= 22R-Hydroxycholesterol
{c}
<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 $\mathrm{C_{27}H_{46}O_2}$. The stereochemical qualifier identifies a spatial arrangement, while the number 22 identifies the modified carbon.<22R-Hydroxycholesterol in ChEBI>[CHEBI:67237, Definition, Formula and systematic names]{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>.<Cholesterol side-chain cleavage enzyme in IUBMB Enzyme Nomenclature>[EC 1.14.15.6, steps 1a–1c]{ref}
Atom counting separates addition of oxygen from loss of carbon. Relative to cholesterol's $\mathrm{C_{27}H_{46}O}$, 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 $2.56\,\text{\AA}$ from the <haem> iron in the reported structure.<Structural basis for three-step sequential catalysis by the cholesterol side chain cleavage enzyme CYP11A1 by Natalia Mast and others>[Abstract, intermediate-bound 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.
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