FIG. 1.

Schematic representation of the different classes of cytochrome P450 systems. Class I systems comprise a FAD-containing flavodoxin reductase, an iron-sulfur protein (ferredoxin), and the P450. The eukaryotic class I enzymes are associated with the mitochondrial membrane. In a class II system, the P450 is partnered with a diflavin reductase, whereas in the class III system, the diflavin reductase is fused to the P450. We propose that the new class IV system is made up of an FMN-containing reductase with a ferredoxin-like center linked to a P450 in a single polypeptide. The N and C termini of the fused class III and IV systems are shown.