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.^[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.