Probing structural and functional domains of human P450c17

Human P450c17 performs at least six chemical transformations, but this spectrum of activity is differentially regulated by structural changes and by redox partner proteins. Furthermore, P450c17 isoforms from different species with approximately 90% amino acid identity exhibit markedly different relative rates for these transformations. Although this phenomenology has been recognized for nearly 20 years, the underlying chemistry and structural basis for these effects are poorly understood. We have constructed a structural model of human P450c17 using computational chemistry to understand informative, naturally occurring human mutations and to provide a rational basis for designing alterations in P450c17 that probe functional domains of the protein. We have mapped with considerable confidence key residues involved in the interaction with redox partner proteins, including K89, R347, and R358, which form positive charges on the "proximal" surface of P450c17. Neutralization of these charges selectively impairs 17, 20-lyase activity without large reductions in 17alpha-hydroxylase activity or 17alpha-hydroxypregnenolone binding. We are now directing our efforts to the identification of key residues in the active site that mediate the substrate specificity and catalytic selectivity of human P450c17.