Open Access Articles- Top Results for Cholesterol side-chain cleavage enzyme

Cholesterol side-chain cleavage enzyme

SymbolsCYP11A1 ; CYP11A; CYPXIA1; P450SCC
External IDsOMIM118485 MGI88582 HomoloGene37347 IUPHAR: 1358 ChEMBL: 2033 GeneCards: CYP11A1 Gene
EC number1.14.15.6
RNA expression pattern
File:PBB GE CYP11A1 204309 at tn.png
More reference expression data
RefSeq (mRNA)NM_000781NM_019779
RefSeq (protein)NP_000772NP_062753
Location (UCSC)Chr 15:
74.63 – 74.66 Mb
Chr 9:
58.02 – 58.03 Mb
PubMed search[1][2]

Cholesterol side-chain cleavage enzyme is commonly referred to as P450scc, where "scc" is an acronym for side-chain cleavage. P450scc is a mitochondrial enzyme that catalyzes conversion of cholesterol to pregnenolone. This is the first reaction in the process of steroidogenesis in all mammalian tissues that specialize in the production of various steroid hormones.[1]

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cholesterol + 3 NADPH + 3 H+ + 3 O2 ⇄ pregnenolone + 4-methylpentanal + 3 NADP+ + 3 H2O

P450scc is a member of the cytochrome P450 superfamily of enzymes (family 11, subfamily A, polypeptide 1). The gene name is CYP11A1.[2]


cholesterol monooxygenase (side-chain-cleaving)
EC number
CAS number 37292-81-2
IntEnz IntEnz view
ExPASy NiceZyme view
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO

The systematic name of this enzyme class is cholesterol,reduced-adrenal-ferredoxin:oxygen oxidoreductase (side-chain-cleaving). Other names include:

Tissue and intracellular localization

The highest level of the cholesterol side-chain cleavage system is found in the adrenal cortex and the corpus luteum.[1] The system is also expressed at high levels in steroidogenic theca cells in the ovary, and Leydig cells in the testis.[1] During pregnancy, the placenta also expresses significant levels of this enzyme system.[3] P450scc is also present at much lower levels in several other tissue types, including the brain.[4] In the adrenal cortex, the concentration of adrenodoxin is similar to that of P450scc, but adrenodoxin reductase is expressed at lower levels.[5]

Immunofluorescence studies using specific antibodies against P40scc system enzymes have demonstrated that proteins are located exclusively within the mitochondria.[6][7] P450scc is associated with the inner mitochondrial membrane, facing the interior (matrix).[8] Adrenodoxin and adrenodoxin reductase are soluble peripheral membrane proteins located inside the mitochondrial matrix that appear to associate with each other primarily through electrostatic interactions.[9]

Mechanism of action

P450scc catalyzes the conversion of cholesterol to pregnenolone in three monooxygenase reactions. These involve 2 hydroxylations of the cholesterol side-chain, which generate, first, 22R-hydroxycholesterol and then 20alpha,22R-dihydroxycholesterol. The final step cleaves the bond between carbons 20 and 22, resulting in the production of pregnenolone and isocaproic aldehyde.

Each monooxygenase step requires 2 electrons (reducing equivalents). The initial source of the electrons is NADPH.[10] The electrons are transferred from NADPH to P450scc via two electron transfer proteins: adrenodoxin reductase[11] and adrenodoxin.[12][13] All three proteins together constitute the cholesterol side-chain cleavage complex.

The involvement of three proteins in cholesterol side-chain cleavage reaction raises the question of whether the three proteins function as a ternary complex as reductase:adrenodoxin:P450. Both spectroscopic studies of adrenodoxin binding to P450scc and kinetic studies in the presence of varying concentrations of adrenodoxin reductase demonstrated that the reductase competes with P450scc for binding to adrenodoxin. These results demonstrated that the formation of a functional ternary complex is not possible.[12] From these studies, it was concluded that the binding sites of adrenodoxin to its reductase and to P450 are overlapping and, as a consequence, adrenodoxin functions as a mobile electron shuttle between reductase and P450.[12] These conclusions have been confirmed by structural analysis of adrenodoxin and P450 complex.[14]

The process of electron transfer from NADPH to P450scc is not tightly coupled; that is, during electron transfer from adrenodoxin reductase via adrenodoxin to P450scc, a certain portion of the electrons leak outside of the chain and react with O2, generating superoxide radicals.[15] Steroidogenic cells include a diverse array of antioxidant systems to cope with the radicals generated by the steroidogenic enzymes.[16]


In each steroidogenic cell, the expression of the P450scc system proteins is regulated by the trophic hormonal system specific for the cell type.[1] In adrenal cortex cells from zona fasciculata, the expression of the mRNAs encoding all three P450scc proteins is induced by corticotropin (ACTH).[7][17] The trophic hormones increase CYP11A1 gene expression through transcription factors such as steroidogenic factor 1 (SF-1), by the α isoform of activating protein 2 (AP-2) in the human, and many others.[17][18] The production of this enzyme is inhibited notably by the nuclear receptor DAX-1.[17]

P450scc is always active, however its activity is limited by the supply of cholesterol in the inner membrane. The supplying of cholesterol to this membrane (from the outer mitochondrial membrane) is, thus, considered the true rate-limiting step in steroid production. This step is mediated primarily by the steroidogenic acute regulatory protein (StAR or STARD1). Upon stimulation of a cell to make steroid, the amount of StAR available to transfer cholesterol to the inner membrane limits how fast the reaction can go (the acute phase). With prolonged (chronic) stimulation, it is thought that cholesterol supply becomes no longer an issue and that the capacity of the system to make steroid (i.e., level of P450scc in the mitochondria) is now more important.

Corticotropin (ACTH) is a hormone that is released from the anterior pituitary in response to stress situations. A study of the steroidogenic capacity of the adrenal cortex in infants with acute respiratory disease demonstrated that indeed during disease state there is a specific increase in the steroidogenic capacity for the synthesis of the glucocorticoid cortisol but not for the mineralocorticoid aldosterone or androgen DHEAS that are secreted from other zones of the adrenal cortex.[19]


Mutations in the CYP11A1 gene result in a steroid hormone deficiency, causing a minority of cases of the rare and potentially fatal condition lipoid congenital adrenal hyperplasia.[20][21][22]

See also


  1. ^ a b c d Hanukoglu I (Dec 1992). "Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis.". J Steroid Biochem Mol Biol 43 (8): 779–804. PMID 22217824. doi:10.1016/0960-0760(92)90307-5. 
  2. ^ "Entrez Gene: CYP11A1 cytochrome P450, family 11, subfamily A, polypeptide 1". 
  3. ^ Strauss JF, Martinez F, Kiriakidou M (Feb 1996). "Placental steroid hormone synthesis: unique features and unanswered questions.". Biol Reprod 54 (2): 303–11. PMID 8788180. doi:10.1095/biolreprod54.2.303. 
  4. ^ Stoffel-Wagner B (Dec 2001). "Neurosteroid metabolism in the human brain.". Eur J Endocrinol 145 (6): 669–79. PMID 11720889. doi:10.1530/eje.0.1450669. 
  5. ^ Hanukoglu I, Hanukoglu Z (May 1986). "Stoichiometry of mitochondrial cytochromes P-450, adrenodoxin and adrenodoxin reductase in adrenal cortex and corpus luteum. Implications for membrane organization and gene regulation.". Eur J Biochem 157 (1): 27–31. PMID 3011431. doi:10.1111/j.1432-1033.1986.tb09633.x. 
  6. ^ Hanukoglu I, Suh BS, Himmelhoch S, Amsterdam A (Oct 1990). "Induction and mitochondrial localization of cytochrome P450scc system enzymes in normal and transformed ovarian granulosa cells." (PDF). J Cell Biol 111 (4): 1373–81. PMC 2116250. PMID 2170421. doi:10.1083/jcb.111.4.1373. 
  7. ^ a b Hanukoglu I, Feuchtwanger R, Hanukoglu A (Nov 1990). "Mechanism of corticotropin and cAMP induction of mitochondrial cytochrome P450 system enzymes in adrenal cortex cells." (PDF). J Biol Chem 265 (33): 20602–8. PMID 2173715. 
  8. ^ Farkash Y, Timberg R, Orly J (April 1986). "Preparation of antiserum to rat cytochrome P-450 cholesterol side chain cleavage, and its use for ultrastructural localization of the immunoreactive enzyme by protein A-gold technique". Endocrinology 118 (4): 1353–65. PMID 3948785. doi:10.1210/endo-118-4-1353. 
  9. ^ Hanukoglu I, Privalle CT, Jefcoate CR (May 1981). "Mechanisms of ionic activation of adrenal mitochondrial cytochromes P-450scc and P-45011 beta." (PDF). J Biol Chem 256 (9): 4329–35. PMID 6783659. 
  10. ^ Hanukoglu I, Rapoport R (1995). "Routes and regulation of NADPH production in steroidogenic mitochondria.". Endocr Res 21 (1-2): 231–41. PMID 7588385. doi:10.3109/07435809509030439. 
  11. ^ Hanukoglu, I.; Gutfinger, T.; Haniu, M.; Shively, JE. (Dec 1987). "Isolation of a cDNA for adrenodoxin reductase (ferredoxin-NADP+ reductase). Implications for mitochondrial cytochrome P-450 systems.". Eur J Biochem 169 (3): 449–55. PMID 3691502. doi:10.1111/j.1432-1033.1987.tb13632.x. 
  12. ^ a b c Hanukoglu I, Jefcoate CR (Apr 1980). "Mitochondrial cytochrome P-450sec. Mechanism of electron transport by adrenodoxin." (PDF). J Biol Chem 255 (7): 3057–61. PMID 6766943. 
  13. ^ Hanukoglu I, Spitsberg V, Bumpus JA, Dus KM, Jefcoate CR (May 1981). "Adrenal mitochondrial cytochrome P-450scc. Cholesterol and adrenodoxin interactions at equilibrium and during turnover." (PDF). J Biol Chem 256 (9): 4321–8. PMID 7217084. 
  14. ^ Cite error: The named reference pmid21636783 was invoked but never defined (see the help page).
  15. ^ Hanukoglu I, Rapoport R, Weiner L, Sklan D (September 1993). "Electron leakage from the mitochondrial NADPH-adrenodoxin reductase-adrenodoxin-P450scc (cholesterol side chain cleavage) system". Arch. Biochem. Biophys. 305 (2): 489–98. PMID 8396893. doi:10.1006/abbi.1993.1452. 
  16. ^ Hanukoglu, I. (2006). "Antioxidant protective mechanisms against reactive oxygen species (ROS) generated by mitochondrial P450 systems in steroidogenic cells.". Drug Metab Rev 38 (1-2): 171–96. PMID 16684656. doi:10.1080/03602530600570040. 
  17. ^ a b c Lavoie HA, King SR (2009). "Transcriptional regulation of steroidogenic genes: STARD1, CYP11A1 and HSD3B.". Exp. Biol. Med. (Maywood) 234 (8): 880–907. PMID 19491374. doi:10.3181/0903-MR-97. 
  18. ^ Guo IC, Shih MC, Lan HC, Hsu NC, Hu MC, Chung BC (2007). "Transcriptional regulation of human CYP11A1 in gonads and adrenals.". J. Biomed. Sci. 14 (4): 509–15. PMID 17594537. doi:10.1007/s11373-007-9177-z. 
  19. ^ Hanukoglu A, Fried D, Nakash I, Hanukoglu I (Nov 1995). "Selective increases in adrenal steroidogenic capacity during acute respiratory disease in infants.". Eur J Endocrinol 133 (5): 552–6. PMID 7581984. doi:10.1530/eje.0.1330552. 
  20. ^ Bhangoo A, Anhalt H, Ten S, King SR (March 2006). "Phenotypic variations in lipoid congenital adrenal hyperplasia.". Pediatr. Endocrinol. Rev. 3 (3): 258–71. PMID 16639391. 
  21. ^ al Kandari H, Katsumata N, Alexander S, Rasoul MA (2006). "Homozygous mutation of P450 side-chain cleavage enzyme gene (CYP11A1) in 46,XY patient with adrenal insufficiency, complete sex reversal, and agenesis of corpus callosum.". J. Clin. Endocr. Metab. 91 (8): 2821–6. PMID 16705068. doi:10.1210/jc.2005-2230. 
  22. ^ Kim CJ, Lin L, Huang N, Quigley CA, AvRuskin TW, Achermann JC, Miller WL (March 2008). "Severe combined adrenal and gonadal deficiency caused by novel mutations in the cholesterol side-chain cleavage enzyme, P450scc". J. Clin. Endocrinol. Metab. 93 (3): 696–702. PMC 2266942. PMID 18182448. doi:10.1210/jc.2007-2330. 

Further reading


Steroid hormone synthesis

Steroidogenesis, showing cholesterol side-chain cleavage enzyme at top.
Steroid hormone synthesis

Additional images

External links

  1. REDIRECT Template:Cholesterol and steroid metabolism enzymes