Cytochrome P450 protein. Molecular model showing the secondary structure of cytochrome P450. This protein 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. This last role makes understanding their activity extremely important in medicinal chemistry.