The Amino Acid Sequence of Rat Liver Glucokinase Deduced from Cloned cDNA

Glucokinase (GCK) in diabetes: from molecular mechanisms to disease pathogenesis

Glucokinase (GCK), a key enzyme in glucose metabolism, plays a central role in glucose sensing and insulin secretion in pancreatic β-cells, as well as glycogen synthesis in the liver. Mutations in the GCK gene have been associated with various monogenic diabetes (MD) disorders, including permanent neonatal diabetes mellitus (PNDM) and maturity-onset diabetes of the young (MODY), highlighting its importance in maintaining glucose homeostasis. Additionally, GCK gain-of-function mutations lead to a rare congenital form of hyperinsulinism known as hyperinsulinemic hypoglycemia (HH), characterized by increased enzymatic activity and increased glucose sensitivity in pancreatic β-cells. This review offers a comprehensive exploration of the critical role played by the GCK gene in diabetes development, shedding light on its expression patterns, regulatory mechanisms, and diverse forms of associated monogenic disorders. Structural and mechanistic insights into GCK's involvement in glucose metabolism are discussed, emphasizing its significance in insulin secretion and glycogen synthesis. Animal models have provided valuable insights into the physiological consequences of GCK mutations, although challenges remain in accurately recapitulating human disease phenotypes. In addition, the potential of human pluripotent stem cell (hPSC) technology in overcoming current model limitations is discussed, offering a promising avenue for studying GCK-related diseases at the molecular level. Ultimately, a deeper understanding of GCK's multifaceted role in glucose metabolism and its dysregulation in disease states holds implications for developing targeted therapeutic interventions for diabetes and related disorders.

Molecular cloning and tissue distribution of glucokinase and glucose-6-phosphatase catalytic subunit paralogs in largemouth bass Micropterus salmoides: Regulation by dietary starch levels and a glucose load

The dysregulation of glucose-G6P (glucose-6-phosphate) interconversion is thought to be one of the main reasons for the low glucose disposal of carnivorous fish, but is not yet well understood in largemouth bass Micropterus salmoides (LMB). In this study, the full length cDNA sequences of genes encoding glucokinase (Gck, catalyzing glucose phosphorylation) and glucose-6-phosphatase catalytic subunit (G6pc, catalyzing glucose dephosphorylation) were cloned by the RACE method from the liver of LMB. Subsequently, the distribution of g6pc and gck as well as their transcriptional regulation by dietary starch levels and a glucose load were investigated. Only one gck gene was identified, while the tandem duplication of g6pca.1 gene was named as g6pca.2 in LMB. The full cDNA sequences of g6pca.1, g6pca.2 and gck in LMB were 1585, 1813 and 2115 bp in length, encoding 478, 352 and 359 amino acids, respectively. Gck was predicted to contain two hexokinase domains, an ATP-binding domain and multiple functional sites, while G6pca.1 and G6pca.2 contained nine transmembrane helices, a PAP2 (type-2 phosphatidic acid phosphatase) domain and multiple functional amino acid sites. Both g6pca.1 and g6pca.2 were predominantly distributed in the liver and to some extent in the intraperitoneal fat, intestine and pyloric caeca, while gck was mainly transcribed in the liver and to some extent in the heart, intestine and brain. Both feeding a high starch diet and a glucose load stimulated the mRNA expression of gck in the liver of LMB. An increase of dietary starch from 9% to 14% down-regulated the transcription of g6pca.1 in the liver of LMB. However, both the mRNA levels of hepatic g6pca.1 and g6pca.2 were sharply up-regulated in LMB during 1-3 h after a glucose load. Overall, the results of this study suggested that the functions of G6pc (G6pca.1 and G6pca.2) and Gck in LMB were highly conserved in evolution. Though hepatic glucose-G6P interconversion was well regulated at the transcript level in LMB fed high starch diets, a futile cycle between glucose and G6P was induced in the liver after a glucose load.

Evolution of glucose utilization: glucokinase and glucokinase regulator protein

Glucose is an essential nutrient that must be distributed throughout the body to provide energy to sustain physiological functions. Glucose is delivered to distant tissues via be blood stream, and complex systems have evolved to maintain the levels of glucose within a narrow physiological range. Phosphorylation of glucose, by glucokinase, is an essential component of glucose homeostasis, both from the regulatory and metabolic point-of-view. Here we review the evolution of glucose utilization from the perspective of glucokinase. We discuss the origin of glucokinase, its evolution within the hexokinase gene family, and the evolution of its interacting regulatory partner, glucokinase regulatory protein (GCKR). Evolution of the structure and sequence of both glucokinase and GCKR have been necessary to optimize glucokinase in its role in glucose metabolism.

Glucokinase and glucokinase regulatory proteins are functionally coexpressed before birth in the rat brain

Our previous description of functional glucokinase (GK) isoforms and their interactions with glucokinase regulatory protein (GKRP) in adult rat and human brains suggested that both participate in glucose sensing in the central nervous system. To determine whether both proteins are coexpressed and active before birth or during early post-natal life, we characterised these molecules in the brains of foetal and post-natal pup rats. We found GK and GKRP mRNAs that were similar to those previously reported in the adult rat brain. Likewise, GK and GKRP gene expression gave rise to proteins of 52 and 69 kDa, respectively. Immunohistochemistry experiments showed the colocalisation of both GK and GKRP proteins in the same brain cells of 21-day-old rat foetuses. Furthermore, coprecipitation of GK and GKRP in the presence of fructose 6-phosphate suggests interactions between both proteins. The presence of GK phosphorylating activity was detected in different brain areas of 21-day-old foetuses with a contribution to the total glucose-phosphorylating activity of between 17.2 +/- 1.7% and 12.4 +/- 3.7%, with the hypothalamus being the region of maximum activity. The hypothalamic GK activity in 21-day-old foetuses has a high apparent K(m) for glucose and no product inhibition by glucose 6-phosphate. Our findings indicate that both proteins may be functionally active before birth and that they can act within a glucose sensor system involved in controlling food intake.

Glucokinase and molecular aspects of liver glycogen metabolism

Conversion of glucose into glycogen is a major pathway that contributes to the removal of glucose from the portal vein by the liver in the postprandial state. It is regulated in part by the increase in blood-glucose concentration in the portal vein, which activates glucokinase, the first enzyme in the pathway, causing an increase in the concentration of glucose 6-P (glucose 6-phosphate), which modulates the phosphorylation state of downstream enzymes by acting synergistically with other allosteric effectors. Glucokinase is regulated by a hierarchy of transcriptional and post-transcriptional mechanisms that are only partially understood. In the fasted state, glucokinase is in part sequestered in the nucleus in an inactive state, complexed to a specific regulatory protein, GKRP (glucokinase regulatory protein). This reserve pool is rapidly mobilized to the cytoplasm in the postprandial state in response to an elevated concentration of glucose. The translocation of glucokinase between the nucleus and cytoplasm is modulated by various metabolic and hormonal conditions. The elevated glucose 6-P concentration, consequent to glucokinase activation, has a synergistic effect with glucose in promoting dephosphorylation (inactivation) of glycogen phosphorylase and inducing dephosphorylation (activation) of glycogen synthase. The latter involves both a direct ligand-induced conformational change and depletion of the phosphorylated form of glycogen phosphorylase, which is a potent allosteric inhibitor of glycogen synthase phosphatase activity associated with the glycogen-targeting protein, GL [hepatic glycogen-targeting subunit of PP-1 (protein phosphatase-1) encoded by PPP1R3B]. Defects in both the activation of glucokinase and in the dephosphorylation of glycogen phosphorylase are potential contributing factors to the dysregulation of hepatic glucose metabolism in Type 2 diabetes.

Co-transfection of GK and mhPINS genes into HepG2 cells confers glucose-stimulated insulin secretion

BACKGROUND

The purpose of this study was to construct an 'artificial beta cell' that can exhibit physiologic glucose-stimulated insulin secretion for the treatment of type 1 diabetes.

METHODS

Retroviral vector containing the glucokinase (GK) gene and mutated human proinsulin (mhPINS) gene was constructed. HepG2 cells were first infected with recombinant retrovirus carrying the GK and mhPINS genes, then selectively cultured with G418 to obtain the positive clones. GK and mhPINS gene transcription and expression were identified by radioimmunity, Western blot and RT-PCR techniques. Finally, the dose-response effect of glucose on insulin secretion from those HepG2 cells that expressed both GK and mhPINS genes was tested with HepG2 cells that only expressed the mhPINS gene as a control.

RESULTS

HepG2 cells with transferred GK and mhPINS genes were selectively cultured with G418 and the positive clones were obtained in 3 weeks. Four clones with GK and mhPINS gene expression were selected from 20 positive clones by radioimmunity and Western blot. We picked up one clone with a strong GK and mhPINS gene expression and named it clone Beta. In clone Beta, differences in insulin secretion at 0.5 and 0.75 mmol/L glucose concentrations were not significant (P>0.05) and differences in insulin secretion at 2.0, 3.0, 4.0, 5.0 and 6.0 mmol/L glucose concentrations were not significant (P>0.05), while there were significant differences in insulin secretion at other glucose concentrations(P<0.05). The artificial beta cell, clone Beta, obtained a glucose-stimulated insulin secretion with maximal insulin secretion at 1.75-2.00 mmol/L glucose concentrations.

DISCUSSION

An artificial beta cell that exhibits glucose-stimulated insulin secretion can be constructed successfully.

The promoter for the gene encoding the catalytic subunit of rat glucose-6-phosphatase contains two distinct glucose-responsive regions

Glucose homeostasis requires the proper expression and regulation of the catalytic subunit of glucose-6-phosphatase (G-6-Pase), which hydrolyzes glucose 6-phosphate to glucose in glucose-producing tissues. Glucose induces the expression of G-6-Pase at the transcriptional and posttranscriptional levels by unknown mechanisms. To better understand this metabolic regulation, we mapped the cis-regulatory elements conferring glucose responsiveness to the rat G-6-Pase gene promoter in glucose-responsive cell lines. The full-length (-4078/+64) promoter conferred a moderate glucose response to a reporter construct in HL1C rat hepatoma cells, which was dependent on coexpression of glucokinase. The same construct provided a robust glucose response in 832/13 INS-1 rat insulinoma cells, which are not glucogenic. Glucose also strongly increased endogenous G-6-Pase mRNA levels in 832/13 cells and in rat pancreatic islets, although the induced levels from islets were still markedly lower than in untreated primary hepatocytes. A distal promoter region was glucose responsive in 832/13 cells and contained a carbohydrate response element with two E-boxes separated by five base pairs. Carbohydrate response element-binding protein bound this region in a glucose-dependent manner in situ. A second, proximal promoter region was glucose responsive in both 832/13 and HL1C cells, with a hepatocyte nuclear factor 1 binding site and two cAMP response elements required for glucose responsiveness. Expression of dominant-negative versions of both cAMP response element-binding protein and CAAT/enhancer-binding protein blocked the glucose response of the proximal region in a dose-dependent manner. We conclude that multiple, distinct cis-regulatory promoter elements are involved in the glucose response of the rat G-6-Pase gene.

Overexpression of regucalcin enhances glucose utilization and lipid production in cloned rat hepatoma H4-II-E cells: Involvement of insulin resistance

The role of regucalcin, which is a regulatory protein in intracellular signaling pathway, in the regulation of glucose utilization and lipid production was investigated using the cloned rat hepatoma H4-II-E cells overexpressing regucalcin. The hepatoma cells (wild-type) and stable regucalcin/pCXN2-transfected cells (transfectant) were cultured for 72 h in a medium containing 10% fetal bovine serum (FBS) to obtain subconfluent monolayers. Cells with subconfluency were cultured for 24 or 72 h in medium containing either vehicle or insulin (10(-8) or 10(-7) M) with or without supplementation of glucose (10, 25, or 50 mg/ml of medium) in the absence of insulin. The production of triglyceride and free fatty acid was significantly increased in transfectants cultured without insulin and glucose supplementation as compared with that of wild-type cells. The supplementation of glucose (10, 25, or 50 mg/ml) caused a remarkable increase in medium glucose consumption, triglyceride, and free fatty acid productions in transfectants cultured without insulin. The presence of insulin (10(-7) M) caused a significant increase in medium glucose consumption, triglyceride, and free fatty acid productions in wild-type cells cultured with glucose supplementation. These increases were significantly prevented in transfectants cultured for 72 h. The expression of acetyl-CoA carboxylase, HMG-CoA reductase, glucokinase, pyruvate kinase, and glyceroaldehyde-3-phosphate dehydrogenase (G3PDH) mRNAs in wild-type cells was not significantly changed by culture with or without glucose supplementation in the presence of insulin. These gene expressions were not significantly changed in transfectants. The expression of glucose transporter 2 mRNA was significantly increased in transfectants as compared with that of wild-type cells. Such an increase was not seen in transfectants cultured in the presence of insulin with or without glucose supplementation. This study demonstrates that overexpression of regucalcin enhances glucose utilization and lipid production in the cloned rat hepatoma H4-II-E cells, and that it regulates the effect of insulin.

Kinetic and proteomic analyses of S-nitrosoglutathione-treated hexokinase A: consequences for cancer energy metabolism

Mammalian hexokinase (HXK) is found at the outer mitochondrial membrane, exposed to mitochondrial oxygen- and nitrogen-radicals. Given the important role of this enzyme in metabolic pathways and diseases, the effect of S-nitrosoglutathione (GSNO) on HXK A structure and activity was studied. To focus on the catalytic domain, yeast HXK A was used because it has a significant homology to the mammalian domain that contains both the regulatory and catalytic sites. Biologically relevant [GSNO]/[HXK] caused a significant decrease in V(max) with glucose (but not with fructose), along with oxidation of 5 Met and nitration of 4 Tyr. Preincubation of HXK with glucose abrogated the effect of GSNO whereas fructose was ineffective. These results are interpreted by considering the tight binding of glucose to the enzyme as opposed to that of fructose. The segment comprised from amino acids 304 to 306 contained the most modifications. Given that this sequence is highly conserved in HXK from various species, a decline in activity is expected when a high-affinity substrate is presented. Considering that changes in primary structure are envisioned at high [GSNO]/[HXK] ratios, like those present under normal conditions, it could be hypothesized that the high concentration of hexokinase present in fast growing tumors may serve not only to sustain high glycolysis rates, but also to minimize protein damage that might result in activity decline, compromising energy metabolism.

Comparison of expression of glucokinase gene and activities of enzymes related to glucose metabolism in livers between dog and cat

Plasma glucose and immunoreactive insulin (IRI) concentrations and activities of enzymes related to glucose metabolism in livers were measured in dogs and cats. Nucleotide sequences of the conserved region of glucokinase (GK) cDNA that contained ATP- and glucose-binding domains were determined in canine liver and feline pancreas for design of the species-specific oligonucleotide primers for reverse transcription-polymerase chain reaction (RT-PCR) analysis. There were no significant differences in plasma glucose and IRI concentrations between dogs and cats. In feline liver, although GK activities were not detected, activities of hexokinase, fructokinase, pyruvate kinase, glucose-6-phosphate dehydrogenase, fructose-1,6-bisphosphatase and glucose-6-phosphatase were significantly higher than those in canine liver. The partial sequences of canine liver GK and feline pancreas GK cDNA were respectively 88% and 89% identical with the rat liver GK cDNA. Expression of GK gene was observed in canine liver and pancreas and feline pancreas with RT-PCR using species specific primers based on the cDNA sequences.

Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination

Chickens are more hyperglycaemic and insulin-resistant than mammals, and in efforts to understand their glucose metabolism we investigated whether glucokinase (GK) is present in chicken liver or pancreas. This enzyme plays a major role in glucose-sensing in mammals and we have examined whether it also contributes to glucose homeostasis in chickens. Using RT-PCR, we cloned and sequenced a partial cDNA fragment (750 bp) from liver and pancreas that showed a high degree of identity with mammalian GK. Using antibodies directed towards human GK, we immunodetected a 50 kDa band in chicken liver and pancreas. The molecular mass of the band and its specific interaction with the antibody suggest that this protein corresponds to a chicken homologue of human GK. We also determined by spectrophotometry a glucokinase-like activity in crude liver homogenates with an apparent half saturating concentration for glucose of 8.6 mM. GK gene and protein expression did not differ between fed and 24 h fasted states but GK-like activity was significantly increased in fed chickens. In conclusion, our study provides evidence for the presence of GK gene and protein in chicken liver and pancreas and shows that the liver enzyme is active.

Identification and characterization of the ATP-binding site in human pancreatic glucokinase

The central role of human pancreatic glucokinase in insulin secretion and, consequently, in maintenance of blood glucose levels has prompted investigation into identification of ATP-binding site residues and examination of ATP- and glucose-binding interactions. Because glucokinase has been resistant to crystallization, computer generated homology models were developed based on the X-ray crystal structure of the COOH-terminal domain of human brain hexokinase 1 bound to glucose and ADP or glucose and glucose-6-phosphate. Human pancreatic glucokinase mutants were designed based upon these models and on ATPase domain sequence conservation to identify and characterize potential glucose and ATP-binding sites. Specifically, mutants Asp78Ala, Thr82Ala, Lys90Ala, Lys102Ala, Gly227Ala, Thr228Ala, Ser336Leu, Ser411Ala, and Ser411Leu were constructed, expressed, purified, and kinetically characterized under steady-state conditions. Compared to their respective wild type controls, several mutants demonstrated dramatic changes in V(max), cooperativity of glucose binding and S(0.5) for ATP and glucose. Results suggest a role for Asp78, Thr82, Gly227, Thr228, and Ser336 in ATP binding and indicate these residues are essential for glucose phosphorylation by human pancreatic glucokinase.

The overexpressed hexahistidine-tagged human hexokinase type III is inhibited by D-glucose

Inhibition by its product, glucose, is a kinetic property of hexokinase type III. In this paper, we report the overexpression in Escherichia coli of human hexokinase type III. The recombinant enzyme was genetically fused with a hexahistidine peptide at the C-terminal end. This modification confers to the product the ability to bind the Ni2+ ion immobilised into agarose by nitrilotriacetic acid (NTA) groups. The purification was performed by one-step column chromatography using ammonium sulphate as stabilising agent. Recombinant hexokinase type III appears as a single band of approximately 100 kDa on a SDS-PAGE gel and shows specific activity of 16 U/mg. Its kinetic parameters are comparable to those of the native enzyme, including the fact that it can be inhibited by glucose. The comparison of these results with the properties of the overexpressed carboxyl-domain led us to suppose that the inhibition site for glucose required the presence of the N-terminal domain.

Beta-glucoside kinase (BglK) from Klebsiella pneumoniae. Purification, properties, and preparative synthesis of 6-phospho-beta-D-glucosides

ATP-dependent beta-glucoside kinase (BglK) has been purified from cellobiose-grown cells of Klebsiella pneumoniae. In solution, the enzyme (EC ) exists as a homotetramer composed of non-covalently linked subunits of M(r) approximately 33,000. Determination of the first 28 residues from the N terminus of the protein allowed the identification and cloning of bglK from genomic DNA of K. pneumoniae. The open reading frame (ORF) of bglK encodes a 297-residue polypeptide of calculated M(r) 32,697. A motif of 7 amino acids (AFD(7)IG(9)GT) near the N terminus may comprise the ATP-binding site, and residue changes D7G and G9A yielded catalytically inactive proteins. BglK was progressively inactivated (t(12) approximately 19 min) by N-ethylmaleimide, but ATP afforded considerable protection against the inhibitor. By the presence of a centrally located signature sequence, BglK can be assigned to the ROK (Repressor, ORF, Kinase) family of proteins. Preparation of (His6)BglK by nickel-nitrilotriacetic acid-agarose chromatography provided high purity enzyme in quantity sufficient for the preparative synthesis (200-500 mg) of ten 6-phospho-beta-d-glucosides, including cellobiose-6'-P, gentiobiose-6'-P, cellobiitol-6-P, salicin-6-P, and arbutin-6-P. These (and other) derivatives are substrates for phospho-beta-glucosidase(s) belonging to Families 1 and 4 of the glycosylhydrolase superfamily. The structures, physicochemical properties, and phosphorylation site(s) of the 6-phospho-beta-d-glucosides have been determined by fast atom bombardment-negative ion spectrometry, thin-layer chromatography, and (1)H and (13)C NMR spectroscopy. The recently sequenced genomes of two Listeria species, L. monocytogenes EGD-e and L. innocua CLIP 11262, contain homologous genes (lmo2764 and lin2907, respectively) that encode a 294-residue polypeptide (M(r) approximately 32,200) that exhibits approximately 58% amino acid identity with BglK. The protein encoded by the two genes exhibits beta-glucoside kinase activity and cross-reacts with polyclonal antibody to (His6)BglK from K. pneumoniae. The location of lmo2764 and lin2907 within a beta-glucoside (cellobiose):phosphotransferase system operon, may presage both enzymatic (kinase) and regulatory functions for the BglK homolog in Listeria species.

Mitochondrial bound type II hexokinase: a key player in the growth and survival of many cancers and an ideal prospect for therapeutic intervention

Despite more than 75 years of research by some of the greatest scientists in the world to conquer cancer, the clear winner is still cancer. This is reflected particularly by liver cancer that worldwide ranks fourth in terms of mortality with survival rates of no more than 3-5%. Significantly, one of the earliest discovered hallmarks of cancer had its roots in Bioenergetics as many tumors were found in the 1920s to exhibit a high glycolytic phenotype. Although research directed at unraveling the underlying basis and significance of this phenotype comprised the focus of cancer research for almost 50 years, these efforts declined greatly from 1970 to 1990 as research into the molecular and cell biology of this disease gained center stage. Certainly, this change was necessary as the new knowledge obtained about oncogenes, gene regulation, and programmed cell death once again placed Bioenergetics in the limelight of cancer research. Thus, we now have a much better molecular understanding of the high glycolytic phenotype of many cancers, the pivotal roles that Type II hexokinase-mitochondrial interactions play in this process to promote tumor cell growth and survival, and how this new knowledge can lead to improved therapies that may ultimately turn the tide on our losing war on cancer.

Expression of glucose transporter-2, glucokinase and mitochondrial glycerolphosphate dehydrogenase in pancreatic islets during rat ontogenesis

To gain better insight into the insulin secretory activity of fetal beta cells in response to glucose, the expression of glucose transporter 2 (GLUT-2), glucokinase and mitochondrial glycerol phosphate dehydrogenase (mGDH) were studied. Expression of GLUT-2 mRNA and protein in pancreatic islets and liver was significantly lower in fetal and suckling rats than in adult rats. The glucokinase content of fetal islets was significantly higher than of suckling and adult rats, and in liver the enzyme appeared for the first time on about day 20 of extrauterine life. The highest content of hexokinase I was found in fetal islets, after which it decreased progressively to the adult values. Glucokinase mRNA was abundantly expressed in the islets of all the experimental groups, whereas in liver it was only present in adults and 20-day-old suckling rats. In fetal islets, GLUT-2 and glucokinase protein and their mRNA increased as a function of increasing glucose concentration, whereas reduced mitochondrial citrate synthase, succinate dehydrogenase and cytochrome c oxidase activities and mGDH expression were observed. These findings, together with those reported by others, may help to explain the decreased insulin secretory activity of fetal beta cells in response to glucose.

One-step purification of a fully active hexahistidine-tagged human hexokinase type I overexpressed in Escherichia coli

The conversion of glucose into glucose 6-phosphate (Glc 6-P)1 traps glucose in a chemical state in which it cannot leave the cell and hence commits glucose to metabolism. In human tissues there are at least three hexokinase isoenzymes responsible for hexose phosphorylation. These enzymes are constituted by a single polypeptide chain with a molecular weight of approximately 100 kDa. Among these isoenzymes, hexokinase type I is the most widely expressed in mammalian tissues and shows reversion of Glc 6-P inhibition by physiological levels of inorganic phosphate. In this work the hexokinase I from human brain was overexpressed in Escherichia coli, as a hexahistidine-tagged protein with the tag extending the C-terminal end. An average of 900 U per liter of culture was obtained. The expressed protein was one-step purified by metal chelate affinity chromatography performed in NTA-agarose column charged with Ni(2+) ions. In order to stabilize the enzymatic activity 0.5 M ammonium sulfate was added to elution buffer. The specific activity of purified hexokinase I was 67.8 U/mg. The recombinant enzyme shows kinetic properties in agreement with those described for the native enzyme, and thus it can be used for biophysical and biochemical investigation.

Chronic intraperitoneal insulin delivery, as compared with subcutaneous delivery, improves hepatic glucose metabolism in streptozotocin diabetic rats

We have previously shown that chronic insulin treatment by the intraperitoneal route normalizes the elevated glucose production (GP) in streptozotocin (STZ) diabetic rats, while insulin delivered by the subcutaneous route only partially normalizes GP. To investigate the biochemical mechanism of the effect of chronic insulin delivery by either route on hepatic glucose metabolism, we measured the hepatic activity of glucose 6-phosphatase (G6Pase) and glucokinase (GK). Four groups of rats were used: (1) nondiabetic rats (N, n = 7), (2) untreated STZ diabetic rats (D, n = 8), (3) diabetic rats treated intraperitoneally (IP, n = 6), or (4) subcutaneously (SC, n = 8) (both 3 U of insulin/d). Glucose levels, higher in D, were normalized by insulin treatment regardless of route. Peripheral insulin levels were lowest in D and highest in SC as expected (N, 162 +/- 18 pmol/L; D, 66 +/- 12; IP, 360 +/- 96; SC, 798 +/- 198). STZ diabetes resulted in a 10-fold decrease in GK (P < .001), and a 2-fold increase in G6Pase activity (P < .01). Both intraperitoneal and subcutaneous treatments normalized G6Pase activity. In contrast, with subcutaneous but not intraperitoneal treatment, GK activity was still 35% less than normal (SC v N, P < .05). Glucose 6-phosphate (G6P) levels did not differ among the groups. In summary: (1) the increase in GP in D reflected increased activity of G6Pase and reduced activity of GK, (2) the partial suppression of GP with subcutaneous insulin treatment reflected correction of increased G6Pase activity, but only partial correction of low GK activity, and (3) the normalization of GP with intraperitoneal insulin treatment reflected correction of both increased G6Pase activity and low GK activity. Our current studies indicate that chronic intraperitoneal insulin treatment is superior to subcutaneous treatment with regard to hepatic glucose metabolism.

Functional glucokinase isoforms are expressed in rat brain

Recently, the description of glucokinase mRNA in certain neuroendocrine cells has opened new ways to characterize this enzyme in the rat brain. In this study, we found glucokinase mRNA and a similar RNA splicing pattern of the glucokinase gene product in rat hypothalamus and pancreatic islets; the mRNA that codes for B1 isoform was the most abundant, with minor amounts of those coding for the B2, P1, P2, P1/B2, and P2/B2 isoforms. Glucokinase gene expression in rat brain gave rise to a protein of 52 kDa with a high apparent Km for glucose and no product inhibition by glucose 6-phosphate, with a contribution to the total glucose phosphorylating activity of between 40 and 14%; the hypothalamus and cerebral cortex were the regions of maximal activity. Low and high Km hexokinases were characterized by several criteria. Also, using RT-PCR analysis we found a glucokinase regulatory protein mRNA similar to that previously reported in liver. These findings indicate that the glucokinase present in rat brain should facilitate the adaptation of this organ to fluctuations in blood glucose concentrations, and the expression of glucokinase and GLUT-2 in the same hypothalamic neurons suggests a role in glucose sensing.

Nuclear import of hepatic glucokinase depends upon glucokinase regulatory protein, whereas export is due to a nuclear export signal sequence in glucokinase

Hepatic glucokinase (GK) moves between the nucleus and cytoplasm in response to metabolic alterations. Here, using heterologous cell systems, we have found that at least two different mechanisms are involved in the intracellular movement of GK. In the absence of the GK regulatory protein (GKRP) GK resides only in the cytoplasm. However, in the presence of GKRP, GK moves to the nucleus and resides there in association with this protein until changes in the metabolic milieu prompt its release. GK does not contain a nuclear localization signal sequence and does not enter the nucleus in a GKRP-independent manner because cells treated with leptomycin B, a specific inhibitor of leucine-rich NES-dependent nuclear export, do not accumulate GK in the nucleus. Instead, entry of GK into the nucleus appears to occur via a piggy-back mechanism that involves binding to GKRP. Nuclear export of GK, which occurs after its release from GKRP, is due to a leucine-rich nuclear export signal within the protein ((300)ELVRLVLLKLV(310)). Thus, GKRP appears to function as both a nuclear chaperone and metabolic sensor and is a critical component of a hepatic GK translocation cycle for regulating the activity of this enzyme in response to metabolic alterations.

Hexokinase 2 from Saccharomyces cerevisiae: regulation of oligomeric structure by in vivo phosphorylation at serine-14

Homodimeric hexokinase 2 from Saccharomyces cerevisiae is known to have two sites of phosphorylation: for serine-14 the modification in vivo increases with glucose exhaustion [Kriegel et al. (1994) Biochemistry 33, 148-152], while for serine-157 it occurs in vitro with ATP in the presence of nonphosphorylateable five-carbon analogues of glucose [Heidrich et al. (1997) Biochemistry 36, 1960-1964]. We show now by site-directed mutagenesis and sedimentation analysis that serine-14 phosphorylation affects the oligomeric state of hexokinase, its substitution by glutamate causing complete dissociation; glutamate exchange for serine-157 does not. Phosphorylation of wild-type hexokinase at serine-14 likewise causes dissociation in vitro. In view of the higher glucose affinity of monomeric hexokinase and the high hexokinase concentration in yeast [Womack, F., and Colowick, S. P. (1978) Arch. Biochem. Biophys. 191, 742-747; Mayes, E. L., Hoggett, J. G., and Kellett, G. L. (1983) Eur. J. Biochem. 133, 127-134], we speculate that the in vivo phosphorylation at serine-14 as transiently occurring in glucose derepression might provide a mechanism to improve glucose utilization from low level and/or that nuclear localization of the monomer might be involved in the signal transduction whereby glucose causes catabolite repression.

Comparison of glucokinase activities in the peripheral leukocytes between dogs and cats

Activities of hexokinase (HK), glucokinase (GK) and pyruvate kinase (PK), were measured. The expression of GK mRNA was investigated using reverse transcription-polymerase chain reaction (RT-PCR) in leukocytes (WBC) of dogs and cats. No significant differences between dogs and cats were found in concentrations of blood glucose and plasma insulin. Dog WBC showed GK activities and the specific fragment with predicted size of 574 bp containing conserved region including glucose- and ATP-binding domains of GK as determined with RT-PCR. However, in cat WBC, the activities and specific fragment of GK were absent. After fasting, the activities and gene expression of GK decreased greatly in the dog WBC. The cat WBC had significantly higher activities of HK and PK than dog WBC.

Quantitative determinations of the steady state transcript levels of hexokinase isozymes and glucose transporter isoforms in normal rat tissues and the malignant tumor cell line AH130

The steady state transcript levels of the four hexokinase (HK) isozymes and four glucose transporter (GLUT) isoforms were determined quantitatively by Northern analysis of RNA samples from rat tissues using synthetic fragments of the RNAs encoding the HK isozymes and GLUT isoforms. Results showed that the levels of HK isozyme transcripts were low in rat tissues, the level of that most highly expressed, the type I isozyme (HKI), in the brain being 0.025% of the total poly(A)+ RNA. A good correlation was found between the reported HK activities and the total amounts of transcripts encoding all HK isozymes in various tissues, showing that the HK activities in tissues can be estimated from the total amount of transcripts encoding HK isozymes. The proposed associated expressions of HK isozymes and GLUT isoforms in particular tissues were confirmed at their transcript levels. The steady state transcript levels of type II HK and the type 1 GLUT isoform in the malignant tumor cell line AH130 were also determined quantitatively.

Yeast hexokinase PII--bound nucleotide conformation at the active site

The structure of adenine nucleotide bound at the active site of yeast hexokinase PII (PII) was studied in the complexes PII x ADPMg(II), PII x ADPMg(II) x Glc and PII x ADPMg(II) x NO3- x Glc using two-dimensional transferred NOE spectroscopy. Binding of the nucleotide ligand to the enzyme resulted in downfield shift of several ligand resonances. Changes in the chemical shift as a function of ligand concentration indicate that various resonances in the bound and free form of the ligand appear to be in fast exchange. The largest chemical shift change between the bound and the free states (delta vM = 88 +/- 9 Hz) at an NMR frequency of 500 MHz was observed for the H2 resonance of the adenine ring. A lower limit for the rate of ligand dissociation from the complex derived from these results is k(off) >> 550 s(-1). Interproton NOEs for various ligand protons in PII x ADPMg(II), PII x ADPMg(II) x Glc and PII x ADPMg(II) x NO3- x Glc complexes were measured at 500 MHz at 10 degrees C. The NOE buildup curves constructed from such measurements made at various mixing times (40, 80, 120, 160 and 200 ms) were analyzed using complete relaxation matrix calculations and various interproton distances were determined. These distances were used in restrained molecular dynamics calculations to derive the conformation of the nucleotide bound at the active site of the enzyme. The results of these calculations indicate that the nucleotide binds in an anti conformation. The glycosidic torsion angle chi (O4'-C1'-N9-C8) determined for the nucleotide in PII x ADPMg(II), PII x ADPMg(II) x Glc and PII x ADPMg(II) x NO3- x Glc complexes are 68 +/- 4 degrees, 52 +/- 4 degrees and 49 +/- 4 degrees respectively. In all these complexes, the ribose pucker is best represented by the unsymmetrical O4'-endo-C1'-exo twist ((o)T1). The observed decrease in the glycosidic torsion angle of the bound nucleotide is attributed to the glucose-induced conformational changes in the enzyme. The structure of the nucleotide derived here is at variance from the one proposed on the basis of X-ray crystallography and model building [Shoham, M. & Steitz, T. A. (1980) J. Mol. Biol. 140, 1-14].

Glycogenin, the primer of glycogen synthesis, binds to actin

We have studied the intracellular localization of glycogenin by fusing green fluorescent protein (GFP) to the N-terminus of rabbit muscle glycogenin and expressing the chimeric protein in C2C12, COS-1 and rat hepatic cells. The fusion protein showed a nuclear and cytosolic distribution and partially co-localized with actin in the cytosol. Disruption of the actin cytoskeleton with cytochalasin D led to a change in the pattern of green fluorescence, which coincided with that observed for the remaining non-depolymerized actin. The distribution of the single point mutant K324A was completely uniform and was not affected by this drug. These findings indicate that rabbit muscle glycogenin binds to actin through the heptapeptide 321DNIKKKL327, a common motif found in other actin-binding proteins, which is located at the C-terminal end of this protein, and suggest that the actin cytoskeleton plays an important role in glycogen metabolism.

Regulation of glucose transporter GLUT-4 and hexokinase II gene transcription by insulin and epinephrine

Transport of glucose across the plasma membrane by GLUT-4 and subsequent phosphorylation of glucose by hexokinase II (HKII) constitute the first two steps of glucose utilization in skeletal muscle. This study was undertaken to determine whether epinephrine and/or insulin regulates in vivo GLUT-4 and HKII gene transcription in rat skeletal muscle. In the first experiment, adrenodemedullated male rats were fasted 24 h and killed in the control condition or after being infused for 1.5 h with epinephrine (30 microg/ml at 1.68 ml/h). In the second experiment, male rats were fasted 24 h and killed after being infused for 2.5 h at 1.68 ml/h with saline or glucose (625 mg/ml) or insulin (39.9 microg/ml) plus glucose (625 mg/ml). Nuclei were isolated from pooled quadriceps, tibialis anterior, and gastrocnemius muscles. Transcriptional run-on analysis indicated that epinephrine infusion decreased GLUT-4 and increased HKII transcription compared with fasted controls. Both glucose and insulin plus glucose infusion induced increases in GLUT-4 and HKII transcription of twofold and three- to fourfold, respectively, compared with saline-infused rats. In conclusion, epinephrine and insulin may regulate GLUT-4 and HKII genes at the level of transcription in rat skeletal muscle.

Transcriptional regulation of hexokinase II in denervated rat skeletal muscle

Hexokinase II protein is augmented in denervated skeletal muscle; therefore, we determined if hexokinase II gene transcription rates and mRNA levels are increased with denervation. The right hindlimb skeletal muscles of male rats were denervated while the left hindlimbs were sham operated. Seventy-two h following surgery, rats were sacrificed and the gastrocnemius and soleus muscles were harvested for nuclear and RNA isolation. Nuclear run-on and ribonuclease protection analyses indicated that denervation increased hexokinase II transcription rates and mRNA levels 42% and 88%, respectively (p < 0.05). Total hexokinase activity rose 23% in denervated gastrocnemius muscle. In conclusion, the increase in hexokinase II gene transcription and mRNA may account for the increase in hexokinase II protein and the subsequent rise in total hexokinase activity in denervated rat skeletal muscle.

Stimulation of glucose-6-phosphatase gene expression by glucose and fructose-2,6-bisphosphate

Glucose-6-phosphatase, a key enzyme in the homeostatic regulation of blood glucose concentration, catalyzes the terminal step in gluconeogenesis and glycogenolysis. Glucose, the product of the glucose-6-phosphatase reaction, dramatically increases the level of glucose-6-phosphatase mRNA transcripts in primary hepatocytes (20-fold), and the maximum response is obtained at a glucose concentration as low as 11 mM. Glucose specifically increases glucose-6-phosphatase mRNA and L-type pyruvate kinase mRNA. In the rat hepatoma-derived cell line, Fao, glucose increases the glucose-6-phosphatase mRNA only modestly (3-fold). In the presence of high glucose concentrations, overexpression of glucokinase in Fao cells via recombinant adenovirus vectors increases lactate production to the level found in primary hepatocytes and increases glucose-6-phosphatase gene expression by 21-fold. Similar overexpression of hexokinase I in Fao cells with high levels of glucose does not increase lactate production nor does it change the response of glucose-6-phosphatase mRNA to glucose. Glucokinase overexpression in Fao cells blunts the previously reported inhibitory effect of insulin on glucose-6-phosphatase gene expression in these cells. Raising the cellular concentration of fructose-2,6-bisphosphate, a potent effector of the direction of carbon flux through the gluconeogenic and glycolytic pathways, also stimulated glucose-6-phosphatase gene expression in Fao cells. Increasing the fructose-2,6-bisphosphate concentration over a 15-fold range (12 +/- 1 to 187 +/- 17 pmol/plate) via an adenoviral vector overexpression system, led to a 6-fold increase (0.32 +/- 0. 03 to 2.2 +/- 0.33 arbitrary units of mRNA) in glucose-6-phosphatase gene expression with a concomitant increase in glycolysis and a decrease in gluconeogenesis. Also, the effects of fructose-2, 6-bisphosphate concentrations on fructose-1,6-bisphosphatase gene expression were stimulatory, leading to a 5-6-fold increase in mRNA level over a 15-fold range in fructose-2,6-bisphosphate level. Liver pyruvate kinase and phosphoenolpyruvate carboxykinase mRNA were unchanged by the manipulation of fructose-2,6-bisphosphate level.

Co-localization of glucokinase with actin filaments

A portion of glucokinase appeared to be co-localized with actin filaments in the cytoplasm of cultured rat hepatocytes incubated with 25 mM glucose. When liver- or islet-type glucokinase was transiently expressed in COS-7 cells, the expressed glucokinase was also co-localized with actin filaments in the cytoplasm of these transfected cells. Although co-localization of glucokinase with actin filaments was not clearly demonstrated in the pancreatic beta-cell line MIN6, islet glucokinase was found to be present in both the nucleus and the cytoplasm, though predominantly in the nucleus. These findings suggest that subcellular localization of glucokinase, including co-localization with actin filaments, may have an important physiological role in metabolic regulation.

Autophosphorylation-inactivation site of hexokinase 2 in Saccharomyces cerevisiae

Hexokinase 2 from Saccharomyces cerevisiae is phosphorylated in vivo at serine-15 [Kriegel et al. (1994) Biochemistry 33, 148-152] and undergoes ATP-dependent autophosphorylation-inactivation in vitro when incubated in the presence of D-xylose [Fernandez et al. (1988) J. Gen. Microbiol. 134, 2493-2498]. This study identifies the site of inactivation by autophosphorylation as serine-158 by observation of a single tryptic peptide difference, peptide sequencing, and size determination by mass spectrometry. Mutation of serine-158 to alanine and cysteine, respectively, prevents autophosphorylation and causes a drastic decrease of the catalytic activity while mutational change to glutamate results in a complete loss of enzyme activity. The catalytically active mutant enzymes display an increased affinity for glucose and exhibit higher K(M) with respect to MgATP. Phosphoserine/phosphothreonine-specific protein phosphatase-2A completely reverses the autophosphorylative inactivation of the wild-type enzyme.

Molecular characterization of glucokinase from Escherichia coli K-12

glk, the structural gene for glucokinase of Escherichia coli, was cloned and sequenced. Overexpression of glk resulted in the synthesis of a cytoplasmic protein with a molecular weight of 35,000. The enzyme was purified, and its kinetic parameters were determined. Its Km values for glucose and ATP were 0.78 and 3.76 mM, respectively. Its Vmax was 158 U/mg of protein. A chromosomal glk-lacZ fusion was constructed and used to monitor glk expression. Under all conditions tested, only growth on glucose reduced the expression of glk by about 50%. A fruR mutation slightly increased the expression of glk-lacZ, whereas the overexpression of plasmid-encoded fruR+ weakly decreased expression. A FruR consensus binding motif was found 123 bp upstream of the potential transcriptional start site of glk. Overexpression of glk interfered with the expression of the maltose system. Repression was strongest in strains that exhibited constitutive mal gene expression due to endogenous induction and, in the absence of a functional MalK protein, the ATP-hydrolyzing subunit of the maltose transport system. It was least effective in wild-type strains growing on maltose or in strains constitutive for the maltose system due to a mutation in malT rendering the mal gene expression independent of inducer. This demonstrates that free internal glucose plays an essential role in the formation of the endogenous inducer of the maltose system.

Gene expression of glucokinase regulatory protein in regenerating rat liver

The activity and messenger RNA (mRNA) levels of glucokinase, and the concentration and mRNA levels of its regulatory protein, were analyzed during liver regeneration. The activity of glucokinase and the concentration of its regulatory protein decreased to 30% and 50%, respectively, after liver resection, remaining low after 1 week. No significant variations in the level of these proteins were found in sham-operated animals. The regulatory protein/glucokinase molar ratio increased during the replicative phase, to a maximum at 48 hours. The mRNA levels of glucokinase and of its regulatory protein decreased rapidly after partial hepatectomy to minimum values at 6 hours (15%) and at 12 hours (4%), respectively, returning to normal values at 24 hours and 168 hours, respectively. Sham-operated animals showed a similar decrease in mRNA levels during the prereplicative phase of liver regeneration, suggesting that the initial effects observed in the gene expression of these proteins were due to surgical stress. During the replicative phase, a specific inhibition of the regulatory protein's gene expression was observed in the regenerating liver. A decrease in the content of regulatory protein and the glucokinase activity, and an increase in the molar ratio of these two proteins correlate with the observed decrease in glycolytic flux, providing further evidence that the phosphorylation of glucose is a control point in the glycolytic/gluconeogenic flux during liver regeneration.

Effect of mutations on the sensitivity of human beta-cell glucokinase to liver regulatory protein

Human beta-cell glucokinase and its liver counterpart displayed a half-saturating concentration of glucose (S0.5) of about 8 mmol/l and a Hill coefficient of 1.7, and were as sensitive to inhibition by the rat liver regulatory protein as the rat liver enzyme. These results indicate that the N-terminal region of glucokinase, which differs among these three enzymes, is not implicated in the recognition of the regulatory protein. They also suggest that the regulatory protein, or a related protein, could modulate the affinity of glucokinase for glucose in beta cells. We have also tested the effect of several mutations, many of which are implicated in maturity onset diabetes of the young. The mutations affected the affinity for glucose and for the regulatory protein to different degrees, indicating that the binding site for these molecules is different. An Asp158 Ala mutation, found in the expression plasmid previously thought to encode the wild-type enzyme, increased the affinity for glucose by about 2.5-fold without changing the affinity for the regulatory protein. The mutations that were found to decrease the affinity for the regulatory protein (Asn166 Arg. Val203 Ala, Asn204 Gln, Lys414 Ala) clustered in the hinge region of glucokinase and nearby in the large and small domains. These results are in agreement with the concept that part of the binding site for the regulatory protein is situated in the hinge region of this enzyme.

Cloning and Biochemical Characterisation of an Aspergillus Niger Glucokinase. Evidence for the Presence of Separate Glucokinase and Hexokinase Enzymes

The Aspergillus niger glucokinase gene glkA has been cloned using a probe generated by polymerase chain reaction with degenerate oligonucleotides. The DNA sequence of the gene was determined, and the deduced amino acid sequence shows significant similarity to other eukaryotic hexokinase and glucokinase proteins, in particular to the Saccharomyces cerevisiae glucokinase protein. The encoded protein was purified from a multicopy glkA transformant, and extensively characterised. The protein has a molecular mass of 54536 Da and a pI of 5.2. The enzyme has high affinity for glucose (K(m) 0.063 mM at pH 7.5) and a relatively low affinity for fructose (K(m) 120 mM at pH 7.5), and in vivo fructose phosphorylation by glucokinase is consequently negligible. The configurations at C1 and C4 of the substrate appear to be essential for substrate specificity. The A. niger glucokinase shows non-competitive inhibition by ADP towards ATP and uncompetitive inhibition by ADP towards glucose. The kcal (turnover number) decreases rapidly below pH 7.5 (56% at pH 7.0 and 17% at pH 6.5) and this may have important implications for the in vivo regulation of activity. In addition, proof is provided for the presence of a second hexosephosphorylating enzyme in A. niger. This enzyme is probably a hexokinase, since unlike glucokinase, this activity is inhibited by trehalose 6-phosphate.

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.

Amino acid conservation in animal glucokinases. Identification of residues implicated in the interaction with the regulatory protein

To delineate the regions of liver glucokinase that are involved in the binding of its regulatory protein and have therefore been conserved throughout evolution, we have cloned the cDNA of the Xenopus laevis enzyme. It contains an open reading frame of 1374 nucleotides and encodes a protein of 458 amino acids, which displays 78 and 79% overall identity to rat and human liver glucokinases, respectively. The conserved regions are predicted to be present mainly in the small domain and the hinge region of glueokinase, and the nonconserved regions in the large domain of the enzyme. We constructed five mutants of Xenopus glucokinase by replacing sets of 2-5 glucokinase-specific residues with their counterparts in the C-terminal half of rat hexokinase I. The affinity for the regulatory protein was not markedly changed for mutants B, D, and E despite a decreased affinity for glucose in mutants B and D. Two other mutants (A and C) were 9- and 250-fold less sensitive to the rat regulator and 40- and 770-fold less sensitive to the Xenopus regulator, respectively, but presented a normal affinity for glucose. The double mutant (A-C) was completely insensitive to inhibition by the regulatory protein. A control mutant (F), obtained by replacing 3 residues that were not conserved in all glucokinases, had a normal affinity for glucose and for the regulatory protein. The property of glucokinase to be inhibited by palmitoyl-CoA was not affected by the mutations described. It is concluded that His-141 to Leu-144, which are located close to the tip of the small domain, as well as Glu-51 and Glu-52, which are present in the large domain of the enzyme close to the hinge region, or nearby residues participate in the binding of the regulatory protein.

Cloning, expression, and characterization of polyphosphate glucokinase from Mycobacterium tuberculosis

Polyphosphate glucokinase from Mycobacterium tuberculosis catalyzes the phosphorylation of glucose using polyphosphate or ATP as the phosphoryl donor. The M. tuberculosis H37Rv gene encoding this enzyme has been cloned, sequenced, and expressed in Escherichia coli. The gene contains an open reading frame for 265 amino acids with a calculated mass of 27,400 daltons. The recombinant polyphosphate glucokinase was purified 189-fold to homogeneity and shown to contain dual enzymatic activities, similar to the native enzyme from H37Ra strain. The high G+C content in the codon usage (64.5%) of the gene and the absence of an E. coli-like promoter consensus sequence are consistent with other mycobacterial genes. Two phosphate binding domains conserved in the eukaryotic hexokinase family were identified in the polyphosphate glucokinase sequence, however, "adenosine" and "glucose" binding motifs were not apparent. In addition, a putative polyphosphate binding region is also proposed for the polyphosphate glucokinase enzyme.

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.

Functional organization of mammalian hexokinase II. Retention of catalytic and regulatory functions in both the NH2- and COOH-terminal halves

The mammalian hexokinase (HK) family includes three closely related 100-kDa isoforms (HKI-III) that are thought to have arisen from a common 50-kDa precursor by gene duplication and tandem ligation. Previous studies of HKI indicated that a glucose 6-phosphate (Glu-6-P)-regulated catalytic site resides in the COOH-terminal half of the molecule and that the NH2-terminal half contains only a Glu-6-P binding site. In contrast, we now show that proteins representing both halves of human and rat HKII have catalytic activity and that each is inhibited by Glu-6-P. The intact enzyme and the NH2- and COOH-terminal halves of the enzyme each increase glucose utilization when expressed in Xenopus oocytes. Mutations corresponding to either Asp-209 or Asp-657 in the intact enzyme completely inactivate the NH2- and COOH-terminal half enzymes, respectively. Mutation of either of these sites results in a 50% reduction of activity in the 100-kDa enzyme. Mutation of both sites results in a complete loss of activity. This suggests that each half of the HKII molecule retains catalytic activity within the 100-kDa protein. These observations indicate that HKI and HKII are functionally distinct and have evolved differently.

Molecular analysis of two hexokinase isoenzymes from Entamoeba histolytica

The zymodemes, electrophoretic patterns of hexokinase, phosphoglucomutase and glucose phosphate isomerase isoenzymes, have been widely used to determine the pathogenicity of Entamoeba histolytica isolates. Although pathogenic and nonpathogenic forms of E. histolytica differ clearly in sequences of many homologous genes, a conversion between pathogenic and nonpathogenic zymodemes has been reported by several laboratories. To approach the question what might be the basis for the observed conversion, we examined the molecular biology of the hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) isoenzymes in pathogenic E. histolytica. We isolated two different cDNAs pHXK1 and pHXK2 coding for polypeptides with significant sequence similarity to hexokinases and deduced molecular masses of 49.8 kDa and 49.4 kDa. The two hexokinase sequences differed by 11% on the amino acid and by 8% on the nucleotide level. Expression of the cDNAs in Escherichia coli as nonfusion proteins gave two polypeptides with hexokinase activity. The recombinant Hxk1 and Hxk2 polypeptides comigrated with the more basic and more acidic isoforms of pathogenic amoebae in starch gel electrophoresis, as well as in low and high resolution isoelectric focussing gels. This identified the observed hexokinase isoenzymes of pathogenic E. histolytica as the products of two genes, hxk1 and hxk2.

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.

Inhibition by recombinant human interleukin-6 of the glucagon-dependent induction of phosphoenolpyruvate carboxykinase and of the insulin-dependent induction of glucokinase gene expression in cultured rat hepatocytes: regulation of gene transcription and messenger RNA degradation

The influence of recombinant human interleukin-6, the major mediator of the inflammatory response in liver, on the glucagon- and insulin-dependent induction of the phosphoenolpyruvate carboxykinase and glucokinase gene, respectively, was monitored on the level of gene transcription, mRNA abundance and enzyme activity in cultured rat hepatocytes. As control markers of the interleukin-6-induced acute-phase response the mRNA levels of the acute phase proteins alpha 2-macroglobulin and beta-fibrinogen were determined. In cultured rat hepatocytes, recombinant human interleukin-6, added simultaneously with glucagon and insulin, lowered the maximal increase in glucagon-induced phosphoenolpyruvate carboxykinase mRNA levels after 2 hr and the maximal increase in glucokinase mRNA levels after 3 hr to about 30%, respectively. It inhibited the glucagon-induced increase in phosphoenolpyruvate carboxykinase gene transcription and phosphoenolpyruvate carboxykinase enzyme activity, as well as the insulin-induced increases in glucokinase gene transcription and glucokinase enzyme activity. Recombinant human interleukin-6 increased the mRNA levels of the acute-phase proteins alpha 2-macroglobulin and beta-fibrinogen gradually over 4 to 6 hr. Recombinant human interleukin-6, added 2 hr after glucagon or 3 hr after insulin at the maximum of the hormone-induced enzyme mRNA levels, almost doubled the decay rate of phosphoenolpyruvate carboxykinase mRNA and glucokinase mRNA. The results show that interleukin-6 induced the expression of inflammatory proteins and simultaneously inhibited the hormone-induced expression of enzymes of intermediary metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)