A recombinant human 'mini'-hexokinase is catalytically active and regulated by hexose 6-phosphates

Metalloglycobiology: The power of metals in regulating glycosylation

The remarkable structural diversity of glycans that is exposed at the cell surface and generated along the secretory pathway is tightly regulated by several factors. The recent identification of human glycosylation diseases related to metal transporter defects opened a completely new field of investigation, referred to herein as "metalloglycobiology", on how metal changes can affect the glycosylation and hence the glycan structures that are produced. Although this field is in its infancy, this review aims to go through the different glycosylation steps/pathways that are metal dependent and that could be impacted by metal homeostasis dysregulations.

Proteomic and phosphoproteomic analysis reveal threonine deficiency increases hepatic lipid deposition in Pekin ducks via reducing STAT phosphorylation

Dietary threonine (Thr) deficiency enhances triglyceride (TG) deposition in the liver of Pekin ducks, which injures hepatic function and impairs growth performance. However, the underlying molecular mechanisms remain unclear. In the present study, we investigated the effects of dietary Thr deficiency on the expressions of proteins and phosphoproteins in liver of Pekin ducks, to identify the underlying molecular changes. A total of 300 one-day-old ducklings were divided into 3 groups with 10 replicates of 10 birds. All ducks were fed corn-wheat-peanut meal diets containing 0.46%, 0.71%, and 0.96% Thr, respectively, from 1 to 21 days of age. Growth performance, serum parameters, hepatic TG content, and expression of genes involved in lipid metabolism of Pekin ducks were determined. A Thr deficiency group (Thr-D, 0.46% Thr) and a Thr sufficiency group (Thr-S, 0.71% Thr) were selected for subsequent proteomic and phosphoproteomic analysis. The results showed that Thr-D reduced the growth performance (P < 0.001), and increased the plasma concentrations of cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and hepatic TG (P < 0.05). Thr-D increased gene expression related to fatty acid and TG synthesis (P < 0.05). A total of 176 proteins and 259 phosphosites (containing 198 phosphoproteins) were observed to be differentially expressed as a result of Thr-D. The upregulated proteins were enriched in the pathway related to amino acid metabolism, peroxisome. The downregulated proteins were enriched in linolenic and arachidonic acid metabolism, and the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway. The upregulated phosphoproteins were enriched in the pathways related to fatty acid biosynthesis, fructose and mannose metabolism, and glycolysis/gluconeogenesis. Thr-D reduced the phosphorylation of STAT1 at S729 and STAT3 at S728, and expression of STAT5B. In contrast, Thr-D increased non-receptor tyrosine-protein kinase (TYK2) expression and STAT1 phosphorylation at S649. Taken together, dietary Thr-D increased hepatic TG accumulation by upregulating the expression of genes and proteins, and phosphoproteins related to fatty acid and triglyceride synthesis. Furthermore, these processes might be regulated by the JAK-STAT signaling pathway, especially the phosphorylation of STAT1 and STAT3.

Baicalein Inhibits the Progression and Promotes Radiosensitivity of Esophageal Squamous Cell Carcinoma by Targeting HIF-1A

Purpose

To explore the mechanism of the effect of baicalein on radioresistance of esophageal cancer, and to provide ideas for the treatment of patients with poor radiotherapy effect of esophageal cancer.

Methods

The glycolytic rate assay kit was used to detect the changes in glycolytic metabolism in esophageal cancer cells after treatment with baicalein, and mass spectrometry was used to detect whether baicalein could affect the level of glycolysis-related metabolites in esophageal cancer cells. The binding of baicalein to the target protein was simulated by molecular docking technique, the protein expression level was detected by Western Blot, and the changes in the cell cycle were detected by flow cytometry.

Results

Radiation combined with baicalein could significantly inhibit the proliferation and migration of esophageal cancer cells compared with that of 6 Gy rays alone. The results of the glycolytic rate assay showed that baicalein could inhibit the glycolysis of esophageal cancer cells. Metabonomic studies showed that baicalein could affect the expression levels of glycolysis-related metabolites. The results of network pharmacology showed that baicalein could target several key glycolysis enzymes and glycolysis-related proteins, such as HIF-1A. The results of the WB experiment showed that glycolysis-related proteins and cycle-related proteins were down-regulated after baicalein treatment.

Conclusion

The main mechanism of baicalein inhibiting radiation resistance of esophageal cancer cells is that targeting HIF-1A protein regulates glucose metabolism and then regulates Cyclin D1/CDK4 axis to change the cell cycle.

Dynamic Change of Intracellular Metabolism of Microglia Evaluated by Transcriptomics in an Alzheimer's Mouse Model

BACKGROUND

Microglia play diverse roles in Alzheimer's disease (AD). Intracellular metabolism has been indicated an important factor in modulating the function of microglia. However, it is not clear whether the intracellular metabolism of microglia changes dynamically in different stages of AD.

OBJECTIVE

To determine whether microglia intracellular metabolism changes dynamically in different stages of AD.

METHODS

Microglia were extracted from APPSwe/PS1dE9 (APP/PS1) mice and wild-type littermates at 2, 4, and 8 months old by fluorescence-activated cell sorting and used for RNA-sequencing analysis and quantitative PCR. Morphologies of amyloid plaques and microglia were detected by immunofluorescence staining.

RESULTS

Compared with control littermates, the microglia of APP/PS1 mice exhibited significant transcriptional changes at 2-month-old before microglia morphological alterations and the plaque formation. The changes continued drastically following age with defined morphological shift of microglia and amyloid plaque enhancement in brains. Further analysis of those genotype and age dependent transcriptomic changes revealed that differentially expressed genes were enriched in pathways related to energy metabolism. Compared with wild-type mice, there were changes of some vital genes related to glucose metabolism and lipid metabolism pathways in APP/PS1 mice at different ages. Glucose metabolism may play a major role in early activation of microglia, and lipid metabolism may be more important in later activation period.

CONCLUSION

Our results showed that microglia actively participate in the pathological progress of AD. The intracellular metabolism of microglia changed significantly in different stages of AD, even preceding amyloid-β deposition.

Epstein-Barr virus immortalization of human B-cells leads to stabilization of hypoxia-induced factor 1 alpha, congruent with the Warburg effect

BACKGROUND

Epstein-Barr virus (EBV) encodes six nuclear transformation-associated proteins that induce extensive changes in cellular gene expression and signaling and induce B-cell transformation. The role of HIF1A in EBV-induced B-cell immortalization has not been previously studied.

METHODS AND FINDINGS

Using Western blotting and Q-PCR, we found that HIF1A protein is stabilized in EBV-transformed lymphoblastoid cells. Western blotting, GST pulldown assays, and immunoprecipitation showed that EBV-encoded nuclear antigens EBNA-5 and EBNA-3 bind to prolylhydroxylases 1 and 2, respectively, thus inhibiting HIF1A hydroxylation and degradation. Immunostaining and Q-PCR showed that the stabilized HIF1A translocates to the nucleus, forms a heterodimer with ARNT, and transactivates several genes involved in aerobic glycolysis. Using biochemical assays and Q-PCR, we also found that lymphoblastoid cells produce high levels of lactate, lactate dehydrogenase and pyruvate.

CONCLUSIONS

Our data suggest that activation of the aerobic glycolytic pathway, corresponding to the Warburg effect, occurs in EBV-transformed lymphoblastoid cells, in contrast to mitogen-activated B-cells.

Regulation and cytoprotective role of hexokinase III

BACKGROUND

Hexokinases (HKs) catalyze the first step in glucose metabolism. Of the three mammalian 100-kDa HK isoforms, HKI and II can bind to mitochondria and protect against cell death. HKIII does not bind mitochondria, and little is known about its regulation or cytoprotective effects. We studied the regulation of HKIII at the transcriptional and protein levels and investigated its role in cellular protection.

METHODOLOGY/PRINCIPAL FINDINGS

We show that like HKII, HKIII expression is regulated by hypoxia, but other factors that regulate HKII expression have no effect on HKIII levels. This transcriptional regulation is partially dependent on hypoxia-inducible factor (HIF) signaling. We also demonstrate regulation at the protein level, as mutations in putative N-terminal substrate binding residues altered C-terminal catalytic activity, suggesting that HKIII activity is governed, in part, by interactions between these two domains. Overexpression of HKIII reduced oxidant-induced cell death, increased ATP levels, decreased the production of reactive oxygen species (ROS), and preserved mitochondrial membrane potential. HKIII overexpression was also associated with higher levels of transcription factors that regulate mitochondrial biogenesis, and greater total mitochondrial DNA content. Attempts to target HKIII to the mitochondria by replacing its N-terminal 32-amino-acid sequence with the mitochondrial-targeting sequence of HKII led to protein aggregation, suggesting that this region is necessary to maintain proper protein folding and solubility.

CONCLUSIONS/SIGNIFICANCE

These results suggest that HKIII is regulated by hypoxia and there are functional interactions between its two halves. Furthermore, HKIII exerts protective effects against oxidative stress, perhaps by increasing ATP levels, reducing oxidant-induced ROS production, preserving mitochondrial membrane potential, and increasing mitochondrial biogenesis.

Glucose phosphorylation and mitochondrial binding are required for the protective effects of hexokinases I and II

Alterations in glucose metabolism have been demonstrated for diverse disorders ranging from heart disease to cancer. The first step in glucose metabolism is carried out by the hexokinase (HK) family of enzymes. HKI and II can bind to mitochondria through their N-terminal hydrophobic regions, and their overexpression in tissue culture protects against cell death. In order to determine the relative contributions of mitochondrial binding and glucose-phosphorylating activities of HKs to their overall protective effects, we expressed full-length HKI and HKII, their truncated proteins lacking the mitochondrial binding domains, and catalytically inactive proteins in tissue culture. The overexpression of full-length proteins resulted in protection against cell death, decreased levels of reactive oxygen species, and possibly inhibited mitochondrial permeability transition in response to H(2)O(2). However, the truncated and mutant proteins exerted only partial effects. Similar results were obtained with primary neonatal rat cardiomyocytes. The HK proteins also resulted in an increase in the phosphorylation of voltage-dependent anion channel (VDAC) through a protein kinase Cepsilon (PKCepsilon)-dependent pathway. These results suggest that both glucose phosphorylation and mitochondrial binding contribute to the protective effects of HKI and HKII, possibly through VDAC phosphorylation by PKCepsilon.

Bovine hexokinase type I: full-length cDNA sequence and characterisation of the recombinant enzyme

This study reports the revised and full-length cDNA sequence of bovine hexokinase type I obtained from bovine brain. Since dissimilarities have been observed between the published bovine hexokinase type I coding sequence (GenBank accession no. M65140) (Genomics 11: 1014-1024, 1991) and an analysed portion of bovine hexokinase type I gene, the entire open reading frame was re-sequenced and the ends of cDNA isolated by rapid amplification of cDNA ends. The coding sequences, when compared with the published bovine hexokinase type I, contained a large number of mismatches that lead to changes in the resulting amino acid sequence. The revisions result in a hexokinase type I cDNA of 3619 bp that encodes a protein of 917 amino acids highly homologous to human hexokinase type I. The expression of the recombinant full-length enzyme demonstrated that it was a catalytically active hexokinase. When characterised for its kinetic and regulatory properties, it displayed the same affinity for glucose and MgATP as the human hexokinase type I and was inhibited by glucose 6-phosphate competitively versus MgATP. The production of the N- and C-terminal recombinant halves of the enzyme followed by comparison with the full-length hexokinase indicated that the catalytic activity is located in the C-terminal domain.

Structural Basis for Allosteric Regulation of the Monomeric Allosteric Enzyme Human Glucokinase

Glucokinase is a monomeric enzyme that displays a low affinity for glucose and a sigmoidal saturation curve for its substrate, two properties that are important for its playing the role of a glucose sensor in pancreas and liver. The molecular basis for these two properties is not well understood. Herein we report the crystal structures of glucokinase in its active and inactive forms, which demonstrate that global conformational change, including domain reorganization, is induced by glucose binding. This suggests that the positive cooperativity of monomeric glucokinase obeys the "mnemonical mechanism" rather than the well-known concerted model. These structures also revealed an allosteric site through which small molecules may modulate the kinetic properties of the enzyme. This finding provided the mechanistic basis for activation of glucokinase as a potential therapeutic approach for treating type 2 diabetes mellitus.

Binding equilibrium isotope effects for glucose at the catalytic domain of human brain hexokinase

We have utilized tritium isotope effects to probe the in vitro binding equilibrium between glucose and human brain hexokinase (E.C.2.7.1.1). Replacing a backbone hydrogen atom in glucose with tritium can significantly increase or decrease the equilibrium association constant. Specifically, the equilibrium tritium isotope effects are 1.027 +/- 0.002, 0.927 +/- 0.0003, 1.027 +/- 0.004, 1.051 +/- 0.001, 0.988 +/- 0.001, and 1.065 +/- 0.003 for [1-t]-, [2-t]-, [3-t]-, [4-t]-, [5-t]-, and [6,6-t(2)]glucose, respectively. We have shown that the existence of prebinding equilibrium isotope effects can contribute to binding isotope effect studies but that this effect is insignificant for glucose binding to hexokinase. The binding isotope effects are interpreted in the context of structural studies of hexokinase-glucose complexes. Ab initio calculations on 2-propanol with or without a hydrogen bonding partner, in steric collision with formaldehyde or methane, and on ethanol, cyclohexanol and 1-hydroxymethyl-tetrahydropyran are presented to clarify the magnitude of isotope effects possible in such interactions and the accompanying changes in free energy. Position-specific binding isotope effects provide direct evidence of the partial deprotonation and activation of O6 by Asp657, of other hydrogen bonding interactions with ionic residues, and of the steric compression of CH2 by the backbone carbonyl of Ser603.

Kinetic and regulatory properties of HK I(+), a modified form of the type I isozyme of mammalian hexokinase in which interactions between the N- and C-terminal halves have been disrupted

A modified form (HK I(+)) of rat Type I hexokinase (HK I) has been expressed. HK I(+) contains a centrally located polyalanine insert which, along with the known helical propensity of adjacent sequence, was expected to lead to alpha-helix formation, with resulting distension of the molecule and disruption of interactions between the N- and C-terminal halves. The properties of HK I(+) are consistent with this expectation and with previous proposals that (1) inhibition of HK I by Glc-6-P or its analogs and antagonism of this inhibition by P(i) result from competition of these ligands for a binding site in the N-terminal half of HK I, with resulting conformational changes propagated through interactions with the catalytic C-terminal half, and (2) binding of Glc-6-P to a site in the C-terminal half of HK I is obstructed by interactions between the halves, present in HK I but not HK I(+).

Crystal structures of mutant monomeric hexokinase I reveal multiple ADP binding sites and conformational changes relevant to allosteric regulation

Hexokinase I, the pacemaker of glycolysis in brain tissue, is composed of two structurally similar halves connected by an alpha-helix. The enzyme dimerizes at elevated protein concentrations in solution and in crystal structures; however, almost all published data reflect the properties of a hexokinase I monomer in solution. Crystal structures of mutant forms of recombinant human hexokinase I, presented here, reveal the enzyme monomer for the first time. The mutant hexokinases bind both glucose 6-phosphate and glucose with high affinity to their N and C-terminal halves, and ADP, also with high affinity, to a site near the N terminus of the polypeptide chain. Exposure of the monomer crystals to ADP in the complete absence of glucose 6-phosphate reveals a second binding site for adenine nucleotides at the putative active site (C-half), with conformational changes extending 15 A to the contact interface between the N and C-halves. The structures reveal distinct conformational states for the C-half and a rigid-body rotation of the N-half, as possible elements of a structure-based mechanism for allosteric regulation of catalysis.

Dual mechanisms for glucose 6-phosphate inhibition of human brain hexokinase

Brain hexokinase (HKI) is inhibited potently by its product glucose 6-phosphate (G6P); however, the mechanism of inhibition is unsettled. Two hypotheses have been proposed to account for product inhibition of HKI. In one, G6P binds to the active site (the C-terminal half of HKI) and competes directly with ATP, whereas in the alternative suggestion the inhibitor binds to an allosteric site (the N-terminal half of HKI), which indirectly displaces ATP from the active site. Single mutations within G6P binding pockets, as defined by crystal structures, at either the N- or C-terminal half of HKI have no significant effect on G6P inhibition. On the other hand, the corresponding mutations eliminate product inhibition in a truncated form of HKI, consisting only of the C-terminal half of the enzyme. Only through combined mutations at the active and allosteric sites, using residues for which single mutations had little effect, was product inhibition eliminated in HKI. Evidently, potent inhibition of HKI by G6P can occur from both active and allosteric binding sites. Furthermore, kinetic data reported here, in conjunction with published equilibrium binding data, are consistent with inhibitory sites of comparable affinity linked by a mechanism of negative cooperativity.

Allosteric regulation of type I hexokinase: A site-directed mutational study indicating location of the functional glucose 6-phosphate binding site in the N-terminal half of the enzyme

The Type I isozyme of mammalian hexokinase has evolved by a gene duplication-fusion mechanism, with resulting internal duplication of sequence and ligand binding sites. However, 1:1 binding stoichiometry indicates that only one of these is available for binding the product inhibitor, Glc-6-P; the location of that site, in the N- or C-terminal half, remains under debate. Recent structural studies (Aleshin et al., Structure 6, 39-50, 1998; Mulichak et al., Nature Struct. Biol. 5, 555-560, 1998) implicated Asp 84 or its analog in the C-terminal half, Asp 532, in binding of Glc-6-P. Zeng et al. (Biochemistry 35, 13157-13164, 1996) demonstrated that mutation of Asp 532 to Lys or Glu did not affect inhibition by the Glc-6-P analog, 1,5-anhydroglucitol-6-P. These same mutations, as well as mutation to Ala, at the Asp 84 position are now shown to result in increased Ki for 1,5-anhydroglucitol-6-P. The ability of Pi to antagonize inhibition by the Glc-6-P analog is severely diminished or abolished by these mutations, suggesting that antagonism is dependent on precise positioning of the inhibitory hexose 6-phosphate. The structure of the enzyme complexed with Glc and Pi has been determined, and shows that Pi occupies the same site as the 6-phosphate group in the complex with Glc-6-P. Thus, antagonism between these ligands results from competition for a common anion binding site in the N-terminal half.

Enzymatic properties of overexpressed human hexokinase fragments

Full-length hexokinase (HK; ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1), a truncate form of the enzyme lacking the first 11 amino acids (HK-11aa) and the 50 kDa C-terminal half ('mini'-HK) containing the catalytic domain, were overexpressed and purified to homogeneity to investigate the influence of the N-terminal region of human hexokinase type I (HK) on its regulatory properties. All forms of the enzyme are catalytically active with the HK-11aa being the most active. All the forms of HK showed the same affinity for glucose and MgATP and were also inhibited by glucose 6-phosphate (Glc 6-P) competitively vs. MgATP with similar Kis (28.5-37 microM). Glucose 1,6-bisphosphate (Glc 1,6-P2) was also a strong inhibitor of all HKs without significant differences among the different truncate forms of the enzyme (Kis 49.5-59 microM). At low concentrations (0-3 mM), Pi was able to reverse the sugar phosphate inhibition of the full-length HK and HK-11aa but not of the 'mini'-HK. In contrast, at high concentrations Pi was an inhibitor of all the hexokinases investigated. These findings confirm that Pi has a low affinity binding site on the C-terminal of HK while counteracts glucose 6-phosphate inhibition by binding to or requiring the N-terminal half of the enzyme. The first 11 N-terminal amino acids influence the specific activity of HK but are unable to affect the kinetic properties investigated.

Expression, purification, and characterization of a recombinant erythroid-specific hexokinase isozyme

Hexokinase type I (HK I; ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1), the predominant glucose-phosphorylating enzyme in red blood cells, exists in human erythrocytes in multiple molecular forms that differ in isoelectric point and are separable by ion-exchange chromatography. The major forms, designated HK Ia, Ib and Ic, have similar kinetic properties but are characterized by different age-dependent decay and different intracellular distribution in reticulocytes. HK Ib, which elutes between HK I and HK II in the DEAE ion-exchange chromatography, appears to be unique to RBCs and different from any other hexokinase isozyme previously described. Indeed, Murakami and Piomelli recently reported the presence of a specific HK isozyme (named HKr) expressed in K562 cells and in human reticulocytes and, moreover, the resolution of the human HK I gene structure provided the direct evidence of an erythroid-specific exon 1. To further investigate the microheterogeneity of HK I in human RBCs we established a prokaryotic expression system for the HKr isozyme, using the pET plasmid, inducible with IPTG. The recombinant HKr, expressed in bacterial cells as a catalytically active enzyme, was purified to homogeneity by a combination of DEAE ionexchange chromatography followed by hydrophobic interaction chromatography and dye-ligand affinity chromatography. The kinetic and chromatographic properties of the homogeneous recombinant HKr suggest that this erythroid-specific HK isozyme in fact corresponds to the HK isoform previously described in human RBCs and referred to as HK Ib.

Regulation of hexokinase I: crystal structure of recombinant human brain hexokinase complexed with glucose and phosphate

Hexokinase I, the pacemaker of glycolysis in brain tissue and red blood cells, is comprised of two similar domains fused into a single polypeptide chain. The C-terminal half of hexokinase I is catalytically active, whereas the N-terminal half is necessary for the relief of product inhibition by phosphate. A crystalline complex of recombinant human hexokinase I with glucose and phosphate (2.8 A resolution) reveals a single binding site for phosphate and glucose at the N-terminal half of the enzyme. Glucose and phosphate stabilize the N-terminal half in a closed conformation. Unexpectedly, glucose binds weakly to the C-terminal half of the enzyme and does not by itself stabilize a closed conformation. Evidently a stable, closed C-terminal half requires either ATP or glucose 6-phosphate along with glucose. The crystal structure here, in conjunction with other studies in crystallography and directed mutation, puts the phosphate regulatory site at the N-terminal half, the site of potent product inhibition at the C-terminal half, and a secondary site for the weak interaction of glucose 6-phosphate at the N-terminal half of the enzyme. The relevance of crystal structures of hexokinase I to the properties of monomeric hexokinase I and oligomers of hexokinase I bound to the surface of mitochondria is discussed.

Identification of a phosphate regulatory site and a low affinity binding site for glucose 6-phosphate in the N-terminal half of human brain hexokinase

Crystal structures of human hexokinase I reveal identical binding sites for phosphate and the 6-phosphoryl group of glucose 6-phosphate in proximity to Gly87, Ser88, Thr232, and Ser415, a binding site for the pyranose moiety of glucose 6-phosphate in proximity to Asp84, Asp413, and Ser449, and a single salt link involving Arg801 between the N- and C-terminal halves. Purified wild-type and mutant enzymes (Asp84 --> Ala, Gly87 --> Tyr, Ser88 --> Ala, Thr232 --> Ala, Asp413 --> Ala, Ser415 --> Ala, Ser449 --> Ala, and Arg801 --> Ala) were studied by kinetics and circular dichroism spectroscopy. All eight mutant hexokinases have kcat and Km values for substrates similar to those of wild-type hexokinase I. Inhibition of wild-type enzyme by 1,5-anhydroglucitol 6-phosphate is consistent with a high affinity binding site (Ki = 50 microM) and a second, low affinity binding site (Kii = 0.7 mM). The mutations of Asp84, Gly87, and Thr232 listed above eliminate inhibition because of the low affinity site, but none of the eight mutations influence Ki of the high affinity site. Relief of 1,5-anhydroglucitol 6-phosphate inhibition by phosphate for Asp84 --> Ala, Ser88 --> Ala, Ser415 --> Ala, Ser449 --> Ala and Arg801 --> Ala mutant enzymes is substantially less than that of wild-type hexokinase and completely absent in the Gly87 --> Tyr and Thr232 --> Ala mutants. The results support several conclusions. (i) The phosphate regulatory site is at the N-terminal domain as identified in crystal structures. (ii) The glucose 6-phosphate binding site at the N-terminal domain is a low affinity site and not the high affinity site associated with potent product inhibition. (iii) Arg801 participates in the regulatory mechanism of hexokinase I.

Structure of the human hexokinase type I gene and nucleotide sequence of the 5' flanking region

This study reports the precise intron/exon boundaries and intron/exon composition of the human hexokinase type I gene. A yeast artificial chromosome containing the hexokinase type I gene was isolated from the yeast artificial chromosome library of the Centre d'Etude du Polymorphisme Humaine. A cosmid sublibrary was created and direct sequencing of the individual cosmids was used to provide the exon/intron organization. The human hexokinase type I gene was found to be composed of 18 exons ranging in size from 63 to 305 bp. Intron 1 is at least 15 kb in length, whereas intron 2 spans at least 10 kb. Overall, the length of the 17 introns ranges from 104 to greater than 15 kb. The entire coding region is contained in at least 75 kb of the gene. The structure of the gene reveals a remarkable conservation of the size of the exons compared with glucokinase and hexokinase type II. Isolation of the 5' flanking region of the gene revealed a 75-90% identity with the rat sequence. Direct evidence of an alternative red-blood-cell-specific exon 1 located upstream of the 5' flanking region of the gene is also provided.

The mechanism of regulation of hexokinase: new insights from the crystal structure of recombinant human brain hexokinase complexed with glucose and glucose-6-phosphate

BACKGROUND

Hexokinase I is the pacemaker of glycolysis in brain tissue. The type I isozyme exhibits unique regulatory properties in that physiological levels of phosphate relieve potent inhibition by the product, glucose-6-phosphate (Gluc-6-P). The 100 kDa polypeptide chain of hexokinase I consists of a C-terminal (catalytic) domain and an N-terminal (regulatory) domain. Structures of ligated hexokinase I should provide a basis for understanding mechanisms of catalysis and regulation at an atomic level.

RESULTS

The complex of human hexokinase I with glucose and Gluc-6-P (determined to 2.8 A resolution) is a dimer with twofold molecular symmetry. The N- and C-terminal domains of one monomer interact with the C- and N-terminal domains, respectively, of the symmetry-related monomer. The two domains of a monomer are connected by a single alpha helix and each have the fold of yeast hexokinase. Salt links between a possible cation-binding loop of the N-terminal domain and a loop of the C-terminal domain may be important to regulation. Each domain binds single glucose and Gluc-6-P molecules in proximity to each other. The 6-phosphoryl group of bound Gluc-6-P at the C-terminal domain occupies the putative binding site for ATP, whereas the 6-phosphoryl group at the N-terminal domain may overlap the binding site for phosphate.

CONCLUSIONS

The binding synergism of glucose and Gluc-6-P probably arises out of the mutual stabilization of a common (glucose-bound) conformation of hexokinase I. Conformational changes in the N-terminal domain in response to glucose, phosphate, and/or Gluc-6-P may influence the binding of ATP to the C-terminal domain.

Expression of hexokinase mRNA in human hippocampus

The expression of hexokinase messenger RNA was evaluated in human hippocampus using in situ hybridization technique. The message showed an uneven distribution with high levels present in the granular cell layer of the dentate gyrus and CA3 region. The detection of specific transcripts was also observed in the lateral geniculate nucleus, the dentate polymorphic cell layer and the parahippocampal gyrus. The data suggest that, in the hippocampus, the expression of hexokinase is higher in neurons than in glial cells and that the rate of glucose metabolism may display considerable variations in the different subregions of this area.

Molecular bases of hexokinase deficiency

Hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1; HK) deficiency is a rare disease where the predominant clinical effect is nonspherocytic hemolytic anemia. We have previously shown that the only patient for which hexokinase deficiency has been so far investigated at molecular level is a double heterozygote carrying a T1667 --> C substitution on one HK type I allele and a 96 bp deletion (concerning nucleotides 577 to 672 in the HK cDNA sequence) in the other allele. To investigate whether these mutations found in the patient with the hexokinase variant referred to as 'HK-Melzo' could be associated with hexokinase deficiency, we have expressed in E. coli the wild-type human hexokinase type I and two different mutants carrying the T --> C nucleotide substitution at position 1667 and the nt 577-672 deletion, respectively. Wild-type human recombinant hexokinase is expressed in bacterial cells as a soluble catalytically active enzyme that, upon purification to homogeneity, exhibited the same kinetic properties of human placenta hexokinase type I. Both mutant hexokinases were also expressed as soluble recombinant proteins under the same conditions, but they showed an impaired catalytic activity with respect to the wild-type enzyme. In particular, the T1667 --> C substitution, causing the amino acid change from Leu529 to Ser, is responsible for the complete loss of the hexokinase catalytic activity, while the 96 bp deletion causes a drastic reduction of the hexokinase activity. Taken together, both mutations explain the hexokinase deficiency found in the patient with the 'HK-Melzo' variant.

Functional organization of mammalian hexokinases: characterization of the rat type III isozyme and its chimeric forms, constructed with the N- and C-terminal halves of the type I and type II isozymes

Previous studies have shown that catalytic function is associated with both halves of the Type II isozyme of mammalian hexokinase, while the Type I isozyme is functionally differentiated into a catalytic C-terminal half and regulatory N-terminal half. The Type III isozyme has now been shown to be similar to the Type I isozyme in its functional organization. Chimeras composed of the N-terminal half of Type III hexokinase and the C-terminal half of either Type I or Type II hexokinase have activities that can be attributed to the C-terminal half and are similar in activity to chimeras composed of the C-terminal half of Type III and the intrinsically inactive N-terminal domain of Type I or the inactivated (by site-directed mutation) N-terminal half of Type II hexokinase. Virtually no activity was seen with chimeras constructed with the N-terminal half of the Type III isozyme and catalytically inactive (by site-directed mutation) C-terminal halves of Type I or Type II hexokinase. Substrate inhibition by Glc is seen only with the Type III isozyme and with chimeric forms containing the C-terminal half of Type III hexokinase and the N-terminal half of Type I or Type II isozyme, the latter inactivated by site-directed mutation; this is attributed to conformational changes induced by binding of Glc to a low affinity site in the N-terminal half, with subsequent effect on catalytic activity of the C-terminal half. These results also provide further insight into the role of interactions (or lack of interactions) between the N- and C-terminal halves in the inhibition of the Type I-III isozymes by Glc-6-P, its antagonism by low concentrations of Pi, and the inhibition seen at higher concentrations of Pi.

Crystallization and preliminary X-ray analysis of human brain hexokinase

Human brain hexokinase type I, expressed in Escherichia coli, has been crystallized from polyethylene glycol 8000 in the presence of inorganic phosphate. The crystals are hexagonal needles of diameter 0.25 mm, diffracting to a resolution of 3.5 A on a rotating-anode/area-detector system. The crystals belong to the space group P3(1)21/P3(2)21 with cell dimensions a = b = 171.5 A and c = 99.4 A. The specific volume of the crystal is 4.2 A3/Da, suggesting an asymmetric unit with a single 100 kDa molecule and a solvent content of 71% by volume or two molecules of hexokinase with a solvent content of 41%. The complex of hexokinase with glucose crystallizes under similar conditions, giving crystals of the same morphology.

Alteration of enzyme function of the type II hexokinase C-terminal half on replacements of restricted regions by corresponding regions of glucokinase

To know the structural properties responsible for the enzymic activity of the 50-kDa C-terminal half of type II hexokinase (HKII-C) derived from rat hepatoma cell line AH130, we constructed cDNAs of HKII-C and its recombinants in which restricted regions containing highly conserved sequences, referred to as regions 2 and 3, were replaced by the corresponding regions of glucokinase. The binding domains of ATP and glucose were proposed to exist in these regions, respectively. Then, the HKII-C and chimera HKII-Cs were overexpressed in Escherichia coli BL21(DE3)pLysS. They all exhibited hexokinase activity, and their activities were inhibited by glucose-6-phosphate (Glc-6-P) competitively for ATP and uncompetitively for glucose. The replacement of region 2 of HKII-C by the corresponding region of glucokinase increased the affinity for glucose and decreased the affinity for Glc-6-P, but it did not significantly affect the affinity for ATP. In contrast, the replacement of region 3 did not cause an appreciable change in hexokinase activity. These findings suggest that region 2 is associated with the binding of ATP and Glc-6-P, and that the latter binding site is located close to the ATP binding site. In addition, region 2 was suggested to be directly related with the binding of glucose and other hexoses.

Purification and characterization of the carboxyl-domain of human hexokinase type III expressed as fusion protein

In mammalian tissues hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) exists as four isoenzymes encoded by distinct genes. These proteins are homologous and are organized in two homologous domains, with the exception of hexokinase type IV which has only one. This organization is believed to be the result of a duplication and tandem fusion event involving the gene encoding for the ancestral hexokinase. In this study, we cloned the carboxyl-domain of human hexokinase type III and expressed it in Escherichia coli as a glutathione S-transferase fusion protein, using the pGEX-2T expression vector. The recombinant protein showed catalytic activity. A comparative study of the kinetic properties of the expressed carboxyl-domain and the enzyme partially purified from human lymphocytes is also shown. The results now allow a better understanding of the role of the carboxyl-domain in determining the catalytic properties of the enzyme.

Verification of the lactase site of rat lactase-phlorizin hydrolase by site-directed mutagenesis

BACKGROUND & AIMS

Lactase-phlorizin hydrolase (LPH) is an intestinal microvillus membrane glycoprotein that hydrolyzes lactose and phlorizin. These enzymatic activities have been assigned to glutamic acid (E) residues 1271 and 1747 in rabbit LPH. The aim of this study was to determine directly if this assignment was correct and if these two amino acids are the only nucleophiles required for LPH enzyme activity.

METHODS

Site-directed mutagenesis of a full-length rat LPH complementary DNA was used to convert the rat homologues E1274 and E1750 to aspartic acid or glycine. Mutants were analyzed by enzyme activity assays.

RESULTS

All tested activities of E1274D and E1274G were virtually unaffected. In contrast, mutations E1750D and E1750G resulted in total loss of lactase and cellobiose activities, leaving only low ONP-glc and ONP-gal hydrolase activities detectable. A double mutant containing both E1274G and E1750G had no activity.

CONCLUSIONS

These studies directly confirm that the two previously identified glutamic acids are essential to the enzymatic activity of rat LPH. Rat lactase activity is not associated with the E1274 site. This study provides the first evidence that rat LPH has its major catalytic site at E1750, representing all of the lactase and the majority of the phlorizin hydrolase activity.

Active site residues of human brain hexokinase as studied by site-specific mutagenesis

The truncated gene of hexokinase, mini-hexokinase, starting with methionine 455 and ending at the C terminus was expressed in Escherichia coli. Mini-hexokinase lost its ability to ameliorate inhibition of glucose-6-P-inhibited mini-hexokinase in the presence of phosphate (P(i)). We suggest that the P(i) site either resides in the N-terminal half of hexokinase I or requires the N-terminal portion of the enzyme. Site-directed mutagenesis was performed to obtain two mutants of mini-hexokinase: C606S and C628S. Both are thought to be associated with the active site of hexokinase I. These mutants exhibited a 3-fold increase in Km for glucose but no change in either the Km for ATP or the kcat. The circular dichroism (CD) spectra showed no differences among the wild-type or mutant enzymes. These results suggest that Cys606 and Cys628 are not involved in glucose binding directly. The putative ATP-binding site of full-length human brain hexokinase may involve Arg539 and Gly679, and these residues were mutated to Ile. For the mutant R539I, the kcat value decreased 114-fold relative to wild-type hexokinase, whereas the Km values for ATP and glucose changed only slightly. No change was observed in the Ki value for 1,5-anhydroglucitol 6-phosphate. CD spectra showed only a slight change in secondary structure. For the mutant G679I, overexpressed hexokinase is insoluble. We suggest that Arg539 is important for catalysis because it stabilizes the transition state product ADP-hexokinase. Gly679 is probably important for proper folding of the protein.

Human beta-cell glucokinase. Dual role of Ser-151 in catalysis and hexose affinity

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.

Localization and age-dependent expression of hexokinase mRNA in the rat brain

Localization of mRNA encoding for the enzyme hexokinase and its regulation in aged animals was carried out in rat brain using the in situ hybridization technique. The highest levels of the hybridization signal were observed in the olfactory bulb, piriform cortex, tenia tecta, hippocampus and granular cells of the cerebellum. Other brain areas and nuclei including cerebral cortex, thalamus, hypothalamus, substantia nigra, subiculum, choroid plexus and superior colliculus displayed moderate to low density of transcripts. Correlation between relative hexokinase content and levels of its mRNA was found only for some brain regions such as caudate-putamen, geniculate nucleus, ventral and lateral thalamic nuclei, superior colliculus and granular cells of the cerebellum. In the cerebral cortex and hippocampus of old animals the expression of hexokinase was significantly increased at 18 and 24 months of age. From the present data we conclude that although hexokinase is an ubiquitous enzyme, sites of synthesis display a discrete and uneven localization in rat CNS and expression, in the aging brain, might be regulated to compensate for reduced oxidative phosphorylation in the brain tissue.

Mammalian glucokinase and its gene

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Intracellular distribution of hexokinase in rabbit brain

Hexokinase in mammalian brain is particulate and usually considered to be bound to the outer mitochondrial membrane. Investigation of rabbit brain mitochondria prepared either by differential centrifugation and discontinuous density gradient centrifugation has provided evidence that this particulate fraction also contains endoplasmic vesicles and synaptosomes. Solubilization of the bound hexokinase by different combinations of detergents and metabolites has proved the existence of different hexokinase binding sites. Electron microscopic examination of hexokinase location by immuno-gold labelling techniques confirmed that hexokinase is indeed predominantly bound to mitochondria but that a significant proportion is also bound to non-mitochondrial membranes. Attempts to quantify this distribution were unsuccessful since different figures were obtained using anti-hexokinase IgG affinity purified on immobilized native or denatured hexokinase. Binding studies of the purified rabbit brain mitochondrial hexokinase to rabbit liver mitochondria and microsomes confirmed that in addition to a binding site on mitochondria there is another binding site on microsomes. The N-terminal sequence of hexokinase has been shown to be important for mitochondria binding and also for microsome binding. These results suggest that the intracellular localization of hexokinase in rabbit brain is not exclusively mitochondrial and that the metabolic role of this enzyme should be reconsidered by including a binding site on the endoplasmic reticulum.