P450 enzymes catalytic cycle

NOS is an important signaling enzyme that synthesizes L-citrulline and nitric oxide (NO) from L-arginine and O2 via two turnovers in a P450-like catalytic cycle (Scheme 2). NOS participates in physiological processes such as neurotransmission, vasodilation, and immune response [54,55]. Improper regulation of NO production can lead to diseases such as septic shock, heart disease, arthritis, and diabetes. 

Fig. 5. Catalytic cycle of cytochrome P450. The substrate HR binds to the resting enzyme A to form intermediate B, which is reduced by one electron to form C and then reacts with dioxygen. The resulting ferric-peroxo intermediate D is reduced by one equivalent to form the transient oxyferrous intermediate E, which proceeds quickly to intermediate F with release of a molecule of water. F is designated Fe(V)=0 to indicate that it is oxidized by two equivalents greater than A and not to imply anything about the true oxidation state of the iron. Intermediate F then transfers an oxygen atom to the substrate to regenerate the resting enzyme. The peroxide shunt refers to the reaction of B with hydrogen peroxide to produce the intermediate F, which can then proceed to product formation.

Studies with synthetic P450 enzyme models have already been mentioned to contribute to the understanding of the mechanisms of P450 action and to provide information on the possible structure and electronic nature of iron porphyrins as intermediates of the catalytic cycle. 

In the catalytic cycle of CYP, reducing equivalents are transferred from NADPH to CYP by a flavoprotein enzyme known as NADPH-cytochrome P450 reductase. The evidence that this enzyme is involved in CYP monooxygenations was originally derived from the observation that cytochrome c, which can function as an artificial electron acceptor for the enzyme, is an inhibitor of such oxidations. This reductase is an essential component in CYP-catalyzed enzyme systems reconstituted from purified components. Moreover antibodies prepared from purified reductase are inhibitors of microsomal... 

In addition to the preceding complexities, the P450 enzymes have some unique characteristics that complicate the design of experimental protocols. Because of the broad substrate selectivities for these enzymes, the enzymes are not optimized for the metabolism of a particular substrate. Therefore, the reaction conditions (i.e., pH, ionic strength, temperature) that result in optimum velocities for a given reaction are dependent on both the enzyme and the substrate. To further complicate matters, the velocities for these enzymes tend to vary greatly with changes in these reaction conditions. This variation may well be due to the dependence of the reaction velocity on several pathways in the catalytic cycle. 

Cytochrome P450 enzymes are haem containing oxygenases they catalyse the introduction of oxygen atoms from 02 into substrates. Of the many possible substrates the most important are molecules in which CH groups are converted to COH groups. The catalytic cycle entails a species in which the iron... 

The key features of the presently accepted catalytic cycle for a cytochrome P450 enzyme are summarized in Fig. 17-E-8. The multistep process includes four stable... 

Figure 8.2 Generalized scheme showing the catalytic cycle of cytochrome P450 enzymes in monooxygenation reactions. Fe = iron atom in P450 heme. RH = substrate. ROH = product. b5 = cytochrome bs. ox and red indicate the reduced and (1 electron) oxidized states of the reductase involved in the electron transfer. See text for details. (From Guengerich, F.P., Client. Res. Toxicol. 14, 611, 2001. With permission.)...

The generally proposed P450 catalytic cycle is shown in Figure 15. The overall features of the mechanism have not changed since the early 1970s. However, recent work showed that the nature of the intermediates and rate-limiting step can differ among enzymes, and that protein structure... 

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