All of the hexokinases can mediate phosphorylation of glucose to glucose6phosphate g6p, which is the first step of both glycogen synthesis and glycolysis. Because the isozymes have different kinetic features, this has permitted them to become optimally adapted to the needs of the specific tissue in which they are the major isozyme. Function of plant hexokinase as a sugar sensor clemson. Hexokinase definition of hexokinase by merriamwebster. Enzymatic assay of hexokinase final assay concentrations.
Hexokinase or glucokinase is an enzyme that convert glucose to glucose 6 phosphate. Hexokinase ii hkii is a predominant isoform in insulinsensitive tissues such as heart, skeletal muscle, and adipose tissues. Plant sugars function not only as metabolic substrates, but also as signal compounds that affect the expression of many different genes involved in most vital plant processes. A change in the conformation of an enzyme in response to substrate binding that renders the enzyme catalytically active. Feb 01, 20 nonspherocytic hemolytic anemia due to hexokinase deficiency has been shown to be caused by mutations in the hk1 gene, which cause at least a partial deficiency of the enzyme hexokinase. Hexokinase 1 hk1, hexokinase 2 hk2, and hexokinase 3 hk3 are allosterically inhibited by their product, glucose6phosphate, but hk4 is not sensitive to glucose6phosphate concentrations wilson, 1995. Hexokinase method definition of hexokinase method by. Hexokinase isoforms i and ii both bind to the outer mitochondrial membrane omm and there is an abundance of evidence that this binding is greatly enhanced by their interaction with vdac, more specifically the most abundant isoform, vdac1.
Hexokinase activity assayhexokinase activity assays for the wild type intact, n and c terminal halves, and mutant hexokinase protein preparations were performed spectrophotometrically in the presence of 0. In addition, the liver contains a glucokinase, which requires a much greater concentration of glucose before it reacts. Atpbinding site of human brain hexokinase as studied by molecular modeling and sitedirected mutagenesis. Hexokinase is present in most all mammalian cells in low concentrations. Hence, when glc6p is not rapidly utilized, that is to say, no longer needed by the cell. Human hexokinase is a m onomer that consists of two domains, an nterminal domain and a cterminal domain, connected by an alpha helix 2. Hexokinase may also be used for the assay of other saccharides, which are convertible to glucose or fructose and is therefore useful in the assay of many glycosides. There is a lot of glucose that gets filtered through the liver, and the liver can turn it into other usable forms like pyuvate, hmp shunt, and glycogen. Its structure was first determined from yeast by tom steitz at yale university. Liver has glucokinase and can run the first step of glycolysis in much higher volume.
In most cells a hexokinase with a high affinity for glucosei. Jul 26, 2019 an enzyme that catalyzes the conversion of atp and a dhexose to adp and a dhexose 6phosphate. The bifunctional role of hexokinase in metabolism and glucose. M were comparable to those from hexokinase of trypanosoma. One atp molecule is cleaved to adp and the phosphate released is added to glucose. The structure and mechanism of hexokinase proteopedia, life. The mechanism of regulation of hexokinase learning. Nonspherocytic hemolytic anemia due to hexokinase deficiency. Atp is relatively specific for hexokinase compared to solubili zation by high salt.
Indeed, proponents of the second mechanism suggest that the 6phosphoryl group of gluc6p binds to the. As in the case of glucose, the h value for mannose. Hexokinase definition is any of a group of enzymes that accelerate the phosphorylation of hexoses as in the formation of glucose6phosphate from glucose and atp in carbohydrate metabolism. A hexokinase is an enzyme that phosphorylates hexoses sixcarbon sugars, forming hexose phosphate.
Early studies on vdac characterized it as the outer membrane hexokinase binding protein 7, 8. However, glucokinase is coded by a separate gene and its distinctive kinetic properties allow it to serve a different set of functions. Hexokinases catalyze the first essential step of glucose metabolism, the conversion of glucose into glucose6phosphate. It is also upregulated in many types of tumors associated with enhanced aerobic glycolysis in tumor cells, the warburg effect. In the process of glycolysis in liver cells glucose is converted to glucose 6 phosphate by glucokinase a hexokinase. Hk3 is found to be overexpressed in malignant follicular thyroid nodules. In most tissues and organisms, glucose is the most important substrate of hexokinases, and glucose 6phosphate the most important product. If youre seeing this message, it means were having trouble loading external resources on our website.
Patients with hemolytic disease, therefore, generally have markedly elevated red cell hexokinase activity, since the. Hexokinase ii integrates energy metabolism and cellular. These are separate, noninterconvertible isozymes womack et al. Glucokinase gk is a hexokinase isozyme, related homologously to at least three other hexokinases.
Cancer and hexokinase if youre seeing this message, it means were having trouble loading external resources on our website. M were comparable to those from hexokinase of trypanosoma cruzi. In most organisms, glucose is the most important substrate of hexokinases, and glucose6phosphate is the most important product. In mammals there are four hexokinase isozymes with different tissue distributions. This directly couples extramitochondrial glycolysis to intramitochondrial oxidative phosphorylation. This enzyme which convert glucose to glucose 6 phosphate exists in two isomer forms. Biologically active shrnas were generated from the plko. Hexokinases have been isolated from the yeast cell in two distinct forms, designated pi and pii schulze et al. The structure and function of human brain hexokinase. Hexokinase 2 hk2, also known as muscle form hexokinase, localizes to the outer membrane of mitochondria and is present in adipose tissue, skeletal muscle, and heart 4. Hexokinase ia is found in all mammalian tissues, and is considered a housekeeping enzyme, unaffected by most physiological, hormonal, and metabolic changes. Request pdf mammalian hexokinases and their abnormal expression in. Hexokinase is the initial enzyme of glycolysis, catalyzing the phosphorylation of glucose by atp to glucose6p. The catalytic properties of hexokinase i are dependent on product inhibition as well as.
Hexokinasemitochondrial interactions regulate glucose. Hexokinase catalyzes the first step in glucose metabolism, using atp for the phosphorylation of glucose to glucose6phosphate. Crystal structure of the hexokinase klhxk1 of kluyveromyces lactis. Hexokinase iib constitutes the principal regulated isoform in many cell types and is increased in many cancers. The primary plant glucose signaling system is somehow mediated by the protein hexokinase. Stereo views of yeast hexokinase yeast hexokinase contains 2 domains connected by a hinge region. Plays a key role in maintaining the integrity of the outer mitochondrial membrane by. Glucokinase is a type of hexokinase hexokinase iv, that specifically phosphorylates glucose a hexose sugar when it is present in high titre level. The enzyme hexokinase hk catalyzes the reaction between glucose and adenosine triphosphate atp to form glucose6phosphate g6p and adenosine diphosphate adp. Purification and characterization of hexokinase from. Hexokinase hk is the gateway enzyme of glucose metabolism. Hexokinase 2, glycogen synthase and phosphorylase play a key role in muscle glycogen supercompensation article pdf available in plos one 77. Hexokinase is the first enzyme in the glycolytic pathway, and it converts glucose into glucose6phosphate.
An enzyme that catalyzes the conversion of atp and a dhexose to adp and a dhexose 6phosphate. Four different forms of hexokinase, designated type hk1, hk2, hk3, and hk4, encoded by different genes, are present in mammalian tissues. Based on the corresponding amino acid positions of the yeast hexokinase pii crystal structure, the s177a mutation appears to occupy a pivotal. Hexokinase definition of hexokinase by medical dictionary. Several recent studies have reported that mitochondrial binding of hexokinase ii is affected by the activity of cyclophilin d 59, 60. It is one of the ratelimiting enzymes of glycolysis. It will be shown that the results obtained are in harmony with the mechanism suggested by trayser and colowick 5. Panels c and f represent the merge images of hkii and arrestin. The structure and mechanism of hexokinase proteopedia, life in 3d. Glucokinase is used in the first step of the metabolism of glucose, during this step the phosphorylation of glucose by atp generates glucose6phosphate and adp.
Hexokinase catalyzes the phosphorylation of glucose, the ratelimiting first step of glycolysis. The mitochondrionbound hexokinase i is believed to optimize the atpadp exchange. The kinetic properties of the pure enzyme were determined and the km values for glucose km 66. Hexokinase definition and meaning collins english dictionary. In the presence of nicotinamide adenine dinucleotide nad, g6p is oxidized by the enzyme glucose6phosphate dehydrogenase g6. Hexokinase is the enzyme which phosphorylates hexose six carbon sugar. Hexokinase may be used for the determination of dglucose 1, dfructose 2 and dsorbitol 3 in food or biological research samples. Hexokinase versus glucokinase hexokinase all tissues non. Nonspherocytic hemolytic anemia due to hexokinase deficiency nsha due to hk1 deficiency is a very rare condition mainly characterized by severe, chronic hemolysis, beginning in infancy. Hexokinase is the initial enzyme of glycolysis, catalyzing the phosphorylation of. The amino acids corresponding to the mitochondrial binding domain 5 have been removed in the. There are two subtypes of mitochondrial hexokinases, hk1 and hk2, each of which has four isozymes designated hexokinase i, ii, iii and iv.
Models for the reaction mechanism of yeast hexokinase hypothesize that glucose binds before atp and then adp is released, followed by glucose6phosphate delafuente and sols, 1970. Light activation of the insulin receptor regulates mitochondrial. Hexokinase hk is a key glycolytic enzyme in prawns. In this report the hill coefficient and the hill plot have been used only to measure the degree of cooperativity with no implication of mechanism. The bifunctional role of hexokinase in metabolism and. The enzymephosphate 2 and enzymeglucose 3 hypotheses relative to yeast hexokinase ap. Dglucose, dmannose, dfructose, sorbitol, and dglucosamine can act as acceptors. Hexokinase 3 also known as hk3 is an enzyme which in humans is encoded by the hk3 gene on chromosome 5. Phylogenetic trees were generated using the neighborjoining. It is also allosterically inhibited by physiological concentrations of its immediate product, glucose6phosphate. Approximately 20 cases of this condition have been described to date. Hexokinase from leishmania mexicana was purified to homogeneity from a glycosomeenriched fraction obtained after a differential centrifugation of promastigote form.
The goal of this research is to identify the molecular and cellular mechanism of glucose signaling by hexokinase. Pdf regulation of hexokinase and glucose6phosphate. Cyclophilin d is an immunophilin that exhibits peptidylprolyl cistrans isomerase ppiase activity and is localized to the mitochondrial matrix. Hexokinase undergoes and inducedfit conformational change when it binds to glucose, which ultimately prevents the hydrolysis of atp. Hexokinase phosphorylates hexoses transfer an inorganic phosphate group from atp to a hexose and form hexose phosphate. Among these, hk1 is the predominant glucose phosphorylating activity in those tissues that share a strict dependence on. Pdf hexokinase 2, glycogen synthase and phosphorylase. Glucokinase hexokinase d is a monomeric cytoplasmic enzyme found in the liver and pancreas that serves to regulate glucose levels in these organs. Mar 09, 20 hexokinase iv is present in the liver, pancreas, hypothalamus, small intestine, and perhaps certain other neuroendocrine cells, and plays an important regulatory role in carbohydrate metabolism. Several studies suggest mitochondrial hexokinase mthk is a major.
In a 3 ml reaction mix, the final concentrations are 8. The glycolytic pathway embdenmeyerhofparnas pathway glycolysis converts one c6 unit glucose to two c3 units pyruvate of lower energy in a process that harnesses the released free energy to synthesize atp from adp and pi. Similar to hexokinases 1 and 2, this allosteric enzyme is inhibited by its product glucose6phosphate. Catalyzes the phosphorylation of hexose, such as dglucose and dfructose, to hexose 6phosphate dglucose 6phosphate and dfructose 6phosphate, respectively by similarity. Hexokinase 2 is a key mediator of aerobic glycolysis and promotes. Mammalian hexokinases and their abnormal expression in cancer.
Hexokinases phosphorylate glucose to produce glucose6phosphate g6p, the first step in most glucose metabolism pathways. If youre behind a web filter, please make sure that the domains. Human hexokinase is an a b protein, with alpha helices and beta sheets interspersed within the same subunit. Hexokinase possesses the ability to transfer an inorganic phosphate group from atp to a substrate. A recent report of the preliminary crystal structure of rat hexokinase i at 3.
Glycolysis the glycolytic pathway the reactions of. Signs and symptoms of hexokinase deficiency are very similar to those of pyruvate kinase deficiency but anemia is generally. The structure and mechanism of hexokinase proteopedia. This message will disappear when all data is loaded. In conjunction with cyclin a and galectin3, hk3 could be used as diagnostic biomarker to screen for malignancy in patients. The kinetic mechanism of brain hexokinase is thought to be random bi bi 11, and a mechanism for its molecular regulation has been proposed 14,15. Oct 24, 2006 hexokinase from leishmania mexicana was purified to homogeneity from a glycosomeenriched fraction obtained after a differential centrifugation of promastigote form. In rapidly growing cancer cells, hexokinase is markedly elevated, and a large fraction 5040% of the total activity is bound to the outer. Binding of noncatalytic atp to human hexokinase i highlights the.
Enzymatic reaction image will open in a new window. Hexokinase is the first key enzyme in glycolysis because it traps glucose within cells. Laboratory procedure manual centers for disease control. Apr 20, 2017 hexokinase or glucokinase is an enzyme that convert glucose to glucose 6 phosphate.
Pdf the increasing of glycolysis in tumors under aerobic conditions is known as warburg phenomenon. Proposed mechanisms by which mitochondrial hexokinases oppose apoptosis. Molecular cloning and functional characterization of a hexokinase. Hexokinase activity assay hexokinase activity assays for the wild type intact, n and c terminal halves, and mutant hexokinase protein preparations were performed spectrophotometrically in the presence of 0. This is a mechanism by which the influx of substrate into the glycolytic pathway is controlled. This is a pdf file of an unedited manuscript that has been accepted for publication. Brain glycolysis is one of the principle pathways for generating energy. Mediates the initial step of glycolysis by catalyzing phosphorylation of dglucose to dglucose 6phosphate by similarity. Suppression of kinetic cooperativity of hexokinase d. Pdf hexokinase 2, glycogen synthase and phosphorylase play. Carbohydrate metabolism references pollardknight d, cornishbowden a. But this enzyme is powerfully inhibited by its product, glc6p. Hexokinase iv is present in the liver, pancreas, hypothalamus, small intestine, and perhaps certain other neuroendocrine cells, and plays an important regulatory role in carbohydrate metabolism. Regulation of hexokinase binding to vdac by cyclophilin d.
301 935 679 427 795 121 157 1300 631 912 661 705 450 1012 476 583 1167 186 1413 410 122 1266 1435 1015 548 1482 1001 104 480 667 532 115 965 185 947 899 1258 943