Hexokinases

Evidence for channeling of intermediates in the oxidative pentose phosphate pathway by soybean and pea nodule extracts, yeast extracts, and purified yeast enzymes

Evidence is presented that intermediates of the oxidative pentose phosphate pathway (OPPP) are channeled from one pathway enzyme to the next. CO2 produced from [1-14C]glucose in the presence of unlabelled pathway intermediates contained much more radioactivity than predicted by a model in which pathway-produced intermediates are in equilibrium with identical molecules in the bulk phase. This was the case whether glucose 6-phosphate (Glc6P), 6-phosphogluconolactone, or 6-phosphogluconate was added. Assumptions involved in calculating the amount of 14CO2 predicted for free mixing of 14C-labelled and unlabelled intermediates are discussed, together with the following results. (a) 14CO2 production by pea nodules in the presence of 3 mM 6-phosphogluconate was higher than in its absence. (b) Apparent channeling of intermediates was much higher for purified yeast enzymes than for yeast extract. (c) 6-Phosphogluconate and 6-phosphogluconolactone were channeled between yeast Glc6P dehydrogenase and 6-phosphogluconate dehydrogenase despite the absence of 6-phosphogluconolactonase in the purified yeast enzyme mixture. (d) When purified yeast hexokinase was physically separated from Glc6P dehydrogenase and 6-phosphogluconate dehydrogenase by a dialysis membrane, there was no apparent channeling. (e) Poly(ethylene glycol), high salt and detergents had little effect on apparent channeling of OPPP intermediates, which is consistent with a stable complex of enzymes. On the other hand, density gradient centrifugation experiments suggested a more transient interaction between the enzymes. Taken together, the results support channeling of OPPP pathway intermediates.

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.

Homologous and heterologous interactions between hexokinase and mitochondrial porin: evolutionary implications

Binding of the Type I isozyme of mammalian hexokinase to mitochondria is mediated by the porin present in the outer mitochondrial membrane. Type I hexokinase from rat brain is avidly bound by rat liver mitochondria while, under the same conditions, there is no significant binding to mitochondria from S. cerevisiae. Previously published work demonstrates the lack of significant interaction of yeast hexokinase with mitochondria from either liver or yeast. Thus, structural features required for the interaction of porin and hexokinase must have emerged during evolution of the mammalian forms of these proteins. If these structural features serve no functional role other than facilitating this interaction of hexokinase with mitochondria, it seems likely that they evolved in synchrony since operation of selective pressures on the hexokinase-mitochondrial interaction would require the simultaneous presence of hexokinase and porin capable of at least minimal interaction, and be responsive to changes in either partner that affected this interaction. Recent studies have indicated that a second type of binding site, which may or may not involve porin, is present on mammalian mitochondria. There are also reports of hexokinase binding to mitochondria in plant tissues, but the nature of the binding site remains undefined.

High glycolysis in gliomas despite low hexokinase transcription and activity correlated to chromosome 10 loss

Loss of chromosome 10 was observed in 10 out of 12 xenografted glioblastomas studied. Chromosome 10 carries the gene coding the hexokinase type 1 isoenzyme (HK-I), which catalyses the first step of glycolysis, which is essential in brain tissue and glioblastomas. We investigated the relationships between the relative chromosome 10 number, the amount of HK-I mRNA, HK-I activity and its intracellular distribution, and glycolysis-related parameters such as the lactate-pyruvate ratio, lactate dehydrogenase (LDH) and ATP contents. Individual tumour HK-I mRNA amounts were 23-65% lower than that of normal human brain and reflected the relative decrease of chromosome 10 number (alpha < 0.01). Total HK activities of individual glioblastomas varied considerably but were constantly (a mean of seven times) lower than that of normal brain tissue. The mitochondria-bound HK-I fraction of individual tumours was generally over 50%, compared with that of normal brain tissue. As shown by lactate - pyruvate ratios, in all the gliomas, glycolysis was elevated to an average of 3-fold that measured in normal brain. An elevated ATP content was also constantly noted. Adaptation of glioblastoma metabolism to the chromosome 10 loss and to the HK-I transcription unit emphasises the critical role of glycolysis in their survival. We hypothesise that HK-I, the enzyme responsible for initiating glycolysis necessary for brain function, may approach its lowest limit in gliomas, thereby opening therapeutic access to pharmacological anti-metabolites affecting energy metabolism and tumour growth.

Identification and characterization of basal and cyclic AMP response elements in the promoter of the rat hexokinase II gene

Hexokinases catalyze the phosphorylation of glucose and initiate cellular glucose metabolism. Hexokinase II (HKII) is the principal hexokinase isoform in skeletal muscle, heart, and adipose tissue. Isoproterenol and exogenous cyclic AMP (cAMP) increase HKII gene transcription in L6 myotubes. Various segments of the HKII promoter that direct the expression of the chloramphenicol acetyltransferase reporter gene were transfected into L6 myotubes to identify basal and cAMP response elements. The 5'-flanking region that extends 90 base pairs upstream of the transcription start site includes a CCAAT box and a cAMP response element (CRE); both contribute to basal promoter activity and each provides an independent, maximal response to cAMP. An inverted CCAAT motif, or Y box, located just upstream of the CCAAT box, contributes to basal promoter activity but is not involved in the cAMP response. Homo- and heterodimers composed of the CRE-binding protein and activating transcription factor-1 bind specifically to the CRE. The Y box and the CCAAT box specifically bind the factor NF-Y (also known as CBF).

Analysis of the signaling pathway involved in the regulation of hexokinase II gene transcription by insulin

The hexokinases, by converting glucose to glucose 6-phosphate, help maintain the glucose concentration gradient that results in the movement of glucose into cells through the facilitative glucose transporters. Hexokinase II (HKII) is the major hexokinase isoform in skeletal muscle, heart, and adipose tissue. Insulin induces HKII gene transcription in L6 myotubes, and this, in turn, increases HKII mRNA and the rates of HKII protein synthesis and glucose phosphorylation in these cells. Inhibitors of distinct insulin signaling pathways were used to dissect the molecular mechanism by which HKII gene expression is induced by insulin in L6 myotubes. Treatment with wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase), or with rapamycin, an inhibitor of the pathway from the insulin receptor to p70/p85 ribosomal S6 protein kinase (p70(s6k)), prevented the induction of HKII mRNA by insulin. In contrast, treatment with PD98059, an inhibitor of mitogen-activated protein kinase activation, had no effect on insulin-induced HKII mRNA. In addition, rapamycin blocked the insulin-induced expression of an HKII promoter-chloramphenicol acetyltransferase fusion gene transiently transfected into L6 myotubes, whereas PD98059 had no such effect. These results suggest that a phosphatidylinositol 3-kinase/p70(s6k)-dependent pathway is required for regulation of HKII gene transcription by insulin and that the Ras-mitogen-activated protein kinase-dependent pathway is probably not involved.

Intracellular pH governs the subcellular distribution of hexokinase in a glioma cell line

Hexokinase plays a key role in regulating cell energy metabolism. Hexokinase is mainly particulate, bound to the mitochondrial outer membrane in brain and tumour cells. We hypothesized that the intracellular pH (pH1) controls the intracellular distribution of hexokinase. Using the SNB-19 glioma cell line, pH1 variations were imposed by incubating cells in a high-K+ medium at different pH values containing specific ionophores (nigericin and valinomycin), without affecting cell viability. Subcellular fractions of cell homogenates were analysed for hexokinase activity. Imposed pH1 changes were verified microspectrofluorimetrically by using the pH1-sensitive probe SNARF-1-AM (seminaphtho-rhodafluor-1-acetoxymethyl ester). Imposition of an acidic pH1 for 30 min strongly decreased the particulate/total hexokinase ratio, from 63% in the control sample to 31%. Conversely, when a basic pH1, was imposed, the particulate/total hexokinase ratio increased to 80%. The glycolytic parameters, namely lactate/pyruvate ratio, glucose 6-phosphate and ATP levels, were measured concomitantly. Lactate/pyruvate ratio and ATP level were both markedly decreased by acidic pH1 and increased by basic pH1. Conversely, the glucose 6-phosphate level was increased by acidic pH1 and decreased by basic pH1. To demonstrate that the change of hexokinase distribution was not due to altered metabolite levels of glycolysis, a pH1 was imposed for a 5 min incubation time. Modification of the hexokinase distribution was similar to that noted after a 30 min incubation, whereas metabolite levels of glycolysis were not affected. These results provide evidence that the intracellular distribution of hexokinase is highly sensitive to variations of the pH1, and regulates hexokinase activity.

Metabolic modulation of hexokinase association with mitochondria in living smooth muscle cells

Hexokinase isoform I binds to mitochondria of many cell types. It has been hypothesized that this association is regulated by changes in the concentrations of specific cellular metabolites. To study the distribution of hexokinase in living cells, fluorophore-labeled functional hexokinase I was prepared. After microinjection into A7r5 smooth muscle cells, hexokinase localized to distinct structures identified as mitochondria. The endogenous hexokinase demonstrated a similar distribution with the use of immunocytochemistry. 2-Deoxyglucose elicited an increase in glucose 6-phosphate (G-6-P) and a decrease in ATP levels and diminished hexokinase binding to mitochondria in single cells. 3-O-methylglucose elicited slowly developing decreases in all three parameters. In contrast, cyanide elicited a rapid decrease in both ATP and hexokinase binding. Analyses of changes in metabolite levels and hexokinase binding indicate a positive correlation between binding and cell energy state as monitored by ATP. On the other hand, only in the presence of 2-deoxyglucose was the predicted inverse correlation between binding and G-6-P observed. Unlike the relatively large changes in distribution observed with the fluorescent-injected hexokinase, cyanide caused only a small decrease in the localization of endogenous hexokinase with mitochondria. These findings suggest that changes in the concentrations of specific metabolites can alter the binding of hexokinase I to specific sites on mitochondria. Moreover, the apparent difference in sensitivity of injected and endogenous hexokinase to changes in metabolites may reflect the presence of at least two classes of binding mechanisms for hexokinase, with differential sensitivity to metabolites.

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.

Effects of low-frequency stimulation on soluble and structure-bound activities of hexokinase and phosphofructokinase in rat fast-twitch muscle

Several glycolytic enzymes exist in muscle as free and structure-bound forms. A fraction of hexokinase (HK) is associated with the outer mitochondrial membrane. Phosphofructokinase (PFK) and aldolase (ALD) bind to F-actin, and AMP deaminase (AMPase) interacts with myosin. Using low-frequency stimulation (10 Hz, 24 h/d), we studied in rat fast-twitch muscle effects of contractile activity on soluble and structure-bound forms of these enzymes. Phosphoglucose isomerase (PGI), a soluble enzyme, was also examined. Fractional extraction was applied to study the intracellular distribution of soluble and bound enzyme activities 5 min, 1 h, 3 h, 1 d, and 7 d after the onset of stimulation. Confirming previous findings, total HK activity increased 7-fold in 7-d-stimulated muscles, whereas PFK, ALD, and PGI were reduced, ranging between 55% and 80% of their normal activities. AMPase activity was unaltered. At the time points studied, no changes were found in the extraction behavior of PGI and AMPase. The fraction of bound ALD increased slightly (12%). However, the distribution of HK and PFK was markedly altered. Bound PFK increased from 50% in the control to 85% in 7-d-stimulated muscles. Bound HK rose from 52% to 83% during the same time period. The increase in PFK binding was steep and occurred mainly within the first minutes and hours. The increase in HK binding occurred with some delay, but was significant in muscles stimulated for more than 1 h. In view of the altered kinetic properties of F-actin-bound PFK (alleviated allosteric inhibition by ATP) and bound HK (elevated catalytic activity), these changes are interpreted as early responses to match the metabolic demands during maximal contractile activity imposed on a muscle not programmed for sustained activity: Enhanced binding of PFK serves to accelerate glycolytic flux immediately after the onset of stimulation, whereas mitochondrial binding of HK facilitates the phosphorylation of exogenous glucose when glycogen stores have been depleted.