Structural basis for pregnenolone biosynthesis by the mitochondrial monooxygenase system

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Steroid hormone synthesis in humans relies on a key enzyme complex involving adrenodoxin reductase (AdR), adrenodoxin (Adx), and cytochrome P450 (CYP11A1).
• This complex is crucial for steroidogenesis and serves as a model for studying electron transfer, catalysis, and C-C bond cleavage by monooxygenases.
• Previous mechanistic understanding was limited by a lack of structural data.

Purpose of the Study:

• To determine the crystal structure of the human Adx-CYP11A1 complex.
• To elucidate the mechanism of electron transfer and catalysis in steroidogenesis.
• To provide structural insights into cholesterol hydroxylation and C-C bond cleavage.

Main Methods:

• X-ray crystallography was used to determine the structure of the Adx-CYP11A1 complex.
• Structures were obtained with cholesterol substrate and reaction intermediates.
• Analysis of the complex's geometry and redox center positioning.

Main Results:

• The crystal structure of the human Adx-CYP11A1 complex, the first for a eukaryotic CYP and its redox partner, was determined.
• The [2Fe-2S] cluster of Adx is positioned 17.4 Å from the heme iron of CYP11A1.
• The structure reveals the mechanism of sequential cholesterol hydroxylations and suggests a mechanism for C-C bond cleavage.

Conclusions:

• The Adx-CYP11A1 structure provides a detailed mechanistic understanding of steroidogenesis.
• Initial protein-protein association is driven by electrostatic forces, leading to an optimized geometry for electron transfer.
• Conserved interaction interfaces suggest this mechanism is common to mitochondrial P450s.