Endocrine Abstracts

Reproduction requires the temporal and spacial control of specific and exact signals, delivered by locally regulated hormonal flux. In the case of androgen biosynthesis, cytochrome P450 17α-hydroxylase, 17,20-lyase (P450c17) is the key enzyme. However, P450c17 is a multi-functional P450, so, it synthesizes cortisol in the zona fascicula of the adrenal cortex and also androgen precursors in the adrenal zona reticularis and the gonads. The regulation of this dual activity has been the focus of research studies over many decades and remains unresolved. Functionally, it is known that each of these reactions of P450c17 require electrons transferred by the electron donor cytochrome P450 oxidoreductase (CPR). The first reaction, the 17α-hydroxylation of its substrate, occurs in all cells where P450c17 is expressed. Remarkably, a second, subsequent reaction, namely the 17,20-lyase activity, only occurs in the zona reticularis and gonads. The specificity of the second reaction is due to a non-redox ‘allosteric’ interaction with the haem-protein cytochrome b5. Surprisingly, cytochrome b5 and cytochrome P450 oxidoreductase have overlapping binding sites on the surface of the P450c17 enzyme. This poses the question as to how cytochrome b5 and cytochrome P450 oxidoreductase interact with P450c17 - structurally, functionally and physiologically? This conundrum can be rationalized based on the observation that P450c17 can homodimersise. A homodimer would allow cytochrome P450 oxidoreductase to bind to one P450c17 of the P450c17 homodimer, whilst cytochrome b5 could bind to the other P450c17 simultaneously, at the surfaces distal to the dimer interface. We present our molecular modelling data that predicts that the P450c17 homodimer is a stable structure and the CPR and cytochrome b5 proteins can assemble on the P450c17 dimer to form a tetrameric ‘CPR:P450c17:P450c17:cyt b5’ assembly, predicted to be the functional complex required for androgen biosynthesis. This model is fully consistent with extensive experimental data published over the last two decades. Predictions derived from this model are currently being tested by a range of in vitro and in vivo experimental approaches.