18.3.2 Postulated reaction cycle of Monooxygenases
The postulated reaction cycle of P450 enzymes is shown in Figure 1 for P450cam
fromPseudomonas putida (Hedegaard and Gunsalus, 1965; Tyson et al., 1972; Le-
wis, 1996), which catalyzes the regio- and stereospecific hydroxylation of camphor
to 5-exo-hydroxycamphor.
It typifies how enzymatic oxygenation is performed under physiological condi-
tions using molecular oxygen as oxidant. In the inactive and substrate-free form
of P450cam (Kaim and Schwederski, 1995) the low-spin FeIII(d^5 ,SĀ¼0.5) center
is six-fold co-ordinated via a porphyrin system, a fifth cysteinate ligand and a water
molecule. Often, water molecules in enzymes which are co-ordinated to metal cen-
ters represent replacement positions for substrates. In the initial reaction step of the
P450cam reaction cycle the water ligand is removed after binding of a camphor
molecule near to the heme center 2 (Poulos et al., 1985, 1987). The camphor binding
induces a change from the low-spin FeIIIconfiguration to a high-spin FeIII-camphor
complex 2 with an iron center far away from the plane porphyrin system (out of plane
18.3 P450 fundamentals 397
Figure 1. Reaction cycle of P450cam fromPseudomonas putida. 1 , P450 aquo complex, low-spin FeIII,
o.p.s; 2 , P450 camphor, high-spin FeIII, o.p.s.; 3 , P450 camphor, high-spin FeII, o.p.s.; 4 , P450 camphor
O 2 - , low-spin FeIII, i.p.s.; 5 , P450 camphor O 2 2ā, low-spin FeIII, i.p.s.; 6 , P450 camphor HO 2 - , low-spin
FeIII, i.p.s.; 7 , P450 camphor, activated oxygen, Fe?, i.p.s.; 8 , P450 camphor adduct, likely low-spin, FeIV
with a cation radical porphyrin or FeIIIwith neutral porphyrin, i.p.s.