= Adrenodoxin
<Adrenodoxin> is an <iron-sulfur cluster>-containing electron-transfer <protein> involved in mitochondrial steroid synthesis. It transfers reducing equivalents to mitochondrial <Cytochrome P450> enzymes, including the <cholesterol side-chain cleavage enzyme>. Its cluster is of the $[2\mathrm{Fe}-2\mathrm S]$ type; the structural presence of that cofactor and its interaction with a partner can be investigated separately from catalytic turnover.<Structural basis for pregnenolone biosynthesis by the mitochondrial monooxygenase system by Natallia Strushkevich and others>[Abstract and Figure1: Adx cofactor and CYP11A1 complex]{ref}
The electron-transfer chain distinguishes the donor from the enzyme doing the substrate conversion. <NADPH> supplies electrons upstream through <adrenodoxin reductase>; reduced <adrenodoxin> then supplies one-electron equivalents to the <cholesterol side-chain cleavage enzyme>.<Cholesterol side-chain cleavage enzyme in IUBMB Enzyme Nomenclature>[Overall reaction and Comments: electron-transfer pathway]{ref}
The fully coupled overall reaction uses six reduced <adrenodoxin> equivalents per <pregnenolone>. Thus an original ideal bookkeeping example producing $2\,\mathrm{\mu mol}$ of <pregnenolone> needs $12\,\mathrm{\mu mol}$ of one-electron donor equivalents. This does not mean twelve micromoles of fresh <protein> must be consumed: a carrier can be reduced again and reused.
A 2011 study also changed adrenodoxin conditions while measuring <carbon monoxide> access to reduced <haem> spectroscopically. This probes an effect of the redox partner on the catalytic protein, beyond merely locating two structures next to one another.<Structural basis for three-step sequential catalysis by the cholesterol side chain cleavage enzyme CYP11A1 by Natalia Mast and others>[Abstract and Figure3 legend: adrenodoxin-dependent spectroscopic comparison]{ref}
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