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hydroxymethylglutaryl-CoA reductase (NADPH)
Identifiers
EC number 1.1.1.34
CAS number 9028-35-7
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures
Gene Ontology AmiGO / EGO
hydroxymethylglutaryl-CoA reductase
Identifiers
EC number 1.1.1.88
CAS number 37250-24-1
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures
Gene Ontology AmiGO / EGO
3-hydroxy-3-methylglutaryl-Coenzyme A reductase
HMGR-1DQ8.png
HMG-CoA reductase
Identifiers
Symbol HMGCR
Entrez 3156
HUGO 5006
OMIM 142910
RefSeq NM_000859
UniProt P04035
Other data
EC number 1.1.1.88
Locus Chr. 5 q13.3-q14

HMG-CoA reductase (or 3-hydroxy-3-methyl-glutaryl-CoA reductase or HMGR) is the rate-controlling enzyme (EC 1.1.1.88) of the mevalonate pathway, the metabolic pathway that produces cholesterol and other isoprenoids. This enzyme is the target of the widely available cholesterol-lowering drugs known collectively as the statins. HMG-CoA reductase is anchored in the membrane of the endoplasmic reticulum, and was long regarded as having seven transmembrane domains, with the active site located in a long carboxyl terminal domain in the cytosol. More recent evidence shows it to contain eight transmembrane domains.[1]

The enzyme commission designation is EC 1.1.1.34 for the NADPH-dependent enzyme, whereas 1.1.1.88 links to an NADH-dependent enzyme.

In humans, the gene for HMG-CoA reductase is located on the long arm of the fifth chromosome (5q13.3-14).[2] Related enzymes having the same function are also present in other animals, plants and bacteria.

Contents

Reaction

HMGCR converts HMG-CoA to mevalonic acid:

Mevalonate pathway

Inhibitors

Drugs

Drugs that inhibit HMG-CoA reductase, known collectively as HMG-CoA reductase inhibitors (or "statins"), are used to lower serum cholesterol as a means of reducing the risk for cardiovascular disease.[3]

These drugs include rosuvastatin (CRESTOR), lovastatin (Mevacor), atorvastatin (Lipitor), pravastatin (Pravachol), fluvastatin (Lescol), and simvastatin (Zocor).[4] Red yeast rice extract contains several naturally occurring cholesterol-lowering statins known as monacolins. The most active of these is monacolin K, or lovastatin (previously sold under the trade name Mevacor, and now available as generic lovastatin).[5]

Vytorin is drug that combines the use simvastatin and ezetimibe, which blocks the formation of cholesterol by the body, along with the absorption of cholesterol in the intestines.[6]

Hormones

HMG-CoA reductase is active when blood glucose is high. The basic functions of insulin and glucagon are to maintain glucose homeostasis. Thus, in controlling blood sugar levels, they indirectly affect the activity of HMG-CoA reductase, but a decrease in activity of the enzyme is caused by an AMP-activated protein kinase, which responds to an increase in AMP concentration, and also to leptin (see 4.4, Phosphorylation of reductase).

Importance

HMG-CoA reductase is a polytopic, transmembrane protein that catalyzes a key step in the mevalonate pathway (MetaCyc mevalonate pathway), which is involved in the synthesis of sterols, isoprenoids and other lipids. In humans, HMG-CoA reductase is the rate-limiting step in cholesterol synthesis and represents the sole major drug target for contemporary cholesterol-lowering drugs.

The medical significance of HMG-CoA reductase has continued to expand beyond its direct role in cholesterol synthesis following the discovery that it can offer cardiovascular health benefits independent of cholesterol reduction.[7] Statins have been shown to have anti-inflammatory properties,[8] most likely as a result of their ability to limit production of key downstream isoprenoids that are required for portions of the inflammatory response. It can be noted that blocking of isoprenoid synthesis by statins has shown promise in treating a mouse model of multiple sclerosis, an inflammatory autoimmune disease.[9]

HMG-CoA reductase is also an important developmental enzyme. Inhibition of its activity and the concomitant lack of isoprenoids that yields can lead to morphological defects.[10]

Regulation

HMG-CoA reductase-Substrate complex (Blue:Coenzyme A, red:HMG, green:NADP)

Regulation of HMG-CoA reductase is achieved at several levels: transcription, translation, degradation and phosphorylation.

Transcription of the reductase gene

Transcription of the reductase gene is enhanced by the sterol regulatory element binding protein (SREBP). This protein binds to the sterol regulatory element (SRE), located on the 5' end of the reductase gene. When SREBP is inactive, it is bound to the ER or nuclear membrane. When cholesterol levels fall, SREBP is released from the membrane by proteolysis and migrates to the nucleus, where it binds to the SRE and transcription is enhanced. If cholesterol levels rise, proteolytic cleavage of SREBP from the membrane ceases and any proteins in the nucleus are quickly degraded.

Translation of mRNA

Translation of mRNA is inhibited by a mevalonate derivative, which has been reported to be farnesol,[11][12]although this role has been disputed.[13]

Degradation of reductase

Rising levels of sterols increase the susceptibility of the reductase enzyme to ER-assosiated degradation (ERAD) and proteolysis. Helices 2-6 (total of 8) of the HMG-CoA reductase transmembrane domain sense the higher levels of cholesterol, which leads to the exposure of Lysine 248. This lysine residue can become ubiquinated by the E3 ligase AMFR, serving as a signal for proteolytic degradation.

Phosphorylation of reductase

Short-term regulation of HMG-CoA reductase is achieved by inhibition by phosphorylation (of Serine 872, in humans[14]). Decades ago it was believed that a cascade of enzymes controls the activity of HMG-CoA reductase: an HMG-CoA reductase kinase was thought to inactivate the enzyme, and the kinase in turn was held to be activated via phosphorylation by HMG-CoA reductase kinase kinase. An excellent review on regulation of the mevalonate pathway by Nobel Laureates Joseph Goldstein and Michael Brown adds specifics: HMG-CoA reductase is phosphorylated and inactivated by an AMP-activated protein kinase, which also phosphorylates and inactivates acetyl-CoA carboxylase, the rate-limiting enzyme of fatty acid biosynthesis.[15] Thus, both pathways utilizing acetyl-CoA for lipid synthesis are inactivated when energy charge is low in the cell, and concentrations of AMP rise. There has been a great deal of research on the identity of upstream kinases that phosphorylate and activate the AMP-activated protein kinase.[16]

Fairly recently, LKB1 has been identified as a likely AMP kinase kinase,[17] which appears to involve calcium/calmodulin signaling. This pathway likely transduces signals from leptin, adiponectin, and other signaling molecules.[16]

See also

References

  1. ^ Roitelman J, Olender EH, Bar-Nun S, Dunn WA, Simoni RD (June 1992). "Immunological evidence for eight spans in the membrane domain of 3-hydroxy-3-methylglutaryl coenzyme A reductase: implications for enzyme degradation in the endoplasmic reticulum". J. Cell Biol. 117 (5): 959–73. doi:10.1083/jcb.117.5.959. PMID 1374417.  
  2. ^ Lindgren V, Luskey KL, Russell DW, Francke U (December 1985). "Human genes involved in cholesterol metabolism: chromosomal mapping of the loci for the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl-coenzyme A reductase with cDNA probes". Proc. Natl. Acad. Sci. U.S.A. 82 (24): 8567–71. doi:10.1073/pnas.82.24.8567. PMID 3866240.  
  3. ^ Farmer JA (1998). "Aggressive lipid therapy in the statin era". Prog Cardiovasc Dis 41 (2): 71–94. doi:10.1016/S0033-0620(98)80006-6. PMID 9790411.  
  4. ^ "Is there a "best" statin drug?". Johns Hopkins Med Lett Health After 50 15 (11): 4–5. January 2004. PMID 14983817.  
  5. ^ Lin YL, Wang TH, Lee MH, Su NW (January 2008). "Biologically active components and nutraceuticals in the Monascus-fermented rice: a review". Appl. Microbiol. Biotechnol. 77 (5): 965–73. doi:10.1007/s00253-007-1256-6. PMID 18038131.  
  6. ^ Flores NA (September 2004). "Ezetimibe + simvastatin (Merck/Schering-Plough)". Curr Opin Investig Drugs 5 (9): 984–92. PMID 15503655.  
  7. ^ Arnaud C, Veillard NR, Mach F (April 2005). "Cholesterol-independent effects of statins in inflammation, immunomodulation and atherosclerosis". Curr Drug Targets Cardiovasc Haematol Disord 5 (2): 127–34. doi:10.2174/1568006043586198. PMID 15853754.  
  8. ^ Sorrentino S, Landmesser U (December 2005). "Nonlipid-lowering effects of statins" (). Curr Treat Options Cardiovasc Med 7 (6): 459–66. doi:10.1007/s11936-005-0031-1. PMID 16283973. http://www.treatment-options.com/1092-8464/7/459.  
  9. ^ Stüve O, Youssef S, Steinman L, Zamvil SS (June 2003). "Statins as potential therapeutic agents in neuroinflammatory disorders". Curr. Opin. Neurol. 16 (3): 393–401. doi:10.1097/01.wco.0000073942.19076.d1 (inactive 2009-11-26). PMID 12858078.  
  10. ^ Thorpe JL, Doitsidou M, Ho SY, Raz E, Farber SA (February 2004). "Germ cell migration in zebrafish is dependent on HMGCoA reductase activity and prenylation". Dev. Cell 6 (2): 295–302. doi:10.1016/S1534-5807(04)00032-2. PMID 14960282.  
  11. ^ Meigs TE, Roseman DS, Simoni RD (April 1996). "Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase degradation by the nonsterol mevalonate metabolite farnesol in vivo". J. Biol. Chem. 271 (14): 7916–22. doi:10.1074/jbc.271.14.7916. PMID 8626470. http://www.jbc.org/cgi/content/full/271/14/7916.  
  12. ^ Meigs TE, Simoni RD (September 1997). "Farnesol as a regulator of HMG-CoA reductase degradation: characterization and role of farnesyl pyrophosphatase". Arch. Biochem. Biophys. 345 (1): 1–9. doi:10.1006/abbi.1997.0200. PMID 9281305.  
  13. ^ Keller RK, Zhao Z, Chambers C, Ness GC (April 1996). "Farnesol is not the nonsterol regulator mediating degradation of HMG-CoA reductase in rat liver". Arch. Biochem. Biophys. 328 (2): 324–30. doi:10.1006/abbi.1996.0180. PMID 8645011.  
  14. ^ Istvan ES, Palnitkar M, Buchanan SK, Deisenhofer J (March 2000). "Crystal structure of the catalytic portion of human HMG-CoA reductase: insights into regulation of activity and catalysis". EMBO J. 19 (5): 819–30. doi:10.1093/emboj/19.5.819. PMID 10698924.  
  15. ^ Goldstein JL, Brown MS (February 1990). "Regulation of the mevalonate pathway". Nature 343 (6257): 425–30. doi:10.1038/343425a0. PMID 1967820.  
  16. ^ a b Hardie DG, Scott JW, Pan DA, Hudson ER (July 2003). "Management of cellular energy by the AMP-activated protein kinase system". FEBS Lett. 546 (1): 113–20. doi:10.1016/S0014-5793(03)00560-X. PMID 12829246.  
  17. ^ Witters LA, Kemp BE, Means AR (January 2006). "Chutes and Ladders: the search for protein kinases that act on AMPK". Trends Biochem. Sci. 31 (1): 13–6. doi:10.1016/j.tibs.2005.11.009. PMID 16356723.  

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