Inner Workings of Cytochrome P450

The active site of cytochrome P450 contains a heme iron center. The iron is tethered to the P450 protein via a thiolate ligand derived from a cysteine residue. This cysteine and several flanking residues are highly conserved in known CYPs and have the formal PROSITE signature consensus pattern [FW] - [SGNH] - x - [GD] - {F} - [RKHPT] - {P} - C - [LIVMFAP] - [GAD]. Because of the vast variety of reactions catalyzed by CYPs, the activities and properties of the many CYPs differ in many aspects. In general, the P450 catalytic cycle proceeds as follows:

1. The substrate binds to the active site of the enzyme, in close proximity to the heme group, on the side opposite to the peptide chain. The bound substrate induces a change in the conformation of the active site, often displacing a water molecule from the distal axial coordination position of the heme iron,[6] and sometimes changing the state of the heme iron from low-spin to high-spin. This gives rise to a change in the spectral properties of the enzyme, with an increase in absorbance at 390 nm and a decrease at 420 nm. This can be measured by difference spectrometry and is referred to as the "type I" difference spectrum (see inset graph in figure). Some substrates cause an opposite change in spectral properties, a "reverse type I" spectrum, by processes that are as yet unclear. Inhibitors and certain substrates that bind directly to the heme iron give rise to the type II difference spectrum, with a maximum at 430 nm and a minimum at 390 nm (see inset graph in figure). If no reducing equivalents are available, this complex may remain stable, allowing the degree of binding to be determined from absorbance measurements in vitro

2. The change in the electronic state of the active site favors the transfer of an electron from NAD(P)H via cytochrome P450 reductase or another associated reductase This takes place by way of the electron transfer chain, as described above, reducing the ferric heme iron to the ferrous state.

3. Molecular oxygen binds covalently to the distal axial coordination position of the heme iron. The cysteine ligand is a better electron donor than histidine, with the oxygen consequently being activated to a greater extent than in other heme proteins. However, this sometimes allows the bond to dissociate, the so-called "decoupling reaction", releasing a reactive superoxide radical, interrupting the catalytic cycle.

4. A second electron is transferred via the electron-transport system, either from cytochrome P450 reductase, ferredoxins, or cytochrome b5, reducing the dioxygen adduct to a negatively charged peroxo group. This is a short-lived intermediate state.

5. The peroxo group formed in step 4 is rapidly protonated twice by local transfer from water or from surrounding amino-acid side chains, releasing one water molecule, and forming a highly reactive iron(V)-oxo species.

6. Depending on the substrate and enzyme involved, P450 enzymes can catalyse any of a wide variety of reactions. A hypothetical hydroxylation is shown in this illustration. After the product has been released from the active site, the enzyme returns to its original state, with a water molecule returning to occupy the distal coordination position of the iron nucleus.

S: An alternative route for mono-oxygenation is via the "peroxide shunt": interaction with single-oxygen donors such as peroxides and hypochlorites can lead directly to the formation of the iron-oxo intermediate, allowing the catalytic cycle to be completed without going through steps 3, 4 and 5. A hypothetical peroxide "XOOH" is shown in the diagram.

C: If carbon monoxide (CO) binds to reduced P450, the catalytic cycle is interrupted. This reaction yields the classic CO difference spectrum with a maximum at 450 nm.

Because most CYPs require a protein partner to deliver one or more electrons to reduce the iron (and eventually molecular oxygen), CYPs are properly speaking part of P450-containing systems of proteins. Five general schemes are known:

* CPR/cyb5/P450 systems employed by most eukaryotic microsomal (i.e., not mitochondrial) CYPs involve the reduction of cytochrome P450 reductase (variously CPR,POR, or CYPOR) by NADPH, and the transfer of reducing power as electrons to the CYP. Cytochrome b5 (cyb5) can also contribute reducing power to this system after being reduced by cytochrome b5 reductase (CYB5R).

* FR/Fd/P450 systems which are employed by mitochondrial and some bacterial CYPs.

* CYB5R/cyb5/P450 systems in which both electrons required by the CYP come from cytochrome b5.

* FMN/Fd/P450 systems originally found in Rhodococcus sp. in which a FMN-domain-containing reductase is fused to the CYP.

* P450 only systems, which do not require external reducing power. Notably these include CYP5 (thromboxane synthase), CYP8, prostacyclin synthase, and CYP74A (allene oxide synthase).