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 soluble one-electron transport protein adrenodoxin (Adx), and a membrane-integrated cytochrome P450 of the CYP11 family. In the 2.3-Å resolution crystal structure of the Adx·AR complex, 580 Å2 of partly polar surface are buried. Main interaction sites are centered around Asp79, Asp76, Asp72, and Asp39 of Adx and around Arg211, Arg240, Arg244, and Lys27 of AR, respectively. In particular, the region around Asp39 defines a new protein interaction site for Adx, similar to those found in plant and bacterial ferredoxins. Additional contacts involve the electron transfer region between the redox centers of AR and Adx and C-terminal residues of Adx. The Adx residues Asp113 to Arg115 adopt 310-helical conformation and engage in loose intermolecular contacts within a deep cleft of AR. Complex 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 Å apart suggesting a possible electron transfer route between these redox centers. The AR·Adx complex represents the first structure of a biologically relevant complex between a ferredoxin and its reductase.