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

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

A protein phosphokinase (EC 2.7.1.1.37) was isolated from baker's yeast (Saccharomyces cerevisiae) after a 17,000-fold purification; the purified enzyme is homogeneous according to the criteria of gel electrophoresis and ultracentrifuge analysis. The enzyme has a high isoelectric point of ca. 9 and appears to exist as a monomer with a molecular weight of 42,000 plus or minus 1500. It is neither stimulated by cyclic 3',5'-AMP, -GMP, -CMP or -ump nor inhibited by the regulatory subunit of rabbit muscle protein kinase (Reimann, E. M., Walsh, D. A., and Krebs, E. G. (1971), J. Biol. Chem. 246, 1986). In the presence of divalent metal ions, preferably Mg-2+ or Mn-2+, the enzyme readily transfers the terminal phosphate group of ATP to phosvitin, alphaS1B- and beta a-casein and an NH2-terminal tryptic peptide derived from beta a-casein, but not to protamine, lysine, or arginine-rich histones or to yeast enzymes such as phosphorylase, phosphofructokinase, or pyruvate carboxylase; serine and polyserine were also inactive as phosphate acceptors. Km values of 0.17 mM for beta a-casein and 0.2 mMfor ATP were determined at 10 mM Mg-2+. The urified yeast protein kinase also catalyzes the reverse reaction, namely, the transfer of phosphate from fully phosphorylated beta a-casein or its NH2-terminal peptide to ADP resulting in the formation of ATP. AMP, GDP, UDP, and CDP did not serve as phosphate acceptors in this reaction. As observed by Rabinowitz and Lipmann (Rabinowitz, M., and Lipmann, F. (1960), J. Biol. Chem. 235, 1043) both reactions have different pHoptima with values of 7.5 for the forward reaction (phosphorylation of the proteins) and ca 5.2 for the formation of ATP; both are differently affected by salts. Phosphorylation of beta a-casein with [gamma-32-P]ATP followed by digestion of the labeled protein with trypsin indicated that all the radioactivity was exclusively introduced in an NH2-terminal peptide possessing the unique sequence: Glu-Ser(P)-Leu-Ser(P)-Ser(P)-Ser(P)-Glu-Glu...(Ribadeau-Dumas, B., Brignon, G., Grosclaude, F., and Mercier, J.-C. (1971), eur J. Biochem. 20, 264). By subjecting beta a-casein and its NH2-terminal peptide to the combined action of almond acid phosphatease and purified yeast protein kinase, it was determined that the phosphorylation and dephosphorylation reactions proceed randomly, i.e., all seryl phosphate residues are equally susceptible and that the rate of phosphorylation decreases drastically as the number of bound phosphate groups in the substrate diminishes.
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PMID:Purification and properties of a yeast protein kinase. 23 75

The isozymes of hexokinase in surgical specimens of human subcutaneous adipose tissue were separated by elution from DEAE-cellulose with linear KCl gradients at ph 7.4. Two peaks of activity were found: Peak 1 eluted at 0.05M KCl, and Peak 2 at 0.19M KCl. Michaelis constants (Km) for glucose were: Peak 1, 6.5 x 10-5M; Peak 2, 1.5 x 10-4M. Peak 2 was more susceptible than Peak 1 to inactivation by trypsin, 0.1 mg/ml, and was protected by 0.1M glucose. Both peaks were protected from heat inactivation (45 degrees) by 0.1M glucose. Peak 2 comprised 66 +/- 5 percent of the total hexokinase activity. No activity with the characteristics of hexokinase III was detected in human fat. In all these characteristics, the isozymes of human adipose tissue closely resemble hexokinases I and II from rats.
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PMID:Hexokinase isozymes of normal human subcutaneous adipose tissue. 68 73

1. The preparation of rat heart mitochondria with Potter-Elvehjem homogenizer results in mitochondria showing stimualtion of respiration induced by Mg2+. This stimualtion is neither caused by adherent hexokinase nor by energy-dependent magnesium accumulation (Mg2+ content in the presence of 10 mM glutamate: 22 nmoles/mg protein; in the presence of glutamate plus antimycin A 21 nmoles/mg protein). 2. The effect of added magnesium is excluded by addition of carboxyatractyloside. This demonstrates the activity of an ATPase outside of the mitochondrial inner membrane. 3. A simple and rapid method for the preparation of Mg2+-insensitive rat heart mitochondria is presented. The minced heart is pressed through a normal syringe and then treated with trypsin. 4. A comparison of mitochondria of both preparations shows that there is no difference in magnesium content and no energy-dependent magnesium influx.
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PMID:Influence of Mg2+-ions on the properties of rat heart mitochondria in dependence on the preparation. 70 27

The effect of thrombin-activated platelets and their release products on the intracellular free calcium concentration ([Ca2+]i) of human polymorphonuclear leukocytes (PMNs) was studied by loading PMNs with a fluorescent indicator of calcium, fura-2. [Ca2+]i of PMNs was transiently elevated by thrombin-activated platelets. The supernatant of thrombin-activated platelets also elicited a transient elevation of [Ca2+]i in PMNs. Pretreatment of the supernatant with hexokinase caused a decrease in the transient [Ca2+]i elevation of PMNs, while hexokinase abrogated the [Ca2+]i elevation of PMNs elicited by 80 mumol/l adenosine triphosphate (ATP). Pretreatment of the supernatant with trypsin also decreased the magnitude of the elevation, while trypsin had no effect on the response to ATP. These findings suggest that thrombin-activated platelets induce a transient [Ca2+]i elevation in PMNs by releasing ATP and some trypsin-sensitive factor(s).
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PMID:Transient calcium elevation in polymorphonuclear leukocytes triggered by thrombin-activated platelets. 159 99

The Type I isozyme of rat hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) is comprised of N- and C-terminal domains, associated with regulatory and catalytic functions, respectively. Extensive sequence similarity between the domains is consistent with evolution of the enzyme by gene duplication and fusion. Cleavage at tryptic sites located in the C-terminal domain is markedly sensitive to ligands present during digestion, while analogous sites in the N-terminal domain are either resistant to trypsin or unaffected by the presence of ligands. These results imply a lack of structural equivalence between the N- and C-terminal domains, with the overall structure of the N-terminal domain being "tighter" and with a major component of ligand-induced conformational changes being focused in the C-terminal domain. Based on a previously proposed structure for brain hexokinase, protection by substrate hexoses is attributed to substrate-induced closing of a cleft in the C-terminal domain. Similar protection at C-terminal cleavage sites results from binding of inhibitory hexose-6-phosphates to the N-terminal domain. In addition, hexose-6-phosphates evoke cleavage at a site, T5, located in a region that has been associated with binding of ATP to the C-terminal domain. Thus, alterations in this region, coupled with reduced accessibility resulting from cleft closure, may account for the mutually exclusive binding of inhibitory hexose-6-phosphates and substrate ATP. In the absence of Mg2+, all nucleoside triphosphates examined (ATP, UTP, CTP, and GTP) protected against digestion by trypsin. In contrast, ATP-Mg2+ stabilized the C-terminal domain but destabilized the N-terminal domain, while the chelated forms of the other nucleoside triphosphates were similar to the unchelated forms in their effect on proteolysis; the unique response to ATP-Mg2+ reflects the specificity for ATP as a substrate.
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PMID:Effect of ligand binding on the tryptic digestion pattern of rat brain hexokinase: relationship of ligand-induced conformational changes to catalytic and regulatory functions. 192 35

Mg2(+)-chelates of several nucleoside triphosphates were shown to increase the inactivation of rat brain hexokinase (ATP:D-hexose-6-phosphotransferase, EC 2.7.1.1) by 0.6 M guanidine hydrochloride, with ATP-Mg2+ having the greatest effect; unchelated forms did not significantly affect inactivation. Since catalytic activity has been associated with the C-terminal half of the molecule, these results were interpreted as indicating a destabilization of this C-terminal region by binding of these chelates to the substrate nucleotide sites, with the particular effectiveness of ATP-Mg2+ reflecting the specificity for this species as a phosphoryl donor. These compounds were also shown to bind to the N-terminal half of the enzyme, as judged by their ability to protect against denaturation by guanidine hydrochloride and subsequent digestion with trypsin. Both free and Mg2(+)-chelated forms afforded protection, with the unchelated nucleotides being most effective; a preference for ATP was seen only with the chelated forms. Thus, it was concluded that the N-terminal half of hexokinase contains a relatively nonspecific nucleotide binding site, distinct from the substrate nucleotide site previously shown to reside in the C-terminal half. On the basis of this same ability to protect the N-terminal half against denaturation and proteolysis, several other polyanionic ligands were shown to bind to this region of the molecule. These included inorganic phosphate, its analogs, sulfate and arsenate, and its homologs, pyrophosphate and tripolyphosphate. All of these anionic ligands were also shown to antagonize inhibition by the glucose 6-phosphate (Glc-6-P) analog, 1,5-anhydroglucitol 6-phosphate. The allosteric site for binding of Glc-6-P has previously been shown to reside in the N-terminal half of the molecule, and it is suggested that the antagonism of inhibition by Glc-6-P (or its analog) by these anionic ligands results from interaction with an anion binding site for which the 6-phosphate group of inhibitory hexose 6-phosphates must compete. A model depicting possible relationships between ligand binding sites on brain hexokinase, and how their interactions might lead to observed regulatory properties, is developed based on these and previous studies of ligand binding as well as evidence that mammalian hexokinases (Mr 100,000) have evolved by duplication and fusion of a gene coding for an ancestral hexokinase with Mr 50,000 and which, like the mammalian enzyme, was sensitive to inhibition by Glc-6-P.
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PMID:Binding of nucleoside triphosphates, inorganic phosphate, and other polyanionic ligands to the N-terminal region of rat brain hexokinase: relationship to regulation of hexokinase activity by antagonistic interactions between glucose 6-phosphate and inorganic phosphate. 230 21

Selective stabilization of either the N- or C-terminal half (by ligands binding to these regions) of rat brain hexokinase against partial denaturation with guanidine hydrochloride and subsequent digestion with trypsin has provided a means for isolating these regions, referred to as N fragment and C fragment, respectively, in quantities adequate for characterization. The N fragment (mol wt 52 kDa) is devoid of catalytic activity. In contrast, the C fragment (mol wt 51 kDa) has a specific activity of about 110 U/mg, nearly twice that (60 U/mg) of the intact 100-kDa enzyme, indicating that the kappa cat is virtually identical for both species. Unlike the parent enzyme, the C fragment is quite sensitive to inhibition by Pi (competitive vs ATP, noncompetitive vs Glc); sulfate and arsenate, but not acetate, inhibit with effectiveness similar to that seen with Pi. The Glc-6-P analog, 1,5-anhydroglucitol-6-P, also inhibits the C fragment (competitive vs ATP, uncompetitive vs Glc). Both N and C fragments bind to Affi-Gel Blue, an affinity matrix bearing a covalently attached analog of ATP, and are eluted by hexose 6-phosphates competitive with nucleotide binding to the parent enzyme. Based on the ability of various hexoses and hexose 6-phosphates (and analogs) to protect against guanidine-induced denaturation and subsequent proteolysis it is concluded that both fragments contain discrete sites for hexoses and hexose 6-phosphates, with specificities resembling those seen for the binding of these ligands to the parent enzyme. Synergistic interactions between the hexose and hexose-6-P binding sites, previously seen with the parent enzyme, are also observed with the C fragment but not the N fragment. The existence of binding sites for hexoses and hexose 6-phosphates on both halves conflicts with previous binding studies demonstrating a single hexose binding site and a single hexose 6-phosphate binding site on the intact 100-kDa enzyme, leading to the conclusion that one of each pair of sites must be latent in the intact enzyme, becoming manifest only in the isolated discrete halves. Several investigators have previously suggested that the 100-kDa mammalian hexokinases evolved by duplication and fusion of a gene encoding an ancestral 50-kDa Glc-6-P-insensitive hexokinase, similar to the present-day yeast enzyme, with sensitivity to Glc-6-P resulting from evolution of a duplicated catalytic site into a regulatory site.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Isolation and characterization of the discrete N- and C-terminal halves of rat brain hexokinase: retention of full catalytic activity in the isolated C-terminal half. 280 17

The adenine nucleotide analog, [3H]pyridoxal 5'-diphospho-5'-adenosine (PLP-AMP), is shown to be a potent and specific inhibitor of yeast hexokinase PII. Evidence that the analog binds specifically at the ATP binding site includes the demonstration that glucose binding enhances PLP-AMP binding and that PLP-AMP and ATP bind competitively with an apparent Ki(PLP-AMP) = 23 microM. In addition, from the relationship between the degree of inhibition and extent of modification, it is estimated that the incorporation of 1 mol of PLP-AMP/mol of subunit is required for complete inhibition. Borohydride reduction of the Schiff's base complex formed between hexokinase and [3H]PLP-AMP gives a stable product. The reduced derivative was digested with trypsin and a single radioactive peptide was isolated by reversed-phase high-pressure liquid chromatography. Amino acid sequence analysis identified Lys-111 as the modified residue. Taking into account the known structures of the binary complexes (Shoham, M., and Steitz, T. A. (1980) J. Mol. Biol. 140, 1-14), the results suggest that Lys-111, located in the smaller of the two lobes of hexokinase, moves into the active site upon formation of the ternary complex.
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PMID:The adenine nucleotide binding site on yeast hexokinase PII. Affinity labeling of Lys-111 by pyridoxal 5'-diphospho-5'-adenosine. 313 29

Glucose 6-phosphate as well as several other hexose mono- and diphosphates were found by kinetic studies to be competitive inhibitors of human hexokinase I (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) versus MgATP. Limited proteolysis by trypsin does not destroy the hexokinase activity but produces as well-defined peptide map when the digested enzyme is electrophoresed in the presence of sodium dodecyl sulfate. MgATP at subsaturating concentration protects hexokinase from trypsin digestion, while phosphorylated sugars, Mg2+, glucose and inorganic phosphate have no effect. Addition of glucose 6-phosphate to the MgATP-hexokinase complex at a concentration 100-times higher than its Ki was not able to reverse the MgATP-induced conformation of hexokinase, suggesting that the binding of glucose 6-phosphate and MgATP are not mutually exclusive. Similar evidence was also obtained by studies of the induced modifications of ultraviolet spectra of hexokinase by the binding of MgATP, glucose 6-phosphate and both compounds. Among a library of monoclonal antibodies produced against rat brain hexokinase I and that recognize human placenta hexokinase I, one (4A6) was found to be able to modify the Ki of glucose 6-phosphate (from 25 to 140 microM) for human hexokinase I. The same antibody also weakens the inhibition by all the other hexoses phosphate studied without affecting the apparent Km for MgATP (from 0.6 to 0.75 mM) or for glucose. These data support the view for the binding of glucose 6-phosphate at a regulatory site on the enzyme.
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PMID:The interaction of phosphorylated sugars with human hexokinase I. 325 34

The pressure-induced dissociation of the isozymes P1 and P2 of hexokinase was investigated by studies of the spectral shift of the intrinsic protein fluorescence and by the fluorescence polarization of dansyl conjugates. The free energy of association of the monomers at atmospheric pressure, Katm, was -14.2 kcal mol-1 at 20 degrees C and -11.4 kcal mol-1 at 0 degrees C. The positive enthalpy indicates that the association of the monomers is entropy-driven, overcoming the negative enthalpy of hydration of the subunit interfaces. At 0 degrees C and 1 bar, glucose stabilizes the association by -1.1 kcal mol-1 and the binding of both adenosine 5'-(beta, gamma-methylenetriphosphate) (AMPPCP) and glucose by an even larger amount, -1.34 kcal mol-1. Paradoxically, adenosine 5'-triphosphate (ATP), or AMPPCP, in the absence of glucose destabilizes the association by +0.34 kcal mol-1, while adenosine 5'-diphosphate (ADP) stabilizes it by -0.6 kcal mol-1. Comparison of dV0, the apparent standard volume of association, at different pHs and temperatures indicates that its value (115-160 mL mol-1) is strongly dependent upon the ionization of a group at the subunit interface with a pK near neutrality. Under dissociating pressures, trypsin action results in permanent dissociation of the dimer, confirming earlier observations of Colowick by less direct methods. The P1 and P2 enzymes differ in Katm and dV0 and markedly so in the effects of salt upon the stability of the dimer.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Dissociation of yeast hexokinase by hydrostatic pressure. 329 47


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