Cloning and expression of a Xenopus liver cDNA encoding a fructose-phosphate-insensitive regulatory protein of glucokinase

Recent advances in the pathogenesis of hereditary fructose intolerance: implications for its treatment and the understanding of fructose-induced non-alcoholic fatty liver disease

Hereditary fructose intolerance (HFI) is a rare inborn disease characterized by a deficiency in aldolase B, which catalyzes the cleavage of fructose 1,6-bisphosphate and fructose 1-phosphate (Fru 1P) to triose molecules. In patients with HFI, ingestion of fructose results in accumulation of Fru 1P and depletion of ATP, which are believed to cause symptoms, such as nausea, vomiting, hypoglycemia, and liver and kidney failure. These sequelae can be prevented by a fructose-restricted diet. Recent studies in aldolase B-deficient mice and HFI patients have provided more insight into the pathogenesis of HFI, in particular the liver phenotype. Both aldolase B-deficient mice (fed a very low fructose diet) and HFI patients (treated with a fructose-restricted diet) displayed greater intrahepatic fat content when compared to controls. The liver phenotype in aldolase B-deficient mice was prevented by reduction in intrahepatic Fru 1P concentrations by crossing these mice with mice deficient for ketohexokinase, the enzyme that catalyzes the synthesis of Fru 1P. These new findings not only provide a potential novel treatment for HFI, but lend insight into the pathogenesis of fructose-induced non-alcoholic fatty liver disease (NAFLD), which has raised to epidemic proportions in Western society. This narrative review summarizes the most recent advances in the pathogenesis of HFI and discusses the implications for the understanding and treatment of fructose-induced NAFLD.

NAT6 acetylates the N-terminus of different forms of actin

All forms of mammalian actin comprise at their N-terminus a negatively charged region consisting of an N-acetylated aspartate or glutamate followed by two or three acidic residues. This structural feature is unique to actins and important for their interaction with other proteins. The enzyme catalyzing the acetylation of the N-terminal acidic residue is thought to be NAA10, an enzyme that acetylates multiple intracellular proteins. We report here that this acetylation is essentially carried out by NAT6 (Fus2), a protein of unknown function. Tests of the activity of human recombinant NAT6 on a series of purified proteins showed that the best substrate had several acidic residues near its N-terminus. Accordingly NAT6 was particularly active on highly acidic peptides with sequences corresponding to the N-terminus of different forms of mammalian actins. Knocking out of NAT6 in two human cell lines led to absence of acetylation of the first residue of mature beta-actin (Asp2) and gamma-actin-1 (Glu2). Complete acetylation of these two actins was restored by re-expression of NAT6, or by incubation of extracts of NAT6-deficient cells with low concentrations of recombinant NAT6, while NAA10 showed much less or no activity in such assays. Alpha-actin-1 expressed in NAT6-knockout cells was not acetylated at its N-terminus, indicating that the requirement of NAT6 for acetylation of actin N-termini also applies to the skeletal muscle actin isoform. Taken together, our findings reveal that NAT6 plays a critical role in the maturation of actins by carrying out the acetylation of their N-terminal acidic residue.

Molecular basis for the role of glucokinase regulatory protein as the allosteric switch for glucokinase

Glucokinase (GK) is a monomeric allosteric enzyme and plays a pivotal role in blood glucose homeostasis. GK is regulated by GK regulatory protein (GKRP), and indirectly by allosteric effectors of GKRP. Despite the critical roles of GK and GKRP, the molecular basis for the allosteric regulation mechanism of GK by GKRP remains unclear. We determined the crystal structure of Xenopus GK and GKRP complex in the presence of fructose-6-phosphate at 2.9 Å. GKRP binds to a super-open conformation of GK mainly through hydrophobic interaction, inhibiting the GK activity by locking a small domain of GK. We demonstrate the molecular mechanism for the modulation of GK activity by allosteric effectors of GKRP. Importantly, GKRP releases GK in a sigmoidal manner in response to glucose concentration by restricting a structural rearrangement of the GK small domain via a single ion pair. We find that GKRP acts as an allosteric switch for GK in blood glucose control by the liver.

Evolution of hepatic glucose metabolism: liver-specific glucokinase deficiency explained by parallel loss of the gene for glucokinase regulatory protein (GCKR)

BACKGROUND

Glucokinase (GCK) plays an important role in the regulation of carbohydrate metabolism. In the liver, phosphorylation of glucose to glucose-6-phosphate by GCK is the first step for both glycolysis and glycogen synthesis. However, some vertebrate species are deficient in GCK activity in the liver, despite containing GCK genes that appear to be compatible with function in their genomes. Glucokinase regulatory protein (GCKR) is the most important post-transcriptional regulator of GCK in the liver; it participates in the modulation of GCK activity and location depending upon changes in glucose levels. In experimental models, loss of GCKR has been shown to associate with reduced hepatic GCK protein levels and activity.

METHODOLOGY/PRINCIPAL FINDINGS

GCKR genes and GCKR-like sequences were identified in the genomes of all vertebrate species with available genome sequences. The coding sequences of GCKR and GCKR-like genes were identified and aligned; base changes likely to disrupt coding potential or splicing were also identified.

CONCLUSIONS/SIGNIFICANCE

GCKR genes could not be found in the genomes of 9 vertebrate species, including all birds. In addition, in multiple mammalian genomes, whereas GCKR-like gene sequences could be identified, these genes could not predict a functional protein. Vertebrate species that were previously reported to be deficient in hepatic GCK activity were found to have deleted (birds and lizard) or mutated (mammals) GCKR genes. Our results suggest that mutation of the GCKR gene leads to hepatic GCK deficiency due to the loss of the stabilizing effect of GCKR.

Evolution of vertebrate glucokinase regulatory protein from a bacterial N-acetylmuramate 6-phosphate etherase

Mammalian GKRP [GK (glucokinase) regulatory protein], a fructose 6-phosphate and fructose 1-phosphate sensitive inhibitor of GK, appears to have resulted from the duplication of a gene similar to bacterial N-acetylmuramate 6-phosphate etherase MurQ. In the present study, we show that several genomes of primitive eukaryotes encode a GKRP-like protein with two MurQ repeats. Recombinant Haemophilus influenzae MurQ and the GKRP homologue of the amoeboflagellate Naegleria gruberi both behaved as excellent N-acetylmuramate 6-phosphate etherases, with Kcat values (83 and 20 s(-1)) at least as high as that reported for Escherichia coli MurQ. In contrast, rat and Xenopus GKRP displayed much lower etherase activities (Kcat=0.08 and 0.05 s(-1) respectively). The etherase activity of rat GKRP was inhibited by ligands (fructose 6-phosphate, fructose 1-phosphate and sorbitol 6-phosphate) known to regulate its interaction with GK and by mutations affecting the binding of these phosphate esters. This indicated that these phosphate esters all bind to a single regulatory site, which evolved from the original catalytic site. Sorbitol 6-phosphate and other phosphate esters also inhibited the etherase activity of Xenopus GKRP, but did not affect its ability to inhibit GK. Thus, unlike what was previously thought, Xenopus GKRP has a binding site for phosphate esters, but this site is uncoupled from the GK-binding site. Taken together, these data indicate that duplication of the murQ gene led to a eukaryotic-type etherase, which subsequently evolved to GKRP by acquiring a new binding specificity while losing most of its etherase activity.

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.

Inhibition of glucokinase translocation by AMP-activated protein kinase is associated with phosphorylation of both GKRP and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

The rate of glucose phosphorylation in hepatocytes is determined by the subcellular location of glucokinase and by its association with its regulatory protein (GKRP) in the nucleus. Elevated glucose concentrations and precursors of fructose 1-phosphate (e.g., sorbitol) cause dissociation of glucokinase from GKRP and translocation to the cytoplasm. In this study, we investigated the counter-regulation of substrate-induced translocation by AICAR (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside), which is metabolized by hepatocytes to an AMP analog, and causes activation of AMP-activated protein kinase (AMPK) and depletion of ATP. During incubation of hepatocytes with 25 mM glucose, AICAR concentrations below 200 microM activated AMPK without depleting ATP and inhibited glucose phosphorylation and glucokinase translocation with half-maximal effect at 100-140 microM. Glucose phosphorylation and glucokinase translocation correlated inversely with AMPK activity. AICAR also counteracted translocation induced by a glucokinase activator and partially counteracted translocation by sorbitol. However, AICAR did not block the reversal of translocation (from cytoplasm to nucleus) after substrate withdrawal. Inhibition of glucose-induced translocation by AICAR was greater than inhibition by glucagon and was associated with phosphorylation of both GKRP and the cytoplasmic glucokinase binding protein, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) on ser-32. Expression of a kinase-active PFK2 variant lacking ser-32 partially reversed the inhibition of translocation by AICAR. Phosphorylation of GKRP by AMPK partially counteracted its inhibitory effect on glucokinase activity, suggesting altered interaction of glucokinase and GKRP. In summary, mechanisms downstream of AMPK activation, involving phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and GKRP are involved in the ATP-independent inhibition of glucose-induced glucokinase translocation by AICAR in hepatocytes.

2-Keto-4-methylthiobutyrate, an intermediate in the methionine salvage pathway, is a good substrate for CtBP1

In the present work, we have studied the kinetic properties of the catalytic domain of CtBP1, a co-repressor belonging to the d-2-hydroxyacid dehydrogenase family and known to reduce pyruvate in the presence of NADH. CtBP1 acted on a variety of alpha-keto acids, for which it displayed biphasic curves with inhibition at elevated concentrations, as observed with other dehydrogenases of the same family. Based on catalytic efficiencies, the best substrate was 2-keto-4-methylthiobutyrate, an intermediate of the methionine salvage pathway. It was about 20-fold better than 2-ketoisocaproate and glyoxylate, and 80-fold better than pyruvate. From these data we conclude that 2-keto-4-methylthiobutyrate may be an important regulator of CtBP activity, possibly linking gene repression to the activity of the methionine salvage and spermine synthesis pathways.

Differences in regulatory properties between human and rat glucokinase regulatory protein

The inhibition of glucokinase by rat and Xenopus GKRPs (glucokinase regulatory protein) is well documented. We report a comparison of the effects of human and rat GKRPs on glucokinase activity. Human GKRP is a more potent inhibitor of glucokinase than rat GKRP in the absence of fructose 6-phosphate or sorbitol 6-phosphate, and has a higher affinity for these ligands. However, human and rat GKRPs have similar affinities for fructose 1-phosphate and chloride. Residues that are not conserved between the rodent and human proteins affect both the affinity for fructose 6-phosphate and sorbitol 6-phosphate and the inhibitory potency of GKRP on glucokinase in the absence of these ligands.

Identification of fructose 6-phosphate- and fructose 1-phosphate-binding residues in the regulatory protein of glucokinase

Glucokinase is inhibited in the liver by a regulatory protein (GKRP) whose effects are increased by Fru-6-P and suppressed by Fru-1-P. To identify the binding site of these phosphate esters, we took advantage of the homology of GKRP to the isomerase domain of GlmS (glucosamine-6-phosphate synthase) and created 12 different mutants of rat GKRP. Mutations of three residues predicted to bind to Fru-6-P resulted in proteins that were approximately 5-fold (S110A) and 50-fold (S179A and K514A) less potent as inhibitors of glucokinase and had an at least 100-fold reduced affinity for the effectors. Mutation of another residue of the putative binding site (T109A) resulted in a 10-fold decrease in the inhibitory power and an inversion of the effect of sorbitol-6-P, a Fru-6-P analog. The replacement of Gly(107), a residue close to the binding site, by cysteine (as in GlmS and Xenopus GKRP) resulted in a protein that had 20 times more affinity for Fru-6-P and 30 times less affinity for Fru-1-P. These results are consistent with GKRP having one single binding site for phosphate esters. They also show that a missense mutation of GKRP can lead to a gain of function.

Analysis of the cooperativity of human beta-cell glucokinase through the stimulatory effect of glucose on fructose phosphorylation

Using overexpressed Escherichia coli sorbitol-6-phosphate dehydrogenase to monitor fructose 6-phosphate formation, we found that the stimulation of fructose phosphorylation by glucose was reduced in two human beta-cell glucokinase mutants with a low Hill coefficient or when the activity of wild type glucokinase was decreased by replacing ATP with poorer nucleotide substrates. Mutation of two other residues, neighboring glucose-binding residues in the catalytic site, also reduced the affinity for glucose as a stimulator of fructose phosphorylation. Among a series of glucose analogs, only 3, all substrates of glucokinase, stimulated fructose phosphorylation; other analogs were either inactive or inhibited glucokinase. Glucose increased the apparent affinity for inhibitors that are glucose analogs but not for the glucokinase regulatory protein or palmitoyl-CoA. These data indicate that the stimulatory effect of glucose on fructose phosphorylation reflects the positive cooperativity for glucose and is mediated by binding of glucose to the catalytic site. They support models involving the existence of two slowly interconverting conformations of glucokinase that differ through their affinity for glucose and for glucose analogs. We show by computer simulation that such a model can account for the kinetic properties of glucokinase, including the differential ability of mannoheptulose and N-acetylglucosamine to suppress cooperativity (Agius, L., and Stubbs, M. (2000) Biochem. J. 346, 413-421).

Overexpression and purification of fructose-1-phosphate kinase from Escherichia coli: application to the assay of fructose 1-phosphate

Fructose 1-phosphate is a metabolite that plays a regulatory role in liver and is best measured using an assay based on its conversion to fructose 1,6-bisphosphate by a bacterial fructose-1-phosphate kinase (Fru1PK). The open reading frame encoding Escherichia coli Fru1PK has been introduced in an expression plasmid (pET3a) based on the T7 promoter-driven system, which was used to overexpress the enzyme. The conditions for the production of soluble Fru1PK were optimized. The purification procedure used involved ammonium sulfate precipitation and chromatography on DEAE-Sepharose and was aimed mostly at stabilizing the enzyme and at freeing Fru1PK from bacterial contaminants that could interfere in the fructose 1-phosphate assay. From a 1-liter culture, more than 50 mg protein is obtained. This preparation can be used in an enzymatic assay that measures specifically fructose 1-phosphate in tissue extracts.

Mice mutant for glucokinase regulatory protein exhibit decreased liver glucokinase: a sequestration mechanism in metabolic regulation

The importance of glucokinase (GK; EC 2.7.1.12) in glucose homeostasis has been demonstrated by the association of GK mutations with diabetes mellitus in humans and by alterations in glucose metabolism in transgenic and gene knockout mice. Liver GK activity in humans and rodents is allosterically inhibited by GK regulatory protein (GKRP). To further understand the role of GKRP in GK regulation, the mouse GKRP gene was inactivated. With the knockout of the GKRP gene, there was a parallel loss of GK protein and activity in mutant mouse liver. The loss was primarily because of posttranscriptional regulation of GK, indicating a positive regulatory role for GKRP in maintaining GK levels and activity. As in rat hepatocytes, both GK and GKRP were localized in the nuclei of mouse hepatocytes cultured in low-glucose-containing medium. In the presence of fructose or high concentrations of glucose, conditions known to relieve GK inhibition by GKRP in vitro, only GK was translocated into the cytoplasm. In the GKRP-mutant hepatocytes, GK was not found in the nucleus under any tested conditions. We propose that GKRP functions as an anchor to sequester and inhibit GK in the hepatocyte nucleus, where it is protected from degradation. This ensures that glucose phosphorylation is minimal when the liver is in the fasting, glucose-producing phase. This also enables the hepatocytes to rapidly mobilize GK into the cytoplasm to phosphorylate and store or metabolize glucose after the ingestion of dietary glucose. In GKRP-mutant mice, the disruption of this regulation and the subsequent decrease in GK activity leads to altered glucose metabolism and impaired glycemic control.

Identification of the cDNA encoding human 6-phosphogluconolactonase, the enzyme catalyzing the second step of the pentose phosphate pathway(1)

We report the sequence of a human cDNA encoding a protein homologous to devB (a bacterial gene often found in proximity to the gene encoding glucose-6-phosphate dehydrogenase in bacterial genomes) and to the C-terminal part of human hexose-6-phosphate dehydrogenase. The protein was expressed in Escherichia coli, purified and shown to be 6-phosphogluconolactonase, the enzyme catalyzing the second step of the pentose phosphate pathway. Sequence analysis indicates that bacterial devB proteins, the C-terminal part of hexose-6-phosphate dehydrogenase and yeast Sol1-4 proteins are most likely also 6-phosphogluconolactonases and that these proteins are related to glucosamine-6-phosphate isomerases.

acs1 of Haemophilus influenzae type a capsulation locus region II encodes a bifunctional ribulose 5-phosphate reductase- CDP-ribitol pyrophosphorylase

The serotype-specific, 5.9-kb region II of the Haemophilus influenzae type a capsulation locus was sequenced and found to contain four open reading frames termed acs1 to acs4. Acs1 was 96% identical to H. influenzae type b Orf1, previously shown to have CDP-ribitol pyrophosphorylase activity (J. Van Eldere, L. Brophy, B. Loynds, P. Celis, I. Hancock, S. Carman, J. S. Kroll, and E. R. Moxon, Mol. Microbiol. 15:107-118, 1995). Low but significant homology to other pyrophosphorylases was only detected in the N-terminal part of Acs1, whereas the C-terminal part was homologous to several short-chain dehydrogenases/reductases, suggesting that Acs1 might be a bifunctional enzyme. To test this hypothesis, acs1 was cloned in an expression vector and overexpressed in Escherichia coli. Cells expressing this protein displayed both ribitol 5-phosphate dehydrogenase and CDP-ribitol pyrophosphorylase activities, whereas these activities were not detectable in control cells. Acs1 was purified to near homogeneity and found to copurify with ribitol 5-phosphate dehydrogenase and CDP-ribitol pyrophosphorylase activities. These had superimposable elution profiles from DEAE-Sepharose and Blue-Sepharose columns. The dehydrogenase activity was specific for ribulose 5-phosphate and NADPH in one direction and for ribitol 5-phosphate and NADP+ in the other direction and was markedly stimulated by CTP. The pyrophosphorylase showed activity with CTP and ribitol 5-phosphate or arabitol 5-phosphate. We conclude that acs1 encodes a bifunctional enzyme that converts ribulose 5-phosphate into ribitol 5-phosphate and further into CDP-ribitol, which is the activated precursor form for incorporation of ribitol 5-phosphate into the H. influenzae type a capsular polysaccharide.

Organization of the human glucokinase regulator gene GCKR

Glucokinase plays an important role in regulating insulin secretion in response to changes in blood glucose levels. As a result, one form of maturity onset diabetes of the young (MODY) results from haploinsufficiency of glucokinase. In both liver and pancreatic islet, glucokinase is allosterically regulated by an inhibitory protein (glucokinase regulatory protein, GCKR). GCKR has therefore become an important gene for functional analysis in type 2 diabetes. To allow genetic assessment of any such role, we have determined the structure of the human GCKR gene. Characterization of P1 and YAC clones containing GCKR shows it to consist of 19 exons spanning 27 kb. RT-PCR, RACE, and RNase protection experiments defined a transcriptional start site for GCKR 66 bp upstream of the initiation codon, but provided no evidence for islet cell specific alternative splicing in the rat. By SSCP screening, a common polymorphic sequence variant has been defined within exon 15 of human GCKR, at nt 1400 of the cDNA. This alters amino acid residue 446 from proline, conserved in rat and Xenopus, to leucine.

Human L-3-phosphoserine phosphatase: sequence, expression and evidence for a phosphoenzyme intermediate

We report the sequence of the cDNA encoding human L-3-phosphoserine phosphatase. The encoded polypeptide contains 225 residues and shows 30% sequence identity with the Escherichia coli enzyme. The human protein was expressed in a bacterial expression system and purified. Similar to known L-3-phosphoserine phosphatases, it catalyzed the Mg2(+)-dependent hydrolysis of L-phosphoserine and an exchange reaction between L-serine and L-phosphoserine. In addition we found that the enzyme was phosphorylated upon incubation with L-[32P]phosphoserine, which indicates that the reaction mechanism proceeds via the formation of a phosphoryl-enzyme intermediate. The sensitivity of the phosphoryl-enzyme to alkali and to hydroxylamine suggests that an aspartyl- or a glutamyl-phosphate was formed. The nucleotide sequence of the cDNA described in this article has been deposited in the EMBL data base under accession number Y10275.

Cloning, sequencing and expression of rat liver 3-phosphoglycerate dehydrogenase

Rat liver d-3-phosphoglycerate dehydrogenase was purified to homogeneity and digested with trypsin, and the sequences of two peptides were determined. This sequence information was used to screen a rat hepatoma cDNA library. Among 11 positive clones, two covered the whole coding sequence. The deduced amino acid sequence (533 residues; Mr 56493) shared closer similarity with Bacillus subtilis 3-phosphoglycerate dehydrogenase than with the enzymes from Escherichia coli, Haemophilus influenzae and Saccharomyces cerevisiae. In all cases the similarity was most apparent in the substrate- and NAD+-binding domains, and low or insignificant in the C-terminal domain. A corresponding 2.1 kb mRNA was present in rat tissues including kidney, brain and testis, whatever the dietary status, and also in livers of animals fed a protein-free, carbohydrate-rich diet, but not in livers of control rats, suggesting transcriptional regulation. The full-length rat 3-phosphoglycerate dehydrogenase was expressed in E. coli and purified. The recombinant enzyme and the protein purified from liver displayed hyperbolic kinetics with respect to 3-phosphoglycerate, NAD+ and NADH, but substrate inhibition by 3-phosphohydroxypyruvate was observed; this inhibition was antagonized by salts. Similar properties were observed with a truncated form of 3-phosphoglycerate dehydrogenase lacking the C-terminal domain, indicating that the latter is not implicated in substrate inhibition or in salt effects. By contrast with the bacterial enzyme, rat 3-phosphoglycerate dehydrogenase did not catalyse the reduction of 2-oxoglutarate, indicating that this enzyme is not involved in human D- or L-hydroxyglutaric aciduria.

PMM (PMM1), the human homologue of SEC53 or yeast phosphomannomutase, is localized on chromosome 22q13

We have cloned the human homologue of SEC53 or yeast phosphomannomutase (HGMW-approved symbol PMM1) from a liver cDNA library. This cDNA encodes a protein of 262 amino acids with a predicted molecular mass of 29 kDa and 54% identity with yeast phosphomannomutase. Expression of the human cDNA in Escherichia coli yielded an active phosphomannomutase, which was purified to homogeneity. Northern blot analysis of human tissues showed strong expression in liver, heart, brain, and pancreas and a lower expression in skeletal muscle. The gene was assigned to chromosome 22q13.1 by the use of hybrid cell lines and by fluorescence in situ hybridization. Most patients presenting with carbohydrate-deficient glycoprotein syndrome type 1 (CDG1 or Jaeken disease) have a greatly reduced phosphomannomutase activity; the gene encoding this enzyme is a likely candidate for CDG1. Since the CDG1 locus maps else where in the genome (16p13), mutations in the phosphomannomutase gene encoded by chromosome 22 are not a major cause of CDG1.

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.

Human glucokinase regulatory protein (GCKR): cDNA and genomic cloning, complete primary structure, and chromosomal localization

Null mutations in the glucokinase (GCK) gene can cause autosomal dominant type 2 diabetes (maturity onset diabetes of the young, MODY); however, MODY is genetically heterogeneous. In both liver and pancreatic islet, glucokinase is subject to inhibition by a regulatory protein (GCKR). Given the role of GCK in MODY, GCKR is itself a candidate type 2 diabetes susceptibility gene. Here we describe the structure of full-length (2.2 kb) cDNA for human GCKR, from the hepatoblastoma cell line HepG2. The human GCKR translation product has 625 amino acids and a predicted molecular weight of 68,700. It has 88% amino acid identity to rat GCKR. Yeast artificial chromosomes (YAC clones) containing human GCKR were isolated, and the gene was mapped to Chromosome (Chr) 2p23 by fluorescent in situ hybridization and somatic cell hybrid analysis.

Heterologous expression of an active rat regulatory protein of glucokinase

The cDNA presumed to encode the rat liver regulatory protein of glucokinase has been expressed in Escherichia coli and a partially soluble protein has been obtained. This recombinant protein was partially purified and found to have the same apparent molecular mass as the regulatory protein purified from rat liver. Like the latter, it inhibited rat liver glucokinase competitively with respect to glucose and its effect was sensitive to fructose 6-phosphate and fructose 1-phosphate.