Molecular recognition and electron transfer in mitochondrial steroid hydroxylase systems

Mitochondrial monooxygenase systems are involved in the biosynthesis of glucocorticoids, mineralocorticoids, bile acids, and 1,25-dihydroxyvitamin D. The reactions are catalyzed by specific P450 enzymes that receive reducing equivalents via NADPH-ferredoxin oxidoreductase (adrenodoxin reductase) and ferredoxin (adrenodoxin). Although the three-dimensional structures of the individual components have not yet been solved, methods of expressing recombinant forms of these enzymes in Escherichia coli have allowed the use of site-directed mutagenesis to investigate the roles of specific amino acids in protein binding interactions, electron transfer, and catalysis. These studies have identified key charged residues in NADPH-ferredoxin oxidoreductase, ferredoxin, and P450scc, which are involved in electrostatic interactions critical for recognition, high-affinity binding, and electron transfer. The finding that the binding sites on ferredoxin for NADPH-ferredoxin oxidoreductase and P450 show significant overlap supports the proposed function for ferredoxin as a mobile electron shuttle between the reductase and P450 enzymes and is consistent with ferredoxin's role in serving multiple P450 isoforms.