 1
1
S I 1 P 2
P 3
Cascade type 4
P450 3
P450 Enzyme
S I P
P 4
P450 1
I 2
NADPH NADP + NADP + NADPH
P 5
Scheme 5.3 General overview of multistep cascades involving P450s. S, substrate; I, inter-
mediate; P, product.
5.2
Physiological Cascade Reactions Involving P450s
5.2.1
Multistep Oxidations Catalyzed by a Single P450
As mentioned above, a number of P450s catalyze sequential reactions, using
more than one molecule of atmospheric O and iterations of the catalytic cycle to
2
carry out multiple oxidations of a single compound. Whereas single P450 oxida-
tions are based on a single two-electron transfer, multistep oxidations consist of
sequential two-electron oxidations. Several multistep P450s are involved in steroid
metabolism, including, for example, CYP11A1, CYP11B2, CYP17A1, CYP51A1,
and CYP19A1 [29].
A number of kinetic and biochemical studies have been undertaken to elucidate
the mechanism of such multistep oxidations. One of the main aspects of these
investigations is whether the ‘‘intermediate product’’ dissociates out of the active
site or the subsequent reaction takes place immediately without dissociation of
the ‘‘intermediate.’’ According to this behavior, Guengerich and colleagues [29]
distinguish between distributive and processive multistep reactions.
For example, mitochondrial cytochrome CYP11A1 (also referred to as
P450scc) cleaves the side chain of cholesterol to form pregnenolone, the
precursor of all steroid hormones [30]. This process consists of three sequential
monooxygenation reactions and proceeds via 22(R)-hydroxycholesterol and
20α,22(R)-dihydroxycholesterol, followed by the cleavage of the C20–C22 bond,
ultimately yielding pregnenolone (Scheme 5.4). Earlier studies had revealed that
