Adrenodoxin reductase-adrenodoxin complex structure suggests electron transfer path in steroid biosynthesis

The steroid hydroxylating system of adrenal cortex mitochondria consists of the membrane-attached NADPH-dependent adrenodoxin reductase (AR), the solu ble one-electron transport protein adrenodoxin (Adx), and a membrane-integr ated cytochrome P450 of the CYP11 family. In the 2.3-Angstrom resolution cr ystal structure of the Adx AR complex, 580 Angstrom (2) of partly polar sur face are buried. Main interaction sites are centered around Asp(79), Asp(76 ), Asp(72), and Asp(39) Of Adx and around Arg(211), Arg(240), Arg(244) and Lys(27) Of AR, respectively. In particular, the region around Asp(39) defin es a new protein interaction site for Adn, similar to those found in plant and bacterial ferredoxins. Additional contacts involve the electron transfe r region between the redox centers of AR and Adx and C-terminal residues of Adx, The Adx residues Asp(113) to Arg(15) adopt 3(10)-helical conformation and engage in loose intermolecular contacts within a deep cleft of AR. Com plex formation is accompanied by a slight domain rearrangement in AR. The [ 2Fe-2S] cluster of Adx and the isoalloxazine rings of FAD of AR are 10 Angs trom apart suggesting a possible electron transfer route between these redo x centers. The AR . Adx complex represents the first structure of a biologi cally relevant complex between a ferredoxin and its reductase.