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.