Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: EC:2.7.1.1 (hexokinase)
5,274 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.2) from rat islets of Langerhans was partially purified by chromatography on DEAE-Cibacron blue F3GA agarose. The enzyme eluted in two separate peaks. Sigmoidal rate dependence was found with respect to glucose (Hill coefficient = 1.5) for both enzyme fractions. Km values for glucose were 5.7 mM for the major fraction and 4.5 mM for the minor fraction. Neither fraction phosphorylated GlcNAc. A GlcNAc kinase (ATP:2-acetamido-2-deoxy-D-glucose 6-phosphotransferase, EC 2.7.1.59)-enriched fraction, prepared by affinity chromatography on Sepharose-N-(6-aminohexanoyl)-GlcNAc, had a Km of 25 microM for GlcNAc. Islet tissue also contained hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) eluting in multiple peaks. The results are consistent with the concept that glucokinase serves as the glucose sensor of pancreatic beta cells.
...
PMID:Chromatographic resolution and kinetic characterization of glucokinase from islets of Langerhans. 633 76

A detailed investigation concerned with localizing hexokinase in the Novikoff ascites tumor is presented. At least 50% of the total hexokinase activity was shown by differential and density gradient centrifugation techniques to be associated with tumor mitochondria. None of this activity was latent. Fractionation of isolated tumor mitochondria with digitonin revealed an outer membrane location for this enzyme. Treatment of tumor mitochondria with glucose 6-phosphate released about 80 to 85% of the hexokinase activity without disrupting the intermembrane compartment. This suggests that at least this proportion of the activity is bound to the outer surface of the outer membrane. Successive treatments did not remove the remaining hexokinase activity. At 30 degrees C, an incubation time of about 10 min with glucose 6-phosphate was required to achieve maximal release. No solubilization occurred at 0-4 degrees C. The isozymes derived from Novikoff mitochondria were identified by anion exchange chromatography as types I and III. Glucokinase activity was not detectable. Evidence is also presented which indicates that the hexokinase obtained from Novikoff mitochondria binds to the outer membrane of rat liver mitochondria. In contrast, the low endogenous hexokinase activity present in isolated liver mitochondria was found not to fractionate with outer membrane markers, but rather with contaminating microsomal membrane markers. Results described here provide the first direct evidence for the submitochondrial location of hexokinase in a tumor. They reveal an outer membrane location and an involvement of two hexokinase isozymes. Because these findings are characteristic of the hepatoma and not observed in control liver preparations, it is suggested that they may be very relevant to the general property of rapidly growing tumors to catabolize large amounts of glucose.
...
PMID:Intracellular localization and properties of particulate hexokinase in the Novikoff ascites tumor. Evidence for an outer mitochondrial membrane location. 688 6

Glucokinase and hexokinase activities were measured in the periportal and perivenous zone of the liver acinus separated by microdissection. A microfluorimetric assay was established for the separate determination of both enzyme activities. Glucokinase activity was about 3.5-fold higher in the perivenous than in the periportal zone in fed male and female rats. after 24 h starvation this gradient was only slightly changed. Hexokinase showed an inverse gradient with about 1.5-fold higher activities in the periportal than in the perivenous zone in both fed and fasted animals. Since glucokinase is restricted to parenchymal cells and hexokinase is present predominantly or even exclusively in non-parenchymal cells, the heterogeneous distribution of glucokinase activity supports the model of a "metabolic zonation of liver parenchyma" with a predominance of glucose uptake in the perivenous and glucose release in the periportal hepatocytes.
...
PMID:Reciprocal distribution of hexokinase and glucokinase in the periportal and perivenous zone of the rat liver acinus. 707 32

Glucokinase is the beta-cell glucose sensor, i.e., the site in glucose metabolism that determines the glucose set-point (sensitivity) for insulin secretion. Hexokinase is also present, but it normally contributes little to glucose metabolism because of end-product inhibition by glucose 6-phosphate. There is a lowered glucose set-point for insulin secretion in 90% pancreatectomized (Px) diabetic rats. We investigated the mechanism by measuring hexokinase and glucokinase activity in islet extracts. Glucokinase activity was minimally raised in Px islets (Vmax 125% of sham-operated control rats). In contrast, hexokinase Vmax was 250% of the control value, suggesting that the increased hexokinase activity caused the beta-cell glucose hypersensitivity. Additional evidence was obtained with a 40-h fast that was performed because of a previous observation that the inhibitory effect of fasting on insulin secretion was impaired in Px rats. Glucokinase activity fell normally in the Px rats (32 +/- 4% reduction in sham vs. 37 +/- 4% in Px rats) as opposed to hexokinase activity, which was unaffected in either group. In summary, a feature of hyperglycemia is upregulated islet hexokinase activity. The result is that hexokinase assumes partial control over the glucose set-point for insulin secretion. As such, regulatory effects on insulin secretion, such as fasting, that are mediated through glucokinase activity may be altered.
...
PMID:Upregulated hexokinase activity in isolated islets from diabetic 90% pancreatectomized rats. 758 32

It was previously reported that insulin biosynthesis in mouse beta TC3 cells was regulated by glucose (Nagamatsu, S., and D. F. Steiner. Endocrinology 130: 748-754, 1992). In the present study, we examined the effect of glucose on the glucose transporter expression and hexokinase activities and determined the relationship between them and glucose-stimulated insulin biosynthesis in beta TC3 cells. Reverse transcriptase-polymerase chain reaction and Northern blot analysis revealed that beta TC3 cells expressed GLUT-1 and GLUT-3 glucose transporter mRNAs, but not GLUT-2. The levels of GLUT-1 and GLUT-3 mRNAs were not affected by glucose (0 or 11 mM glucose) over a period of 48 h. Immunoprecipitation of metabolically labeled beta TC3 cells with specific antibodies against GLUT-1 or GLUT-3 proteins revealed no effect of glucose on the biosynthesis of glucose transporters. Hexokinase [low Michaelis constant (Km) hexokinase] activity from cells incubated in 11 mM glucose for 48 h increased nearly twofold compared with cells maintained in 0 mM glucose, although the amount of cellular hexokinase protein detected by immunoblot analysis was unchanged between 0 and 11 mM glucose conditions. Glucokinase (high Km hexokinase) activity, in contrast, was not affected by glucose. Preincubation of beta TC3 cells with 2-deoxyglucose to inhibit hexokinase, thereby inhibiting all glycolysis, resulted in the decrease of glucose-stimulated insulin biosynthesis. Thus, in mouse beta TC3 cells that do not express GLUT-2, there is a close relationship between hexokinase activity and glucose-stimulated insulin biosynthesis, but not between the glucose transporter and glucose-stimulated insulin biosynthesis.
...
PMID:Glucose transporter expression and functional role of hexokinase in insulin biosynthesis in mouse beta TC3 cells. 765 30

Glucokinase is distinguished from yeast hexokinase and low Km mammalian hexokinases by its low affinity for glucose and its cooperative behavior, even though glucose binding residues and catalytic residues are highly conserved in all of these forms of hexokinase. The roles of Ser-151 and Asn-166 as determinants of hexose affinity and cooperative behavior of human glucokinase have been evaluated by site-directed mutagenesis, expression and purification of the wild-type and mutant enzymes, and steady-state kinetic analysis. Mutation of Asn-166 to arginine increased apparent affinity for both glucose and ATP by a factor of 3. Mutation of Ser-151 to cysteine, alanine, or glycine lowered the Km for glucose by factors of 2-, 26-, and 40-fold, respectively, decreased Vmax, abolished cooperativity for glucose, and also decreased Km for mannose and fructose. The Ser-151 mutants had hexose Km values similar to those of yeast hexokinase, hexokinase I, and the recombinantly expressed COOH-terminal half of hexokinase I. However, the Ki values for the competitive inhibitors, N-acetylglucosamine and glucose-6-P, were unchanged, suggesting that Ser-151 is not important for inhibitor binding. Mutation of Ser-151 also increased the Km for ATP about 5-fold and abolished the enzyme's low ATPase activity, which indicates it is essential for ATP hydrolysis. The substrate-induced change in intrinsic fluorescence of S151A occurred at a much lower glucose concentration than that for wild-type enzyme. The results implicate a dual role for Ser-151 as a determinant of hexose affinity and catalysis, exclusive of the glucose-induced conformational change, and suggest that the low hexose affinity of glucokinase is dependent on interaction of Ser-151 with other regions of the protein.
...
PMID:Human beta-cell glucokinase. Dual role of Ser-151 in catalysis and hexose affinity. 773 Mar 77

Glucokinase and phosphoenolpyruvate carboxykinase are key enzymes of glucose metabolism in the rat liver. The former is considered to be instrumental in regulating glucose hepatic release/uptake according to the glycaemia level, and cytosolic phosphoenolpyruvate carboxykinase is a major flux-generating enzyme for gluconeogenesis. The level of expression of both enzymes and the regulation of their mRNAs in the human liver cell were investigated. Surgical biopsies of liver from patients undergoing partial hepatectomies and parenchymal hepatocytes derived from the biopsies were used to assay glucokinase, hexokinase and phosphoenolpyruvate carboxykinase activities. Hepatocytes were placed in culture and the actions of insulin, glucagon and cAMP on glucokinase and phosphoenolpyruvate carboxykinase mRNAs were studied. The main results are: (a) glucokinase accounts for 95% of the glucose phosphorylation activity of human hepatocytes, although this fact is masked in assays of total liver tissue; (b) glucokinase activity is set at a lower level in human hepatocytes than in rat hepatocytes, and vice-versa for the gluconeogenic enzyme phosphoenolpyruvate carboxykinase; and (c) as previously shown in rat liver, glucokinase and phosphoenolpyruvate carboxykinase mRNAs are regulated in a reciprocal fashion in human hepatocytes, insulin inducing the first enzyme and repressing the latter, whereas glucagon has opposite effects. These data have interesting implications with respect to metabolic regulation and intracellular hormone signaling in the human liver.
...
PMID:Glucokinase and cytosolic phosphoenolpyruvate carboxykinase (GTP) in the human liver. Regulation of gene expression in cultured hepatocytes. 773 62

Human beta-cell glucokinase recognition and phosphorylation of different sugars was investigated by steady-state kinetic analysis, measurements of substrate-induced intrinsic fluorescence changes, and molecular modeling and calculation of interaction energies. Measurements of kcat/Km showed that glucokinase phosphorylated the sugars in the order glucose = mannose > deoxyglucose > fructose = glucosamine. The mode of binding of these sugars to the open conformation of glucokinase was predicted from molecular modeling. Glucokinase is predicted to form similar interactions with the 6-OH, 4-OH, and 1-OH groups of all these sugars. The interactions of the 2-OH and 3-OH groups differ and depend on the type of sugar and reflect differences in cooperative behavior. For example, glucose and deoxyglucose exhibited cooperative behavior with Hill coefficients of 1.8 and 1.5, respectively, while mannose and fructose demonstrated Michaelis-Menten behavior. Galactose, allose, and 2,5-anhydroglucitol were not substrates under the assay conditions used, and the alpha- and beta-anomers of methylglucose were poor substrates with Km's greater than 1000 mM. Glucokinase exhibited an ATPase activity which was 1/2000th that of the rate of the kinase reaction, and unlike yeast hexokinase, it was not affected by the addition of lyxose. Glucosamine was a low affinity inhibitor as well as a substrate, while N-acetylglucosamine and mannoheptulose were high-affinity inhibitors. The change in intrinsic fluorescence that was induced by glucose, mannose, and mannoheptulose had the opposite sign for glucosamine, which implies a very different mode of binding from the other sugars. The calculated interaction energies of glucokinase with glucose, mannose, deoxyglucose, and fructose agree very well with the measured values of kcat/Km, which indicates that these sugars are recognized by binding to the open conformation of glucokinase.
...
PMID:Sugar specificity of human beta-cell glucokinase: correlation of molecular models with kinetic measurements. 774 12

Glucokinase regulates insulin secretion by controlling the rate of glucose phosphorylation. In this report we utilize islets transgenic for high affinity yeast hexokinase to examine the role of glucose phosphorylation on other beta cell functions. Normal pancreatic islets responded to culture in low glucose by lowering insulin synthetic rates, becoming depleted of insulin and insulin mRNA, losing competence to respond to glucose with increased insulin secretion, and lowering glucokinase levels by one-half. In transgenic islets, increased high affinity hexokinase activity provided significant protection against reductions in all parameters of insulin synthesis and helped preserve the competence of beta cells to secrete insulin. The transgenic hexokinase also increased the rate of glucose utilization. These results demonstrate that glucose phosphorylation and presumably glucokinase mediate these glucose regulated responses. Of the parameters measured, only the change in glucokinase activity did not show an effect of the yeast hexokinase transgene. We also found that yeast hexokinase transgene expression was regulated 10-fold by glucose. This is the first demonstration of glucose inducibility of the insulin promoter in transgenic mice.
...
PMID:Insulin synthesis, secretory competence, and glucose utilization are sensitized by transgenic yeast hexokinase. 819 37

Glucokinase (EC 2.7.1.2) activity of B-cells was measured in extracted pancreatic islets isolated from lean and obese fa/fa Zucker rats and maintained in primary culture overnight. Formation of [14C]glucose phosphoric esters from D-[U-14C]glucose was measured in the presence of unlabelled glucose from 0.05 to 0.50 mM for hexokinase (EC 2.7.1.1) activity, and 8.0-16.0 mM unlabelled glucose for glucokinase activity. Eadie-Hofstee analysis revealed that hexokinase kinetic parameters (Vmax and Km) for [14C]glucose phosphoric ester formation were similar in lean- and fa/fa-rat islets. For glucokinase, there was no difference in Vmax. between phenotypes. A non-significant tendency to increased sensitivity to glucose was noted in the fa/fa-rat islets (P = 0.13). In lean-rat islets, the glucokinase inhibitor mannoheptulose (3 mM) decreased Vmax. by 80% and increased the apparent Km from 3.3 +/- 0.7 mM to 12.2 +/- 2.0 mM (P < 0.05). There was no difference in Km or Vmax. in mannoheptulose-treated versus control islets from fa/fa rats. This lack of effect was consistent with reported effects of mannoheptulose on insulin secretion from fa/fa-rat islets [Chan, MacPhail and Mitton (1993) Can. J. Physiol. Pharmacol. 71, 34-39]. The data from glucose and mannoheptulose experiments support the hypothesis that glucokinase function is altered in fa/fa Zucker rats and may contribute to fasting hyperinsulinaemia in vivo in these animals.
...
PMID:Glucokinase activity in isolated islets from obese fa/fa Zucker rats. 824 Feb 76


<< Previous 1 2 3 4 5 6 7 8 Next >>

---