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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.

4-Methylpentanal is a branched six-carbon aldehyde with molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{12}\mathrm{O}$. Its condensed structure is $\mathrm{CHO}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{C}{\mathrm{H}}_3)-\mathrm{C}{\mathrm{H}}_3$. The aldehyde carbon is numbered 1, so the branch is a methyl group at carbon 4 of the five-carbon parent chain.^[ref]

The name makes a useful atom-counting example. A pentanal parent chain supplies five carbons, and the methyl substituent supplies a sixth. The hydrogen count is $1+2+2+1+3+3=12$, including the hydrogen attached to the aldehyde carbon. Five in the parent-chain name does not mean that the entire molecule contains only five carbons.

4-Methylpentanal also provides a chemically identifiable coproduct of cholesterol side-chain cleavage. The cholesterol side-chain cleavage enzyme makes pregnenolone and 4-Methylpentanal, separating the original 27-carbon skeleton into 21- and six-carbon products.^[ref] In an ideal fully coupled reaction, one mole of pregnenolone corresponds to one mole of this aldehyde. Detecting both products can therefore test the proposed material balance more specifically than measuring total steroid-related signal alone.

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.

Pregnenolone is a 21-carbon steroid with a position-3 beta-oriented hydroxyl group, a position-20 ketone and a double bond between carbon positions 5 and 6. Its molecular formula is ${\mathrm{C}}_{21}{\mathrm{H}}_{32}{\mathrm{O}}_2$.^[ref]

A key early step of steroid hormone synthesis converts cholesterol into pregnenolone. The cholesterol side-chain cleavage enzyme first hydroxylates the substrate at two positions and then cleaves the side chain, producing pregnenolone and 4-Methylpentanal. This is a sequence of oxygen-dependent reactions with electron transfer, not simply spontaneous breaking of a carbon chain.^[ref]

The products illustrate a compact material-balance check. The 27 carbons of cholesterol divide into 21 in pregnenolone and six in the aldehyde product: $27=21+6$. This count alone does not balance oxygen or hydrogen, but it makes clear why a pathway can retain the four-ring steroid framework while shortening its side chain. Identifying pregnenolone in a sample establishes a particular chemical intermediate; its presence alone does not measure the rate of every downstream hormone-producing reaction.