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22R-Hydroxycholesterol is a cholesterol derivative bearing an additional hydroxyl group at carbon 22, with $R$ configuration at that position. Its neutral molecular formula is ${\mathrm{C}}_{27}{\mathrm{H}}_{46}{\mathrm{O}}_2$. The stereochemical qualifier identifies a spatial arrangement, while the number 22 identifies the modified carbon.^[ref]

It is the first intermediate in the three-step reaction sequence of the cholesterol side-chain cleavage enzyme. A second hydroxylation precedes cleavage into pregnenolone and 4-Methylpentanal.^[ref]

Atom counting separates addition of oxygen from loss of carbon. Relative to cholesterol's ${\mathrm{C}}_{27}{\mathrm{H}}_{46}\mathrm{O}$, the intermediate gains one oxygen without changing the carbon count. Hydroxylating a carbon–hydrogen position replaces C–H by C–OH; it does not necessarily add an extra net hydrogen to the molecular formula.

A 2011 X-ray crystallography study observed the intermediate bound to the enzyme, with its hydroxyl oxygen $2.56\text{A˚}$ from the haem iron in the reported structure.^[ref] This gives a concrete position for a chemically specified intermediate. It does not mean that every bound molecule has already completed the subsequent reactions.

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.^[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.^[ref]

The fully coupled overall reaction uses six reduced adrenodoxin equivalents per pregnenolone. Thus an original ideal bookkeeping example producing $2\mu \mathrm{mol}$ of pregnenolone needs $12\mu \mathrm{mol}$ 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.^[ref]

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.^[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 $1\mu \mathrm{mol}$ conversion consumes $3\mu \mathrm{mol}$ 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.^[ref]

In 2011, Natalia Mast and colleagues combined X-ray crystallography with solution spectroscopy. Their $2.5\text{A˚}$ structure contained 22R-Hydroxycholesterol, the first intermediate, whose hydroxyl oxygen lay $2.56\text{A˚}$ from the haem iron. Changing adrenodoxin conditions altered carbon monoxide access to reduced haem in a spectroscopic test.^[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.

Natalia Mast is the first author of a 2011 study explaining how the cholesterol side-chain cleavage enzyme carries out successive reactions converting cholesterol toward pregnenolone. The investigators studied the enzyme bound to the first oxygenated intermediate, combining a crystal structure with solution measurements.^[ref]

The active site forms an elongated cavity leading toward the haem group. The intermediate's placement and interactions suggested that movement within this cavity permits successive reactions while the sterol remains associated with the enzyme. Solution spectroscopy tested changes in its position.^[ref]

The example illustrates why one catalyst can support a multistep transformation without requiring the intermediate to diffuse freely after every step. A bound structure proposes spatial constraints; measurements in solution test whether the arrangement can change, rather than assuming that a crystal captures the only functional position.