241 human active and 13 inactive phosphatases in total;
194 phosphatases have substrate data;
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336 protein substrates;
83 non-protein substrates;
1215 dephosphorylation interactions;
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299 KEGG pathways;
876 Reactome pathways;
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last scientific update: 11 Mar, 2019
last maintenance update: 01 Sep, 2023
Cytoplasm NucleusNote=Under low glucose concentrations, GCK associates with GKRPand the inactive complex is recruited to the hepatocyte nucleus
Function (UniProt annotation)
Catalyzes the initial step in utilization of glucose bythe beta-cell and liver at physiological glucose concentrationGlucokinase has a high Km for glucose, and so it is effective onlywhen glucose is abundant The role of GCK is to provide G6P forthe synthesis of glycogen Pancreatic glucokinase plays animportant role in modulating insulin secretion Hepaticglucokinase helps to facilitate the uptake and conversion ofglucose by acting as an insulin-sensitive determinant of hepaticglucose usage
Glycolysis is the process of converting glucose into pyruvate and generating small amounts of ATP (energy) and NADH (reducing power). It is a central pathway that produces important precursor metabolites: six-carbon compounds of glucose-6P and fructose-6P and three-carbon compounds of glycerone-P, glyceraldehyde-3P, glycerate-3P, phosphoenolpyruvate, and pyruvate [MD:M00001]. Acetyl-CoA, another important precursor metabolite, is produced by oxidative decarboxylation of pyruvate [MD:M00307]. When the enzyme genes of this pathway are examined in completely sequenced genomes, the reaction steps of three-carbon compounds from glycerone-P to pyruvate form a conserved core module [MD:M00002], which is found in almost all organisms and which sometimes contains operon structures in bacterial genomes. Gluconeogenesis is a synthesis pathway of glucose from noncarbohydrate precursors. It is essentially a reversal of glycolysis with minor variations of alternative paths [MD:M00003].
Carbon metabolism is the most basic aspect of life. This map presents an overall view of central carbon metabolism, where the number of carbons is shown for each compound denoted by a circle, excluding a cofactor (CoA, CoM, THF, or THMPT) that is replaced by an asterisk. The map contains carbon utilization pathways of glycolysis (map00010), pentose phosphate pathway (map00030), and citrate cycle (map00020), and six known carbon fixation pathways (map00710 and map00720) as well as some pathways of methane metabolism (map00680). The six carbon fixation pathways are: (1) reductive pentose phosphate cycle (Calvin cycle) in plants and cyanobacteria that perform oxygenic photosynthesis, (2) reductive citrate cycle in photosynthetic green sulfur bacteria and some chemolithoautotrophs, (3) 3-hydroxypropionate bi-cycle in photosynthetic green nonsulfur bacteria, two variants of 4-hydroxybutyrate pathways in Crenarchaeota called (4) hydroxypropionate-hydroxybutyrate cycle and (5) dicarboxylate-hydroxybutyrate cycle, and (6) reductive acetyl-CoA pathway in methanogenic bacteria.
Insulin binding to its receptor results in the tyrosine phosphorylation of insulin receptor substrates (IRS) by the insulin receptor tyrosine kinase (INSR). This allows association of IRSs with the regulatory subunit of phosphoinositide 3-kinase (PI3K). PI3K activates 3-phosphoinositide-dependent protein kinase 1 (PDK1), which activates Akt, a serine kinase. Akt in turn deactivates glycogen synthase kinase 3 (GSK-3), leading to activation of glycogen synthase (GYS) and thus glycogen synthesis. Activation of Akt also results in the translocation of GLUT4 vesicles from their intracellular pool to the plasma membrane, where they allow uptake of glucose into the cell. Akt also leads to mTOR-mediated activation of protein synthesis by eIF4 and p70S6K. The translocation of GLUT4 protein is also elicited through the CAP/Cbl/TC10 pathway, once Cbl is phosphorylated by INSR.Other signal transduction proteins interact with IRS including GRB2. GRB2 is part of the cascade including SOS, RAS, RAF and MEK that leads to activation of mitogen-activated protein kinase (MAPK) and mitogenic responses in the form of gene transcription. SHC is another substrate of INSR. When tyrosine phosphorylated, SHC associates with GRB2 and can thus activate the RAS/MAPK pathway independently of IRS-1.
Pancreatic beta cells are specialised endocrine cells that continuously sense the levels of blood sugar and other fuels and, in response, secrete insulin to maintain normal fuel homeostasis. Glucose-induced insulin secretion and its potentiation constitute the principal mechanism of insulin release. Glucose is transported by the glucose transporter (GLUT) into the pancreatic beta-cell. Metabolism of glucose generates ATP, which inhibits ATP-sensitive K+ channels and causes voltage-dependent Ca2+ influx. Elevation of [Ca2+]i triggers exocytotic release of insulin granules. Insulin secretion is further regulated by several hormones and neurotransmitters. Peptide hormones, such as glucagon-like peptide 1 (GLP-1), increase cAMP levels and thereby potentiate insulin secretion via the combined action of PKA and Epac2. Achetylcholine (ACh), a major parasympathetic neurotransmitter, binds to Gq-coupled receptors and activates phospholipase C- (PLC-), and the stimulatory effects involve activation of protein kinase C (PKC), which stimulates exocytosis. In addition, ACh mobilizes intracellular Ca2+ by activation of IP3 receptors.
Prolactin (PRL) is a polypeptide hormone known to be involved in a wide range of biological functions including osmoregulation, lactation, reproduction, growth and development, endocrinology and metabolism, brain and behavior, and immunomodulation. PRL mediates its action through PRLR, a transmembrane protein of the hematopoietin cytokine receptor superfamily. At the protein level, the long PRLR isoform (long-R) and several short PRLR isoforms (short-R) have been detected. Acting through the long-R, PRL activates many signaling cascades including Jak2/Stat, the major cascade, Src kinase, phosphatidylinositol-3-kinase (PI3K)/AKT, and mitogen-activated protein kinase (MAPK) pathways. PRL cannot activate Jak2/Stat5 through the short-R, but can activate pathways including MAPK and PI3K pathways.
Glucagon is conventionally regarded as a counterregulatory hormone for insulin and plays a critical anti-hypoglycemic role by maintaining glucose homeostasis in both animals and humans. To increase blood glucose, glucagon promotes hepatic glucose output by increasing glycogenolysis and gluconeogenesis and by decreasing glycogenesis and glycolysis in a concerted fashion via multiple mechanisms. Glucagon also stimulates hepatic mitochondrial beta-oxidation to supply energy for glucose production. Glucagon performs its main effect via activation of adenylate cyclase. The adenylate-cyclase-derived cAMP activates protein kinase A (PKA), which then phosphorylates downstream targets, such as cAMP response element binding protein (CREB) and the bifunctional enzyme 6-phosphofructo-2-kinase/ fructose-2,6-bisphosphatase (one of the isoforms being PFK/FBPase 1, encoded by PFKFB1).
Insulin resistance is strongly associated with type II diabetes. Diabetogenicfactors including FFA, TNFalpha and cellular stress induce insulin resistance through inhibition of IRS1 functions. Serine/threonine phosphorylation, interaction with SOCS, regulation of the expression, modification of the cellular localization, and degradation represent the molecular mechanisms stimulated by them. Various kinases (ERK, JNK, IKKbeta, PKCzeta, PKCtheta and mTOR) are involved in this process.The development of type II diabetes requires impaired beta-cell function. Chronic hyperglycemia has been shown to induce multiple defects in beta-cells. Hyperglycemia has been proposed to lead to large amounts of reactive oxygen species (ROS) in beta-cells, with subsequent damage to cellular components including PDX-1. Loss of PDX-1, a critical regulator of insulin promoter activity, has also been proposed as an important mechanism leading to beta-cell dysfunction.Although there is little doubt as to the importance of genetic factors in type II diabetes, genetic analysis is difficult due to complex interaction among multiple susceptibility genes and between genetic and environmental factors. Genetic studies have therefore given very diverse results. Kir6.2 and IRS are two of the candidate genes. It is known that Kir6.2 and IRS play central roles in insulin secretion and insulin signal transmission, respectively.
About 2-5% of type II diabetic patients suffer from a monogenic disease with autosomal dominant inheritance. This monogenic form of type II diabetes is called maturity onset diabetes of the young (MODY).We now know that MODY is caused by heterozygous mutations in at least five genes encoding transcription factors: HNF4alpha (MODY1), HNF1alpha (MODY3), PDX1 (MODY4), HNF1beta (MODY5) and NEUROD1 (MODY6). MODY2, which is so far the only subtype not related to a transcription factor, is caused by mutations in the glucokinase gene. Mutations of MODY transcription factor genes lead to abnormal expression of genes involved in pancreatic islet development and metabolism.
Malignant transformation of cells requires specific adaptations of cellular metabolism to support growth and survival. In the early twentieth century, Otto Warburg established that there are fundamental differences in the central metabolic pathways operating in malignant tissue. He showed that cancer cells consume a large amount of glucose, maintain high rate of glycolysis and convert a majority of glucose into lactic acid even under normal oxygen concentrations (Warburg's Effects). More recently, it has been recognized that the 'Warburg effect' encompasses a similarly increased utilization of glutamine. From the intermediate molecules provided by enhanced glycolysis and glutaminolysis, cancer cells synthesize most of the macromolecules required for the duplication of their biomass and genome. These cancer-specific alterations represent a major consequence of genetic mutations and the ensuing changes of signalling pathways in cancer cells. Three transcription factors, c-MYC, HIF-1 and p53, are key regulators and coordinate regulation of cancer metabolism in different ways, and many other oncogenes and tumor suppressor genes cluster along the signaling pathways that regulate c-MYC, HIF-1 and p53.
Glucokinase (GCK1) is negatively regulated by glucokinase regulatory protein (GKRP), which reversibly binds the enzyme to form an inactive complex. Binding is stimulated by fructose 6-phosphate and sorbitol 6-phosphate (hence high concentrations of these molecules tend to reduce GCK1 activity) and inhibited by fructose 1-phosphate (hence a high concentration of this molecule tends to increase GCK1 activity). Once formed, the complex is translocated to the nucleus. In the presence of high glucose concentrations, the nuclear GCK1:GKRP complex dissociates, freeing GCK1 to return to the cytosol. The free GKRP is thought also to return to the cytosol under these conditions, but this return has not been confirmed experimentally. Possible physiological roles for this sequestration process are to decrease futile cycling between glucose and glucose 6 phosphate in hepatocytes under low-glucose conditions, and to decrease the lag between a rise in intracellular glucose levels and the onset of glucose phosphorylation in both hepatocytes and pancreatic beta cells (Brocklehurst et al. 2004; Shiota et al. 1999)
Two transcription factors, PDX1 and HNF1A, play key roles in maintaining the gene expression pattern characteristic of mature beta cells in the endocrine pancreas. Targets of these regulatory molecules include genes encoding insulin, the GLUT2 glucose transporter, the liver- (and pancreas) specific form of pyruvate kinase and other transcription factors including HNF4A, HNF4G, and FOXA3. PDX1 expression in turn is controlled by the activities of MAFA, FOXA2, and PAX6, and negatively regulated via AKT (Chakrabarti and Mirmira 2003; Servitja and Ferrer 2004)
Cytosolic glucokinase (GCK) (and three isoforms of hexokinase) catalyse the irreversible reaction of alpha-D-glucose (Glc) and ATP to form alpha-D-glucose-6-phosphate (G6P) and ADP, the first step in glycolysis. In the body, GCK is found only in hepatocytes and pancreatic beta cells. GCK and the hexokinase enzymes differ in that GCK has a higher Km than the hexokinases and is less readily inhibited by the reaction product. As a result, GCK should be inactive in the fasting state when glucose concentrations are low but in the fed state should have an activity proportional to glucose concentration. These features are thought to enable efficient glucose uptake and retention in the liver, and to function as a sensor of glucose concentration coupled to insulin release in pancreatic beta cells. Defects in GCK are can cause maturity-onset diabetes of the young 2 (MODY2; MIM:125851), a heritable early onset form of type II diabetes (Hussain 2010, Osbak et al. 2009)
The nuclear pore complex (NPC) trafficks cargo across the nuclear membrane. Nucleoprotein TPR functions as a scaffolding element in the nuclear phase of the NPC essential for normal nucleocytoplasmic transport of proteins and mRNAs. The complex glucokinase (GCK1) and glucokinase regulatory protein (GKRP) can be translocated to the nucleus via the NPC. Defects in TPR may confer susceptibility towards thyroid papillary carcinona (TPC; MIM:18850), a common tumor of the thyroid that typically arises as an irregular, solid or cystic mass from otherwise normal thyroid tissue (Vriens et al. 2009, Bonora et al. 2010)
The reactions of glycolysis (e.g., van Wijk and van Solinge 2005) convert glucose 6-phosphate to pyruvate. The entire process is cytosolic. Glucose 6-phosphate is reversibly isomerized to form fructose 6-phosphate. Phosphofructokinase 1 catalyzes the physiologically irreversible phosphorylation of fructose 6-phosphate to form fructose 1,6-bisphosphate. In six reversible reactions, fructose 1,6-bisphosphate is converted to two molecules of phosphoenolpyruvate and two molecules of NAD+ are reduced to NADH + H+. Each molecule of phosphoenolpyruvate reacts with ADP to form ATP and pyruvate in a physiologically irreversible reaction. Under aerobic conditions the NADH +H+ can be reoxidized to NAD+ via electron transport to yield additional ATP, while under anaerobic conditions or in cells lacking mitochondria NAD+ can be regenerated via the reduction of pyruvate to lactate
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