Glucokinase
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Location (UCSC) | n/a | Chr 11: 5.85 – 5.9 Mb | |||||||
PubMed search | [2] | [3] |
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Glucokinase | |||||||||
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KEGG | KEGG entry | ||||||||
MetaCyc | metabolic pathway | ||||||||
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PDB structures | RCSB PDB PDBe PDBsum | ||||||||
Gene Ontology | AmiGO / QuickGO | ||||||||
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Glucokinase (
Glucokinase (GK) is a hexokinase isozyme, related homologously to at least three other hexokinases.[4] All of the hexokinases can mediate phosphorylation of glucose to glucose-6-phosphate (G6P), which is the first step of both glycogen synthesis and glycolysis. However, glucokinase is coded by a separate gene and its distinctive kinetic properties allow it to serve a different set of functions. Glucokinase has a lower affinity for glucose than the other hexokinases do, and its activity is localized to a few cell types, leaving the other three hexokinases as more important preparers of glucose for glycolysis and glycogen synthesis for most tissues and organs. Because of this reduced affinity, the activity of glucokinase, under usual physiological conditions, varies substantially according to the concentration of glucose.[5]
Nomenclature
Alternative names for this enzyme are: human hexokinase IV, hexokinase D, and ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1 (previously 2.7.1.2). The common name, glucokinase, is derived from its relative specificity for glucose under physiologic conditions.
Some
Another mammalian glucose kinase,
Catalysis
Substrates and products
The principal
ATP participates in the reaction in a form complexed to magnesium (Mg) as a cofactor. Furthermore, under certain conditions, glucokinase, like other hexokinases, can induce phosphorylation of other hexoses (6 carbon sugars) and similar molecules. Therefore, the general glucokinase reaction is more accurately described as:[6]
Among the hexose substrates are mannose, fructose, and glucosamine, but the affinity of glucokinase for these requires concentrations not found in cells for significant activity.[8]
Kinetics
Two important kinetic properties distinguish glucokinase from the other hexokinases, allowing it to function in a special role as glucose sensor.
- Glucokinase has a lower affinity for glucose than the other hexokinases. Glucokinase changes conformation and/or function in parallel with rising glucose concentrations in the physiologically important range of 4–10
- Glucokinase is not inhibited by its product, glucose-6-phosphate.[9] This allows continued signal output (e.g., to trigger insulin release) amid significant amounts of its product[10]
These two features allow it to regulate a "supply-driven" metabolic pathway. That is, the rate of reaction is driven by the supply of glucose, not by the demand for end products.
Another distinctive property of glucokinase is its moderate
Because of this cooperativity, the kinetic interaction of glucokinase with glucose does not follow classical
The S0.5 and nH extrapolate to an "inflection point" of the curve describing enzyme activity as a function of glucose concentration at about 4 mmol/L.[12] In other words, at a glucose concentration of about 72 mg/dL, which is near the low end of the normal range, glucokinase activity is most sensitive to small changes in glucose concentration.
The kinetic relationship with the other substrate, MgATP, can be described by classical Michaelis-Menten kinetics, with an affinity at about 0.3–0.4 mmol/L, well below a typical intracellular concentration of 2.5 mmol/L. The fact that there is nearly always an excess of ATP available implies that ATP concentration rarely influences glucokinase activity.
The maximum specific activity (kcat, also known as the turnover rate) of glucokinase when saturated with both substrates is 62/s.[9]
The pH optimum of human glucokinase was identified only recently and is surprisingly high, at pH 8.5–8.7.[13]
A "minimal mathematical model" has been devised based on the above kinetic information to predict the beta cell glucose phosphorylation rate (BGPR) of normal ("wild type") glucokinase and the known mutations. The BGPR for wild type glucokinase is about 28% at a glucose concentration of 5 mmol/L, indicating that the enzyme is running at 28% of capacity at the usual threshold glucose for triggering insulin release.
Mechanism
The
These sulfhydryl groups are quite sensitive to the oxidation status of the cells, making glucokinase one of the components most vulnerable to oxidative stress, especially in the beta cells.
Interactive pathway map
Click on genes, proteins and metabolites below to link to respective articles.[§ 1]
- ^ The interactive pathway map can be edited at WikiPathways: "GlycolysisGluconeogenesis_WP534".
Structure
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Glucokinase is a
This is about half the size of the other mammalian hexokinases, which retain a degree of dimeric structure. Several sequences and the three-dimensional structure of the key active sites. The ATP binding domain, for example, are shared with hexokinases, bacterial glucokinases, and other proteins, and the common structure is termed an actin fold.
Genetics
Human glucokinase is coded for by the GCK gene on chromosome 7. This single autosomal gene has 10 exons.[17] [18] Genes for glucokinase in other animals are homologous to human GCK.[9][19]
A distinctive feature of the gene is that it begins with two
The first promoter from the 5' end, referred to as the "upstream" or neuroendocrine promoter, is active in pancreatic islet cells, neural tissue, and enterocytes (small intestine cells) to produce the "neuroendocrine isoform" of glucokinase.[20] The second promoter, the "downstream" or liver promoter, is active in hepatocytes and directs production of the "liver isoform."[21] The two promoters have little or no sequence homology and are separated by a 30 kbp sequence which has not yet been shown to incur any functional differences between isoforms.[5] The two promoters are functionally exclusive and governed by distinct sets of regulatory factors, so that glucokinase expression can be regulated separately in different tissue types.[5] The two promoters correspond to two broad categories of glucokinase function: In liver, glucokinase acts as the gateway for the "bulk processing" of available glucose, while, in the neuroendocrine cells, it acts as a sensor, triggering cell responses that affect body-wide carbohydrate metabolism.
Distribution among organ systems
Glucokinase has been discovered in specific cells in four types of mammalian tissue: liver, pancreas,
- The predominant cells of the liver are the hepatocytes, and GK is found exclusively in these cells. During digestion of a carbohydrate meal, when blood glucose is plentiful and insulin levels are high, hepatocytes remove glucose from the blood and store it as glycogen. After completion of digestion and absorption, the liver manufactures glucose from both non-glucose substrates (gluconeogenesis) and glycogen (glycogenolysis), and exports it into the blood, to maintain adequate blood glucose levels during fasting. Because GK activity rises rapidly as the glucose concentration rises, it serves as a central metabolic switch to shift hepatic carbohydrate metabolism between fed and fasting states. Phosphorylation of glucose to glucose-6-phosphate by GK facilitates storage of glucose as glycogen and disposal by glycolysis. The separate liver promoter allows glucokinase to be regulated differently in hepatocytes than in the neuroendocrine cells.
- Neuroendocrine cells of the pancreas, gut, and brain share some common aspects of glucokinase production, regulation, and function.[22] These tissues are collectively referred to as "neuroendocrine" cells in this context.
- Beta cells and alpha cells of the pancreatic islets
- Beta cells release insulin in response to rising levels of glucose. Insulin enables many types of cells to import and use glucose, and signals the liver to synthesize glycogen. Alpha cells produce less glucagon in response to rising glucose levels, and more glucagon if blood glucose is low. Glucagon serves as a signal to the liver to break down glycogen and release glucose into the blood. Glucokinase in beta cells serves as a glucose sensor, amplifying insulin secretion as blood glucose rises.
- In the pancreatic beta-cell, glucokinase is a key regulator enzyme. Glucokinase is very important in the regulation of insulin secretion and has been known as the pancreatic beta-cell sensor. Mutations in the gene encoding glucokinase can cause both hyperglycemia and hypoglycemia because of its central role in the regulation of insulin release.[23]
- Glucose-sensitive neurons of the hypothalamus
- In response to rising or falling levels of glucose, cells in the hypothalamus polarize or depolarize. Among the neuroendocrine reactions of the pituitary.
- In response to rising or falling levels of glucose, cells in the hypothalamus polarize or depolarize. Among the neuroendocrine reactions of the
- Enterocytes of the small intestine
- This is the least-understood of the glucokinase sensor systems. It seems likely that responses to incoming glucose during digestion play a role in the incretin amplification of insulin secretion during a meal, or in the generation of satiety signals from gut to brain.
- Beta cells and alpha cells of the pancreatic islets
Distribution among species
Liver glucokinase occurs widely but not universally throughout vertebrate species. The gene structure and amino acid sequence are highly conserved among most mammals (e.g., rat and human glucokinase is more than 80% homologous). However, there are some unusual exceptions: For example, it has not been discovered in
Function and regulation
Most of the glucokinase in a mammal is found in the liver, and glucokinase provides approximately 95% of the hexokinase activity in hepatocytes. Phosphorylation of glucose to
When ample glucose is available, glycogen synthesis proceeds at the periphery of the hepatocytes until the cells are replete with glycogen. Excess glucose is then increasingly converted into
G6P, the product of glucokinase, is the principal substrate of glycogen synthesis, and glucokinase has a close functional and regulatory association with glycogen synthesis. When maximally active, GK and glycogen synthase appears to be located in the same peripheral areas of hepatocyte cytoplasm in which glycogen synthesis occurs. The supply of G6P affects the rate of glycogen synthesis not only as the primary substrate, but by direct stimulation of glycogen synthase and inhibition of glycogen phosphorylase.
Glucokinase activity can be rapidly amplified or damped in response to changes in the glucose supply, typically resulting from eating and fasting. Regulation occurs at several levels and speeds, and is influenced by many factors that affect mainly two general mechanisms:
- Glucokinase activity can be amplified or reduced in minutes by actions of the glucokinase regulatory protein (GKRP). The actions of this protein are influenced by small molecules such as glucose and fructose.
- The amount of glucokinase can be increased by synthesis of new protein. Insulin is the principal signal for increased transcription, operating mainly by way of a transcription factor called sterol regulatory element binding protein-1c (SREBP1c) in the liver. This occurs within an hour after a rise in insulin levels, as after a carbohydrate meal.[citation needed]
Transcriptional
Insulin acting via the
2) also stimulates GK transcription, it seems by way of Akt2 rather than SREBP1c. It is not known whether this effect is one of the downstream effects of activation of insulin receptors or independent of insulin action. Levels of F2,6P
2 play other amplifying roles in glycolysis in hepatocytes.
Other transacting factors suspected of playing a role in liver cell transcription regulation include:
- Hepatic nuclear factor-4-alpha (HNF4α) is an orphan nuclear receptor important in the transcription of many genes for enzymes of carbohydrate and lipid metabolism. It activates GCK transcription.
- Upstream stimulatory factor 1 (USF1) is another basic helix-loop-helix zipper (bHLHZ) transactivator.
- Hepatic nuclear factor 6 (glucose-6-phosphatase and phosphoenolpyruvate carboxykinase.
Hormonal and dietary
Insulin is by far the most important of the hormones that have direct or indirect effects on glucokinase expression and activity in the liver. Insulin appears to affect both glucokinase transcription and activity through multiple direct and indirect pathways. While rising portal vein glucose levels increase glucokinase activity, the concomitant rise of insulin amplifies this effect by induction of glucokinase synthesis. Glucokinase transcription begins to rise within an hour of rising insulin levels. Glucokinase transcription becomes nearly undetectable in prolonged starvation, severe carbohydrate deprivation, or untreated insulin-deficient diabetes.
The mechanisms by which insulin induces glucokinase may involve both of the major intracellular pathways of insulin action, the extracellular signal-regulated kinase (ERK 1/2) cascade, and the phosphoinositide 3-kinase (PI3-K) cascade. The latter may operate via the FOXO1 transactivator.
However, as would be expected given its antagonistic effect on glycogen synthesis,
Other hormones such as
Hepatic
Glucokinase can be rapidly activated and inactivated in hepatocytes by a novel regulatory protein (glucokinase regulatory protein), which operates to maintain an inactive reserve of GK, which can be made quickly available in response to rising levels of portal vein glucose.[24]
Pancreatic
Although most of the glucokinase in the body is in the liver, smaller amounts in the beta and alpha cells of the pancreas, certain hypothalamic neurons, and specific cells (enterocytes) of the gut play an increasingly appreciated role in regulation of carbohydrate metabolism. In the context of glucokinase function, these cell types are collectively referred to as neuroendocrine tissues, and they share some aspects of glucokinase regulation and function, especially the common neuroendocrine promoter. Of the neuroendocrine cells, the beta cells of the pancreatic islets are the most-studied and best-understood. It is likely that many of the regulatory relationships discovered in the beta cells will also exist in the other neuroendocrine tissues with glucokinase.
A signal for insulin
In islet
It is as a signal for insulin release that glucokinase exerts the largest effect on blood sugar levels and overall direction of carbohydrate metabolism. Glucose, in turn, influences both the immediate activity and the amount of glucokinase produced in the beta cells.
Regulation in beta cells
Glucose immediately amplifies glucokinase activity by the cooperativity effect.
A second important rapid regulator of glucokinase activity in beta cells occurs by direct protein-protein interaction between glucokinase and the "bifunctional enzyme" (phosphofructokinase-2/fructose-2,6-bisphosphatase), which also plays a role in the regulation of glycolysis. This physical association stabilizes glucokinase in a catalytically favorable conformation (somewhat opposite the effect of GKRP binding) that enhances its activity.
In as little as 15 minutes, glucose can stimulate GCK transcription and glucokinase synthesis by way of insulin. Insulin is produced by the beta cells, but some of it acts on beta cell B-type
Transcription of the GCK gene is initiated through the "upstream," or neuroendocrine, promoter. This promoter, in contrast to the liver promoter, has elements homologous to other insulin-induced gene promoters. Among the probable transacting factors are Pdx-1 and
Association with insulin secretory granules
Much, but not all, of the glucokinase found in the cytoplasm of beta cells is associated with insulin secretory granules and with mitochondria. The proportion thus "bound" falls rapidly in response to rising glucose and insulin secretion. It has been suggested that binding serves a purpose similar to the hepatic glucokinase regulatory protein—protecting glucokinase from degradation so that it is rapidly available as the glucose rises. The effect is to amplify the glucokinase response to glucose more rapidly than transcription could do so.[25]
Suppression of glucagon in alpha cells
It has also been proposed that glucokinase plays a role in the glucose sensing of the pancreatic alpha cells, but the evidence is less consistent, and some researchers have found no evidence of glucokinase activity in these cells. Alpha cells occur in pancreatic islets, mixed with beta and other cells. While beta cells respond to rising glucose levels by secreting insulin, alpha cells respond by reducing glucagon secretion. When blood glucose concentration falls to hypoglycemic levels, alpha cells release glucagon. Glucagon is a protein hormone that blocks the effect of insulin on hepatocytes, inducing glycogenolysis, gluconeogenesis, and reduced glucokinase activity in hepatocytes. The degree to which glucose suppression of glucagon is a direct effect of glucose via glucokinase in alpha cells, or an indirect effect mediated by insulin or other signals from beta cells, is still uncertain.
Hypothalamic
While all
Glucokinase has been found in the brain in largely the same areas that contain glucose-sensing neurons, including both of the hypothalamic nuclei. Inhibition of glucokinase abolishes the ventromedial nucleus response to a meal. However, brain glucose levels are lower than plasma levels, typically 0.5–3.5 mmol/L. Although this range is matches the sensitivity of the glucose-sensing neurons, it is below the optimal inflection sensitivity for glucokinase. The presumption, based on indirect evidence and speculation, is that neuronal glucokinase is somehow exposed to plasma glucose levels even in the neurons.
Enterocytes and incretin
While glucokinase has been shown to occur in certain cells (enterocytes) of the small intestine and stomach, its function and regulation have not been worked out. It has been suggested that here, also, glucokinase serves as a glucose sensor, allowing these cells to provide one of the earliest metabolic responses to incoming carbohydrates. It is suspected that these cells are involved in incretin functions.
Clinical significance
Because insulin is one of, if not the most important, regulators of glucokinase synthesis,
At least 497 mutations of the human glucokinase gene GCK have been discovered, that can change the efficiency of glucose binding and phosphorylation, increasing or decreasing the sensitivity of beta cell insulin secretion in response to glucose, and producing clinically significant hyperglycemia or hypoglycemia.[26]
Diabetes mellitus
GCK mutations reduce the functional efficiency of the glucokinase molecule.
Hyperinsulinemic hypoglycemia
Some mutations have been found to enhance insulin secretion. Heterozygosity for gain of function mutations reduces the threshold glucose that triggers insulin release. This creates hypoglycemia of varying patterns, including transient or persistent congenital hyperinsulinism, or fasting or reactive hypoglycemia appearing at an older age. The most recent overview of GCK mutation that were observed in patients claimed 17 GCK mutations to cause hyperinsulinemic hypoglycemia.[27]
Homozygosity for gain of function mutations has not been found.
Research
Several
References
- ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000041798 - Ensembl, May 2017
- ^ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- ^ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- PMID 16233797.
- ^ PMID 18726182.
- ^ ISBN 3-8055-7744-3.
- PMID 14975750.
- ISBN 3-8055-7744-3.
- ^ ISBN 978-0-471-37494-7.
- ^ PMID 8549869.
- PMID 16768451.
- PMID 9519733.
- PMID 31388064.
- PMID 15466045.
- PMID 10480597.
- PMID 15016359.
Beautiful structural pictures illustrating the conformational changes and potential regulatory mechanisms
- PMID 1740341.
- PMID 1502186.
- ISBN 3-8055-7744-3.
- ^ PMID 2682629.
- PMID 2557341.
- PMID 8106409.
- S2CID 10764260.
- ISBN 1-57059-207-1.
This is the most detailed treatment of liver glucokinase
- PMID 15331544.
- PMID 31819097.
- ^ PMID 28842611.
- ^ "TTP399 - VTV Therapeutics". VTV Therapeutics Corporate Website. Retrieved April 8, 2021.
- S2CID 21028951.
- S2CID 40490126.
- S2CID 39201131.
External links
- Glaser B (2013-01-24). "Familial Hyperinsulinism". GeneReviews. Seattle WA: University of Washington, Seattle. PMID 20301549. NBK1375.
- De León DD, Stanley CA (23 January 2014). "Permanent Neonatal Diabetes Mellitus". In Adam MP, Ardinger HH, Pagon RA, et al. (eds.). GeneReview. Seattle WA: University of Washington, Seattle. PMID 20301620. NBK1447.
External links
- Glucokinase at the U.S. National Library of Medicine Medical Subject Headings (MeSH)