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82-462: 3DAX , 3SN5 , 3V8D 1581 13122 ENSG00000167910 ENSMUSG00000028240 P22680 Q64505 NM_000780 NM_007824 NP_000771 NP_031850 Cholesterol 7 alpha-hydroxylase also known as cholesterol 7-alpha-monooxygenase or cytochrome P450 7A1 (CYP7A1) is an enzyme that in humans is encoded by the CYP7A1 gene which has an important role in cholesterol metabolism. It

164-477: A Streptomyces sp. have micromolar affinity for LXR-α. LXR-623 ( WAY 252623 ) CAS: [875787-07-8]. LXR-RXR nuclear receptor heterodimers function as transcriptional regulators for genes involved in lipid metabolism, lipid homeostasis, and inflammation. Target genes of LXRs are involved in cholesterol and lipid metabolism regulation, including: The importance of LXRs in physiological lipid and cholesterol metabolism suggests that they may influence

246-487: A catalytic triad , stabilize charge build-up on the transition states using an oxyanion hole , complete hydrolysis using an oriented water substrate. Enzymes are not rigid, static structures; instead they have complex internal dynamic motions – that is, movements of parts of the enzyme's structure such as individual amino acid residues, groups of residues forming a protein loop or unit of secondary structure , or even an entire protein domain . These motions give rise to

328-489: A conformational ensemble of slightly different structures that interconvert with one another at equilibrium . Different states within this ensemble may be associated with different aspects of an enzyme's function. For example, different conformations of the enzyme dihydrofolate reductase are associated with the substrate binding, catalysis, cofactor release, and product release steps of the catalytic cycle, consistent with catalytic resonance theory . Substrate presentation

410-858: A common partner of two nuclear receptors acting in opposite directions with regard to fatty acid metabolism. So both LXR and PPAR-α compete for the limited pool of RXR and this dynamic equilibrium determines the direction of lipid metabolism. Developing new potent and effective LXR agonists without the undesirable side effects may be beneficial for clinical usage. In this regard, DMHCA was reported to reduce atherosclerosis in apolipoprotein E-deficient mice without inducing hypertriglyceridemia and liver steatosis. Treatment with T0901317 decreases amyloidal beta production in an Alzheimer's disease mouse model. However, both T0901317 and GW3965 have been reported to increase plasma and liver triglycerides in some mice models, indicating that T0901317 and GW3965 may not be

492-474: A first step and then checks that the product is correct in a second step. This two-step process results in average error rates of less than 1 error in 100 million reactions in high-fidelity mammalian polymerases. Similar proofreading mechanisms are also found in RNA polymerase , aminoacyl tRNA synthetases and ribosomes . Conversely, some enzymes display enzyme promiscuity , having broad specificity and acting on

574-897: A high- cholesterol diet. LXRβ knockout mice are unaffected by a high- cholesterol diet, suggesting that LXRα and LXRβ have separate roles. LXRs regulate fatty acid synthesis by modulating the expression of sterol regulatory element binding protein-1c (SREBP-1c). LXRs also regulate lipid homeostasis in the brain . LXRα and LXRβ double knockout mice develop neurodegenerative changes in brain tissue. LXRβ knockout mice results in adult-onset motor neuron degeneration in male mice. Adiponectin induces ABCA1 -mediated reverse cholesterol transport by activation of PPAR-γ and LXRα/β. LXR agonists are effective for treatment of murine models of atherosclerosis , diabetes , anti- inflammation , Alzheimer's disease , and cancer. Treatment with LXR agonists (hypocholamide, T0901317, GW3965, or N,N-dimethyl-3beta-hydroxy-cholenamide (DMHCA)) lowers

656-464: A quantitative theory of enzyme kinetics, which is referred to as Michaelis–Menten kinetics . The major contribution of Michaelis and Menten was to think of enzyme reactions in two stages. In the first, the substrate binds reversibly to the enzyme, forming the enzyme-substrate complex. This is sometimes called the Michaelis–Menten complex in their honor. The enzyme then catalyzes the chemical step in

738-439: A range of different physiologically relevant substrates. Many enzymes possess small side activities which arose fortuitously (i.e. neutrally ), which may be the starting point for the evolutionary selection of a new function. To explain the observed specificity of enzymes, in 1894 Emil Fischer proposed that both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. This

820-454: A rapid repression of the CYP7A1 gene. Regulation of CYP7A1 occurs at several levels including synthesis. Bile acids, steroid hormones, inflammatory cytokines, insulin, and growth factors inhibit CYP7A1 transcription through the 5′-upstream region of the promoter. The average life of this enzyme is between two and three hours. Activity can be regulated by phosphorylation-dephosphorylation. CYP7A1

902-438: A singular substrate, reaction or both together, that means, that the enzymes can catalyze all reactions wherein the substrate can experience. The enzyme cholesterol 7 alpha hydroxylase catalyzes the reaction that converts cholesterol into cholesterol 7 alpha hydroxylase reducing and oxidizing that molecule. Click on genes, proteins and metabolites below to link to respective articles. Deficiency of this enzyme will increase

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984-451: A species' normal level; as a result, enzymes from bacteria living in volcanic environments such as hot springs are prized by industrial users for their ability to function at high temperatures, allowing enzyme-catalysed reactions to be operated at a very high rate. Enzymes are usually much larger than their substrates. Sizes range from just 62 amino acid residues, for the monomer of 4-oxalocrotonate tautomerase , to over 2,500 residues in

1066-446: A steady level inside the cell. For example, NADPH is regenerated through the pentose phosphate pathway and S -adenosylmethionine by methionine adenosyltransferase . This continuous regeneration means that small amounts of coenzymes can be used very intensively. For example, the human body turns over its own weight in ATP each day. As with all catalysts, enzymes do not alter the position of

1148-442: A thermodynamically unfavourable one so that the combined energy of the products is lower than the substrates. For example, the hydrolysis of ATP is often used to drive other chemical reactions. Enzyme kinetics is the investigation of how enzymes bind substrates and turn them into products. The rate data used in kinetic analyses are commonly obtained from enzyme assays . In 1913 Leonor Michaelis and Maud Leonora Menten proposed

1230-469: A wide range of mechanisms to restrict their accumulation in tissues and blood. The discovery of farnesoid X receptor (FXR) which is located in the liver, has opened new insights. Bile acid activation of FXR represses the expression of CYP7A1 via, raising the expression of small heterodimer partner (SHP, NR0B2), a non-DNA binding protein. The increased abundance of SHP causes it to associate with liver receptor homolog (LRH)-1, an obligate factor required for

1312-457: Is k cat , also called the turnover number , which is the number of substrate molecules handled by one active site per second. The efficiency of an enzyme can be expressed in terms of k cat / K m . This is also called the specificity constant and incorporates the rate constants for all steps in the reaction up to and including the first irreversible step. Because the specificity constant reflects both affinity and catalytic ability, it

1394-838: Is orotidine 5'-phosphate decarboxylase , which allows a reaction that would otherwise take millions of years to occur in milliseconds. Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction. Enzymes differ from most other catalysts by being much more specific. Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, and activators are molecules that increase activity. Many therapeutic drugs and poisons are enzyme inhibitors. An enzyme's activity decreases markedly outside its optimal temperature and pH , and many enzymes are (permanently) denatured when exposed to excessive heat, losing their structure and catalytic properties. Some enzymes are used commercially, for example, in

1476-414: Is a cytochrome P450 enzyme, which belongs to the oxidoreductase class, and converts cholesterol to 7-alpha-hydroxycholesterol , the first and rate limiting step in bile acid synthesis. The inhibition of cholesterol 7-alpha-hydroxylase (CYP7A1) represses bile acid biosynthesis. Sequence comparisons indicated a huge similarity between cytochromes P450 identified in man and bacteria, and suggested that

1558-421: Is a process where the enzyme is sequestered away from its substrate. Enzymes can be sequestered to the plasma membrane away from a substrate in the nucleus or cytosol. Or within the membrane, an enzyme can be sequestered into lipid rafts away from its substrate in the disordered region. When the enzyme is released it mixes with its substrate. Alternatively, the enzyme can be sequestered near its substrate to activate

1640-492: Is also a synergy between plasma low-density lipoprotein cholesterol (LDL-C) and risks of coronary artery disease (CAD). Glucose signaling also induces CYP7A1 gene transcription by epigenetic regulation of the histone acetylation status. Glucose induction of bile acid synthesis have an important implication in metabolic control of glucose, lipid, and energy homeostasis under normal and diabetic conditions. CYP7A1-rs3808607 and apolipoprotein E (APOE) isoform are associated with

1722-448: Is defective in Tangier disease . A common feature of many metabolic pathways is their control by retinoid X receptor (RXR) heterodimers. LXR heterodimerises with RXR. Promiscuous RXR also heterodimerises with PPAR members. PPAR-α plays a pivotal role in fatty acid catabolism in liver by upregulating the expression of numerous genes involved in mitochondrial fatty acid oxidation. Thus RXR is

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1804-437: Is described by "EC" followed by a sequence of four numbers which represent the hierarchy of enzymatic activity (from very general to very specific). That is, the first number broadly classifies the enzyme based on its mechanism while the other digits add more and more specificity. The top-level classification is: These sections are subdivided by other features such as the substrate, products, and chemical mechanism . An enzyme

1886-749: Is fully specified by four numerical designations. For example, hexokinase (EC 2.7.1.1) is a transferase (EC 2) that adds a phosphate group (EC 2.7) to a hexose sugar, a molecule containing an alcohol group (EC 2.7.1). Sequence similarity . EC categories do not reflect sequence similarity. For instance, two ligases of the same EC number that catalyze exactly the same reaction can have completely different sequences. Independent of their function, enzymes, like any other proteins, have been classified by their sequence similarity into numerous families. These families have been documented in dozens of different protein and protein family databases such as Pfam . Non-homologous isofunctional enzymes . Unrelated enzymes that have

1968-490: Is important for the synthesis of bile acid and the regulation of cholesterol levels. Cholesterol 7 alpha-hydroxylase is the rate-limiting enzyme in the synthesis of bile acid from cholesterol via the classic pathway, catalyzing the formation of 7α-hydroxycholesterol . The unique detergent properties of bile acids are essential for the digestion and intestinal absorption of hydrophobic nutrients. Bile acids have powerful toxic properties like membrane disruption and there are

2050-473: Is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase . Examples are lactase , alcohol dehydrogenase and DNA polymerase . Different enzymes that catalyze the same chemical reaction are called isozymes . The International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers (for "Enzyme Commission") . Each enzyme

2132-418: Is often referred to as "the lock and key" model. This early model explains enzyme specificity, but fails to explain the stabilization of the transition state that enzymes achieve. In 1958, Daniel Koshland suggested a modification to the lock and key model: since enzymes are rather flexible structures, the active site is continuously reshaped by interactions with the substrate as the substrate interacts with

2214-462: Is only one of several important kinetic parameters. The amount of substrate needed to achieve a given rate of reaction is also important. This is given by the Michaelis–Menten constant ( K m ), which is the substrate concentration required for an enzyme to reach one-half its maximum reaction rate; generally, each enzyme has a characteristic K M for a given substrate. Another useful constant

2296-404: Is seen. This is shown in the saturation curve on the right. Saturation happens because, as substrate concentration increases, more and more of the free enzyme is converted into the substrate-bound ES complex. At the maximum reaction rate ( V max ) of the enzyme, all the enzyme active sites are bound to substrate, and the amount of ES complex is the same as the total amount of enzyme. V max

2378-403: Is the ribosome which is a complex of protein and catalytic RNA components. Enzymes must bind their substrates before they can catalyse any chemical reaction. Enzymes are usually very specific as to what substrates they bind and then the chemical reaction catalysed. Specificity is achieved by binding pockets with complementary shape, charge and hydrophilic / hydrophobic characteristics to

2460-415: Is under development for certain indications (e.g., Cushing's syndrome ) as a replacement for ketoconazole with reduced toxicity and improved tolerability and safety . Enzyme Enzymes ( / ˈ ɛ n z aɪ m z / ) are proteins that act as biological catalysts by accelerating chemical reactions . The molecules upon which enzymes may act are called substrates , and the enzyme converts

2542-476: Is upregulated by the nuclear receptor LXR (liver X receptor) when cholesterol (to be specific, oxysterol) levels are high. The effect of this upregulation is to increase the production of bile acids and reduce the level of cholesterol in hepatocytes. It is downregulated by sterol regulatory element-binding proteins (SREBP) when plasma cholesterol levels are low. Bile acids provide feedback inhibition of CYP7A1 by at least two different pathways, both involving

Cholesterol 7 alpha-hydroxylase - Misplaced Pages Continue

2624-790: Is useful for comparing different enzymes against each other, or the same enzyme with different substrates. The theoretical maximum for the specificity constant is called the diffusion limit and is about 10 to 10 (M s ). At this point every collision of the enzyme with its substrate will result in catalysis, and the rate of product formation is not limited by the reaction rate but by the diffusion rate. Enzymes with this property are called catalytically perfect or kinetically perfect . Example of such enzymes are triose-phosphate isomerase , carbonic anhydrase , acetylcholinesterase , catalase , fumarase , β-lactamase , and superoxide dismutase . The turnover of such enzymes can reach several million reactions per second. But most enzymes are far from perfect:

2706-611: The DNA polymerases ; here the holoenzyme is the complete complex containing all the subunits needed for activity. Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme. Coenzymes transport chemical groups from one enzyme to another. Examples include NADH , NADPH and adenosine triphosphate (ATP). Some coenzymes, such as flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamine pyrophosphate (TPP), and tetrahydrofolate (THF), are derived from vitamins . These coenzymes cannot be synthesized by

2788-504: The cholesterol level in serum and liver and inhibits the development of atherosclerosis in murine disease models. Synthetic LXR agonist GW3965 improves glucose tolerance in a murine model of diet-induced obesity and insulin resistance by regulating genes involved in glucose metabolism in liver and adipose tissue. GW3965 inhibits the expression of inflammatory mediators in cultured macrophage and inflammation in mice. Aberrant LXR signaling in macrophages due to

2870-528: The fibrate drug class is mediated through the inhibition of transcription of this enzyme. This inhibition leads to more cholesterol in the bile, which is the body's only route of cholesterol excretion. This also increases the risk of cholesterol gallstone formation. Inhibition of CYP7A1 is thought to be involved in or responsible for the hepatotoxicity associated with ketoconazole . The levorotatory enantiomer of ketoconazole, levoketoconazole , shows 12-fold reduced potency in inhibition of this enzyme, and

2952-511: The law of mass action , which is derived from the assumptions of free diffusion and thermodynamically driven random collision. Many biochemical or cellular processes deviate significantly from these conditions, because of macromolecular crowding and constrained molecular movement. More recent, complex extensions of the model attempt to correct for these effects. Enzyme reaction rates can be decreased by various types of enzyme inhibitors. A competitive inhibitor and substrate cannot bind to

3034-508: The oxidized cholesterol 7-ketocholesterol promotes the inflammation that leads to atherosclerosis . For this reason, 7-ketocholesterol is a therapeutic target for the prevention and treatment of atherosclerosis. When lipogenesis is increased by pharmacological activation of the liver X receptor, hepatic VLDL production is increased 2.5-fold, and the liver produces large TG-rich VLDL particles. Glucose induces expression of LXR target genes involved in cholesterol homeostasis like ABCA1 which

3116-512: The 3-hydroxyl group and stabilize an oxysterol. LXRα and LXRβ form heterodimers with the obligate partner retinoid X receptor ( RXR ), which is activated by 9-cis-13,14-dihydroretinoic acid. The LXR/RXR heterodimer can be activated with either an LXR agonist (oxysterols) or a RXR agonist (9-cis-13,14-dihydroretinoic acid). Oxysterols, the oxygenated derivatives of cholesterol , such as 22(R)-hydroxycholesterol , 24(S)-hydroxycholesterol , 27-hydroxycholesterol , and cholestenoic acid , are

3198-488: The LXRβ gene is located on chromosome 19q 13.3. While the expression of LXRα and LXRβ in various tissues overlap the tissue distribution pattern of these two isoforms differ considerably. LXRα expression is restricted to liver , kidney , intestine , fat tissue, macrophages , lung , and spleen and is highest in liver , hence the name liver X receptor α (LXRα). LXRβ is expressed in almost all tissues and organs hence

3280-400: The ability to carry out biological catalysis, which is often reflected in their amino acid sequences and unusual 'pseudocatalytic' properties. Enzymes are known to catalyze more than 5,000 biochemical reaction types. Other biocatalysts are catalytic RNA molecules , also called ribozymes . They are sometimes described as a type of enzyme rather than being like an enzyme, but even in

3362-432: The active conformation of the helices. The phenyl group of T-0901317 extends toward the β-sheet side of the binding pocket and partially occupies it. The unoccupied section contains hydrophilic, polar residues E267 and R305. H421 and W443 anchor the 22-, 24-, or 27-hydroxyl group of an oxysterol to the binding pocket via hydrogen bonding and electrostatic interactions. The conformational flexibility of R305 allows it to bind

Cholesterol 7 alpha-hydroxylase - Misplaced Pages Continue

3444-437: The active site and are involved in catalysis. For example, flavin and heme cofactors are often involved in redox reactions. Enzymes that require a cofactor but do not have one bound are called apoenzymes or apoproteins . An enzyme together with the cofactor(s) required for activity is called a holoenzyme (or haloenzyme). The term holoenzyme can also be applied to enzymes that contain multiple protein subunits, such as

3526-502: The active site. Organic cofactors can be either coenzymes , which are released from the enzyme's active site during the reaction, or prosthetic groups , which are tightly bound to an enzyme. Organic prosthetic groups can be covalently bound (e.g., biotin in enzymes such as pyruvate carboxylase ). An example of an enzyme that contains a cofactor is carbonic anhydrase , which uses a zinc cofactor bound as part of its active site. These tightly bound ions or molecules are usually found in

3608-407: The animal fatty acid synthase . Only a small portion of their structure (around 2–4 amino acids) is directly involved in catalysis: the catalytic site. This catalytic site is located next to one or more binding sites where residues orient the substrates. The catalytic site and binding site together compose the enzyme's active site . The remaining majority of the enzyme structure serves to maintain

3690-578: The average values of k c a t / K m {\displaystyle k_{\rm {cat}}/K_{\rm {m}}} and k c a t {\displaystyle k_{\rm {cat}}} are about 10 5 s − 1 M − 1 {\displaystyle 10^{5}{\rm {s}}^{-1}{\rm {M}}^{-1}} and 10 s − 1 {\displaystyle 10{\rm {s}}^{-1}} , respectively. Michaelis–Menten kinetics relies on

3772-494: The binding pocket include E267, R305, H421, and W443. The binding pocket accommodates oxysterols of molecular volumes up to 400 Å and T-0901317 easily positions itself with a molecular volume of 304 Å . H421 forms a hydrogen bond with T-0901317's hydroxyl head group which lowers the pKa of the H421 imidazole side chain. As a result, the imidazole side chain interacts electrostatically with π-electrons of W443's indole side chain to stabilize

3854-419: The biochemical characterisation of individual P450 enzymes: Cholesterol 7 alpha hydroxylase consists of 491 amino acids , which on folding forms 23 alpha helices and 26 beta sheets . Cholesterol 7 alpha-hydroxylase is a cytochrome P450 heme enzyme that oxidizes cholesterol in the position 7 using molecular oxygen . It is an oxidoreductase. CYP7A1 is located in the endoplasmic reticulum (ER) and

3936-502: The body de novo and closely related compounds (vitamins) must be acquired from the diet. The chemical groups carried include: Since coenzymes are chemically changed as a consequence of enzyme action, it is useful to consider coenzymes to be a special class of substrates, or second substrates, which are common to many different enzymes. For example, about 1000 enzymes are known to use the coenzyme NADH. Coenzymes are usually continuously regenerated and their concentrations maintained at

4018-471: The chemical equilibrium of the reaction. In the presence of an enzyme, the reaction runs in the same direction as it would without the enzyme, just more quickly. For example, carbonic anhydrase catalyzes its reaction in either direction depending on the concentration of its reactants: The rate of a reaction is dependent on the activation energy needed to form the transition state which then decays into products. Enzymes increase reaction rates by lowering

4100-425: The conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. French chemist Anselme Payen was the first to discover an enzyme, diastase , in 1833. A few decades later, when studying the fermentation of sugar to alcohol by yeast , Louis Pasteur concluded that this fermentation was caused by a vital force contained within

4182-444: The decades since ribozymes' discovery in 1980–1982, the word enzyme alone often means the protein type specifically (as is used in this article). An enzyme's specificity comes from its unique three-dimensional structure . Like all catalysts, enzymes increase the reaction rate by lowering its activation energy . Some enzymes can make their conversion of substrate to product occur many millions of times faster. An extreme example

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4264-454: The development of metabolic disorders such as hyperlipidemia and atherosclerosis . Evidence for this idea has been observed by recent studies that linked LXR activity to the pathogenesis of atherosclerosis. LXRα knockout mice are healthy when fed with a low- cholesterol diet. However, LXRα knockout mice develop enlarged fatty livers , degeneration of liver cells, high cholesterol levels in liver , and impaired liver function when fed

4346-458: The early name UR (ubiquitous receptor). The different pattern of expression suggests that LXRα and LXRβ have different roles in regulating physiological function. Crystal structure of human liver X receptor β (LXRβ) forms a heterodimer with its partner retinoid X receptor α (RXRα) on its cognate element an AGGTCA direct repeat spaced by 4 nucleotides showing an extended X-shaped arrangement with DNA- and ligand-binding domains crossed. In contrast,

4428-433: The energy of the transition state. First, binding forms a low energy enzyme-substrate complex (ES). Second, the enzyme stabilises the transition state such that it requires less energy to achieve compared to the uncatalyzed reaction (ES ). Finally the enzyme-product complex (EP) dissociates to release the products. Enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive"

4510-587: The enzyme urease was a pure protein and crystallized it; he did likewise for the enzyme catalase in 1937. The conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley , who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin . These three scientists were awarded the 1946 Nobel Prize in Chemistry. The discovery that enzymes could be crystallized eventually allowed their structures to be solved by x-ray crystallography . This

4592-483: The enzyme at the same time. Often competitive inhibitors strongly resemble the real substrate of the enzyme. For example, the drug methotrexate is a competitive inhibitor of the enzyme dihydrofolate reductase , which catalyzes the reduction of dihydrofolate to tetrahydrofolate. The similarity between the structures of dihydrofolate and this drug are shown in the accompanying figure. This type of inhibition can be overcome with high substrate concentration. In some cases,

4674-403: The enzyme. As a result, the substrate does not simply bind to a rigid active site; the amino acid side-chains that make up the active site are molded into the precise positions that enable the enzyme to perform its catalytic function. In some cases, such as glycosidases , the substrate molecule also changes shape slightly as it enters the active site. The active site continues to change until

4756-427: The enzyme. For example, the enzyme can be soluble and upon activation bind to a lipid in the plasma membrane and then act upon molecules in the plasma membrane. Allosteric sites are pockets on the enzyme, distinct from the active site, that bind to molecules in the cellular environment. These molecules then cause a change in the conformation or dynamics of the enzyme that is transduced to the active site and thus affects

4838-429: The extent of reduction in circulating LDL cholesterol in response to plant sterol consumption and could serve as potential predictive genetic markers to identify individuals who would derive maximum LDL cholesterol lowering with plant sterol consumption. Genetic variations in CYP7A1 influence its expression and thus may affect the risk of gallstone disease and gallbladder cancer. One of the many lipid lowering effects of

4920-400: The farnesoid X receptor, FXR. In the liver, bile acids bound to FXR induce small heterodimer partner , SHP which binds to LRH-1 and so inhibits the transcription of the enzyme. In the intestine, bile acids/FXR stimulate production of FGF15/19 (depending on species), which then acts as a hormone in the liver via FGFR4 . One feature of enzymes is their high specificity. They are specific on

5002-423: The inhibitor can bind to a site other than the binding-site of the usual substrate and exert an allosteric effect to change the shape of the usual binding-site. Liver X receptor Two isoforms of LXR have been identified and are referred to as LXRα and LXRβ . The liver X receptors are classified into subfamily 1 ( thyroid hormone receptor -like) of the nuclear receptor superfamily, and are given

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5084-448: The membrane-associated and hydrophobic nature of the enzyme system impeded purification, and the number of proteins involved could not be accurately counted. Advances in mRNA purification in the early 1980s allowed to isolate the first cDNA encoding a complete cytochrome P450 (CYP) protein, and thereafter, results of many cloning studies have revealed a large number of different enzymes. Advances in molecular biology and genomics facilitated

5166-468: The mixture. He named the enzyme that brought about the fermentation of sucrose " zymase ". In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are usually named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate (e.g., lactase is the enzyme that cleaves lactose ) or to

5248-429: The natural ligands for LXR. After activation, LXR binds to LXR response element (LXRE), usually a variant of the idealized sequence AGGTCAN4AGGTCA, in the promoters of LXRs' target genes . Some synthetic LXR agonists have been developed, including non steroidal LXR agonists T0901317 and GW3965. The hexacyclic aromatic ketones, (-)anthrabenzoxocinone and (-)bischloroanthrabenzoxocinone ((-)-BABX) derived from

5330-399: The nuclear receptor nomenclature symbols NR1H3 (LXRα) and NR1H2 (LXRβ) respectively. LXRα and LXRβ were discovered separately between 1994-1995. LXRα isoform was independently identified by two groups and initially named RLD-1 and LXR, whereas four groups identified the LXRβ isoform and called it UR, NER, OR-1, and RIP-15. The human LXRα gene is located on chromosome 11p 11.2, while

5412-538: The parallel domain arrangement of other NRs bind an AGGTCA direct repeat spaced by 1 nucleotide. The LXRβ core binds DNA via canonical contacts and auxiliary DNA contacts that enhance affinity for the response element. Crystal structure of human liver X receptor α (LXRα) also forms a heterodimer with its partner retinoid X receptor β (RXRβ). The LXRα-RXRβ heterodimer (PDB 1UHL) binds synthetic LXR oxysterol agonist T-0901317. The ligand-binding pocket predominantly consists of hydrophobic residues. The most critical residues to

5494-403: The possibility of cholesterol gallstones. Disruption of CYP7A1 from classic bile acid synthesis in mice leads to either increased postnatal death or a milder phenotype with elevated serum cholesterol. The latter is similar to the case in humans, where CYP7A1 mutations associate with high plasma low-density lipoprotein and hepatic cholesterol content, as well as deficient bile acid excretion. There

5576-528: The precise orientation and dynamics of the active site. In some enzymes, no amino acids are directly involved in catalysis; instead, the enzyme contains sites to bind and orient catalytic cofactors . Enzyme structures may also contain allosteric sites where the binding of a small molecule causes a conformational change that increases or decreases activity. A small number of RNA -based biological catalysts called ribozymes exist, which again can act alone or in complex with proteins. The most common of these

5658-406: The reaction and releases the product. This work was further developed by G. E. Briggs and J. B. S. Haldane , who derived kinetic equations that are still widely used today. Enzyme rates depend on solution conditions and substrate concentration . To find the maximum speed of an enzymatic reaction, the substrate concentration is increased until a constant rate of product formation

5740-733: The reaction rate of the enzyme. In this way, allosteric interactions can either inhibit or activate enzymes. Allosteric interactions with metabolites upstream or downstream in an enzyme's metabolic pathway cause feedback regulation, altering the activity of the enzyme according to the flux through the rest of the pathway. Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity. Cofactors can be either inorganic (e.g., metal ions and iron–sulfur clusters ) or organic compounds (e.g., flavin and heme ). These cofactors serve many purposes; for instance, metal ions can help in stabilizing nucleophilic species within

5822-410: The same enzymatic activity have been called non-homologous isofunctional enzymes . Horizontal gene transfer may spread these genes to unrelated species, especially bacteria where they can replace endogenous genes of the same function, leading to hon-homologous gene displacement. Enzymes are generally globular proteins , acting alone or in larger complexes . The sequence of the amino acids specifies

5904-412: The structure which in turn determines the catalytic activity of the enzyme. Although structure determines function, a novel enzymatic activity cannot yet be predicted from structure alone. Enzyme structures unfold ( denature ) when heated or exposed to chemical denaturants and this disruption to the structure typically causes a loss of activity. Enzyme denaturation is normally linked to temperatures above

5986-519: The substrate is completely bound, at which point the final shape and charge distribution is determined. Induced fit may enhance the fidelity of molecular recognition in the presence of competition and noise via the conformational proofreading mechanism. Enzymes can accelerate reactions in several ways, all of which lower the activation energy (ΔG , Gibbs free energy ) Enzymes may use several of these mechanisms simultaneously. For example, proteases such as trypsin perform covalent catalysis using

6068-402: The substrates into different molecules known as products . Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps. The study of enzymes is called enzymology and the field of pseudoenzyme analysis recognizes that during evolution, some enzymes have lost

6150-405: The substrates. Enzymes can therefore distinguish between very similar substrate molecules to be chemoselective , regioselective and stereospecific . Some of the enzymes showing the highest specificity and accuracy are involved in the copying and expression of the genome . Some of these enzymes have " proof-reading " mechanisms. Here, an enzyme such as DNA polymerase catalyzes a reaction in

6232-411: The superfamily cytochrome P450 first originated from a common ancestral gene some three billion years ago. The superfamily cytochrome P450 was named in 1961, because of the 450-nm spectral peak pigment that cytochrome P450 has when reduced and bound to carbon monoxide. In the early 1960s, P450 was thought to be one enzyme, and by the mid 1960s it was associated with drug and steroid metabolism. However,

6314-399: The synthesis of antibiotics . Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, and enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew. By the late 17th and early 18th centuries, the digestion of meat by stomach secretions and

6396-427: The transcription of CYP7A1. Furthermore, there is an "FXR/SHP-independent" mechanism that also represses CYP7A1 expression. This "FXR/SHP-independent" pathway involves the interaction of bile acids with liver macrophages, which finally induces the expression and secretion of cytokines. These inflammatory cytokines, which include tumor necrosis factor alpha and interleukin-1beta, act upon the liver parenchymal cells causing

6478-438: The type of reaction (e.g., DNA polymerase forms DNA polymers). The biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others (such as Nobel laureate Richard Willstätter ) argued that proteins were merely carriers for the true enzymes and that proteins per se were incapable of catalysis. In 1926, James B. Sumner showed that

6560-486: The yeast cells called "ferments", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells." In 1877, German physiologist Wilhelm Kühne (1837–1900) first used the term enzyme , which comes from Ancient Greek ἔνζυμον (énzymon)  ' leavened , in yeast', to describe this process. The word enzyme

6642-581: Was first done for lysozyme , an enzyme found in tears, saliva and egg whites that digests the coating of some bacteria; the structure was solved by a group led by David Chilton Phillips and published in 1965. This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail. Enzymes can be classified by two main criteria: either amino acid sequence similarity (and thus evolutionary relationship) or enzymatic activity. Enzyme activity . An enzyme's name

6724-451: Was used later to refer to nonliving substances such as pepsin , and the word ferment was used to refer to chemical activity produced by living organisms. Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin , he found that sugar was fermented by yeast extracts even when there were no living yeast cells in

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