Cytochrome P450 17A1 (CYP17A1) is an important drug target for castration resistant prostate cancer. It is a bi-functional enzyme, catalyzing production of glucocorticoid precursors by hydroxylation of pregnene-nucleus, and androgen biosynthesis by a second C-C lyase step, at the expense of glucocorticoid production. Cytochrome b(5) (cyt b(5)) is known to be a key regulator of the androgen synthesis reaction in vivo, by a mechanism that is not well understood. Two hypotheses have been proposed for the mechanism by which cyt b(5) increases androgen biosynthesis. Cyt b(5) could act as an allosteric effector, binding to CYP17A1 and either changing its selective substrate affinity or altering the conformation of the P450 to increase the catalytic rate or decrease unproductive uncoupling channels. Alternatively, cyt b(5) could act as a redox donor for supply of the second electron in the P450 cycle, reducing the oxyferrous complex to form the reactive peroxo-intermediate. To understand the mechanism of lyase enhancement by cyt b(5), we generated a redox-inactive form of cyt b(5), in which the heme is replaced with a Manganese-protoperphyrin IX (Mn-b(5)), and investigated enhancement of androgen producing lyase reaction by CYP17A1. Given the critical significance of a stable membrane anchor for all of the proteins involved and the need for controlled stoichiometric ratios, we employed the Nanodisc system for this study. The redox inactive form was observed to have no effect on the lyase reaction, while reactions with the normal heme-iron containing cyt b(5) were enhanced similar to 5 fold as compared to reactions in the absence of cyt b(5). We also performed resonance Raman measurements on ferric CYP17A1 bound to Mn-b(5). Upon addition of Mn-b(5) to Nanodisc reconstituted CYP17A1, we observed clear evidence for the formation of a b(5)-CYP17A1 complex, as noted by changes in the porphyrin modes and alteration in the proximal Fe-S vibrational frequency. Thus, although Mn-b(5) binds to CYP17A1, it is unable to enhance the lyase reaction, strongly suggesting that cyt b(5) has a redox effector role in enhancement of the CYP17A1 mediated lyase reaction necessary for androgen synthesis. Published by Elsevier Inc.