Towards a unifying mechanism for CYP17 mutations that cause isolated 17,20-lyase deficiency

Cytochrome P450c17 (CYP17) is a single hemoprotein that catalyzes both the 17alpha-hydroxylase and 17,20-lyase reactions in all species thus far examined. Severe defects in CYP17 cause classical 17-hydroxylase deficiency, but other defects result in partial or selective deficiency states. One such variant is the syndrome of isolated 17,20-lyase deficiency. Recent detailed studies of the biochemical properties of the mutant CYP17 enzymes from patients with isolated 17,20-lyase deficiency demonstrate that alterations in the interaction of CYP17 with its redox partner proteins P450-oxidoreductase and cytochrome b5 form the biochemical basis for these selective enzyme defects. Site-directed mutagenesis studies have confirmed that neutralization of any of several positive charges on the redox partner binding surface results in selective disruption of 17,20-lyase activity. In one case diagnosed as isolated 17,20-lyase deficiency, the identified mutation did not map to the redox partner binding surface; however, we have shown that this mutation cannot be the cause of isolated 17,20-lyase deficiency in this patient. These consistent results have prompted us to propose a paradigm in which neutralization of positive charges in the redox partner binding surface of CYP17 may be the predominant if not sole mechanism leading to isolated 17,20-lyase deficiency.