The primary physiological rôle of the P450 family is that of a monooxygenase. The catalytic reaction can be summarised,
where can be one of a large range of possible substrates.
The specificity of a given P450 is determined by the contact residues that define the active site of the enzyme. These can vary widely between different P450s, however the principal component of the active site of all P450s is a haem moiety. The iron ion of the haem moiety is the site of the catalytic reaction, and is also responsible for the strong 450nm absorption peak in combination with .
The accepted catalytic cycle can be seen in Figure 5.3. This begins when the substrate binds to the active site (1). If the reaction is to proceed further this displaces a water molecule that forms a ligand to the haem iron atom in unbound P450. This is accompanied by a change in the spin of the ion from a low spin () state in which the 5 3d electrons are maximally paired, to a high spin () state in which the electrons are maximally unpaired. This in turn causes a change in the redox potential of the iron from approximately -300 mV to approximately -170 mV. This is sufficient to make the reduction of the iron by the redox-partner of the cytochrome, usually NADPH or NADH, thermodynamically favourable (2). This is followed by the binding of an molecule to a separate site adjacent to the ion (3). This state is not stable and is easily autooxidised releasing , however if the transfer of a second electron occurs (4), the catalytic reaction continues. The then reacts with protons from the surrounding solvent to form which is released leaving an activated oxygen atom (5) which then may react with the substrate molecule (6) resulting in an hydroxylated from of the substrate (7). This entire reaction cycle usually takes between 1 and 10 seconds.
Figure 5.3: Schematic diagram of cytochrome P450 monooxygenase reaction with substrate RH and product ROH.