Cytochrome P450 reductase molecule. Computer artwork showing the secondary structure of a molecule of cytochrome P450 reductase. This protein facilitates the action of cytochrome P450, an enzyme that plays a crucial role in metabolism in animals (including humans), fungi, plants and bacteria. There are many different types of cytochrome P450 protein, but they all have a similar function; that of catalysing oxidation and reduction reactions. They are involved in manufacturing and breaking down hormones (such as testosterone), metabolising vitamin D and detoxifying substances (including drugs) in the liver.