= Cholesterol side-chain cleavage enzyme
The <cholesterol side-chain cleavage enzyme>, also designated CYP11A1, is a <Cytochrome P450> that converts <cholesterol> into <pregnenolone> and <4-Methylpentanal>. Its official accepted name is cholesterol monooxygenase (side-chain-cleaving), EC 1.14.15.6. Two hydroxylations precede cleavage between carbon positions 20 and 22.<Cholesterol side-chain cleavage enzyme in IUBMB Enzyme Nomenclature>[overall reaction, steps 1a–1c and Comments]{ref}
<Adrenodoxin> supplies electrons to the enzyme; <NADPH> is an upstream electron source. For the fully coupled net reaction, each <pregnenolone> formed requires three oxygen molecules and six reduced adrenodoxins. Thus an ideal $1\,\mathrm{\mu mol}$ conversion consumes $3\,\mathrm{\mu mol}$ oxygen and six micromoles of one-electron donor equivalents. The carbon balance is $27=21+6$, partitioning the original steroid and its shortened side chain.<Cholesterol side-chain cleavage enzyme in IUBMB Enzyme Nomenclature>[Reaction and electron-transfer pathway]{ref}
In 2011, <Natalia Mast> and colleagues combined <X-ray crystallography> with solution spectroscopy. Their $2.5\,\text{\AA}$ structure contained <22R-Hydroxycholesterol>, the first intermediate, whose hydroxyl oxygen lay $2.56\,\text{\AA}$ from the <haem> iron. Changing adrenodoxin conditions altered <carbon monoxide> access to reduced haem in a spectroscopic test.<Structural basis for three-step sequential catalysis by the cholesterol side chain cleavage enzyme CYP11A1 by Natalia Mast and others>[Abstract and Figure 3 legend]{ref}
Together, these measurements connect substrate positioning with a perturbation in solution. The proposed movement of intermediates during successive reactions is a mechanistic interpretation of the combined evidence; a single structure is a snapshot of one bound state.
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