The aims of this proposal are to compare the structure/function aspects of NADPH-cytochrome P450 reductase (CYPOR) and the three isoforms of nitric oxide synthase (NOS). The crystallization and determination of the 3D structures of the flavoproteins of these enzyme systems, which are required for electron transfer from NADPH to their heme counterparts, have been fully achieved with CYPOR and partially accomplished with the NOS flavoprotein. Proposed studies include site-directed mutagenesis and chimeric construction to examine the mechanisms of electron transfer and regulation in these FAD- and FMN-requiring enzymes. We are also interested in exploring the mechanistic bases of recently identified mutations of the human CYPOR that cause altered steroidogenesis, leading to Antley-Bixler syndrome in humans.