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

Area of Science:

• Biochemistry
• Structural Biology
• Mitochondrial Function

Background:

• The adrenal cortex utilizes a steroid hydroxylating system involving adrenodoxin reductase (AR), adrenodoxin (Adx), and CYP11 enzymes.
• Understanding the structural basis of electron transfer between AR and Adx is crucial for comprehending steroidogenesis.

Purpose of the Study:

• To determine the high-resolution crystal structure of the Adx.AR complex.
• To elucidate the interaction sites and molecular mechanisms governing complex formation and electron transfer.

Main Methods:

• X-ray crystallography at 2.3-A resolution.
• Analysis of protein-protein interaction surfaces and residue contacts.
• Comparison with known ferredoxin-reductase interaction sites.

Main Results:

• The crystal structure revealed significant buried surface area (580 A(2)) with specific interaction sites involving aspartate residues on Adx and arginine/lysine residues on AR.
• A novel interaction site on Adx, centered around Asp(39), was identified, similar to those in plant and bacterial ferredoxins.
• The [2Fe-2S] cluster of Adx and the FAD of AR are positioned approximately 10 A apart, suggesting a viable electron transfer pathway.

Conclusions:

• The Adx.AR complex structure provides a detailed molecular model for electron transfer in steroid hydroxylation.
• This work represents the first structural characterization of a biologically relevant ferredoxin-reductase complex, offering insights into enzyme-substrate interactions and electron transport mechanisms.