title: Cytochrome P-450scc a review of the specificity and properties of the cholesterol binding site.
abstract: Cytochrome P-450scc is unusual among members of this class of enzymes in showing a high degree of substrate specificity. Features of the cholesterol structure which are particularly important for binding include the 3 beta-hydroxyl, the delta 5-ring configuration, and the side-chain organization in the 20-22 region. Regarding the ring system, binding appears to require planarity and limited size at the 4-5-6 carbons (the A-B ring juncture). In the region of the 3 beta-hydroxyl, a "cleft" in the binding site extends about 4 A beyond the hydroxyl and can accommodate two additional ether-linked carbons. Evidence indicates that an enzyme residue hydrogen-bonds to the oxygen of the 3 beta hydroxyl, providing much of the energy for the initial enzyme-substrate interaction. The cytochrome shows less specificity for the side-chain structure, except in the region of carbons 20-22 where hydroxylation/side-chain cleavage takes place. The binding cleft for the side-chain is limited to approximately the length of the isocaproic group but can accommodate structural variations beyond the 22-position. Evidence indicates that the region near the 20-22 bond is more limited in size, and that an amino acid residue near the heme iron binds strongly and stereospecifically to the 22R-hydroxyl of the cleavage intermediates, 22R-hydroxycholesterol and 20 alpha, 22R-dihydroxycholesterol. The 22R-hydrogen of cholesterol is very close to the heme iron (approximately 3 A), while the 22S-hydrogen is slightly further (about 4 A). The size and bonding properties of the steroid binding/active site suggest a mechanism which accounts for the stereospecificity and sequence of reactions catalyzed by cytochrome P-450scc.
mesh_terms: D000302:Adrenal Cortex / Q000201:enzymology / Q000648:ultrastructure; D000818:Animals; D001665:Binding Sites; D002784:Cholesterol / Q000378:metabolism*; D002788:Cholesterol Esters / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D008928:Mitochondria / Q000201:enzymology; D008958:Models, Molecular; D013010:Sonication; D013379:Substrate Specificity
chemical_list: D002788:Cholesterol Esters; D007527:Isoenzymes; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol