 Cytochrome b5 acts as a facilitator for 17,20 lyase activity of CYP17A1 and can donate a second electron to some P450s.In humans the production of testosterone via pregnenolone to17-OHPreg and DHEA by the CYP17A1 requires POR.Human P450c17 is phosphorylated on serine and threonine residues by a cAMP-dependent protein kinase.