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At least 1,495 records · Page 83Linked to original sources
PubMed1993

Estimates of uridine diphosphate hexoses in erythrocytes: implications for galactosemia.

Impurities in a reagent dehydrogenase caused overestimates of erythrocytic uridine diphosphate glucose and accounted for clinically important differences in results between those of one group of investigators using enzymatic methods and those of two other groups using enzymatic methods, high-performance liquid chromatography, and nuclear magnetic resonance. These data have relevance for the current debate regarding the pathophysiologic changes in galactosemia.

Drug Contamination↗
PubMed1995

Hexose phosphate binding sites of fructose 6-phosphate,2-kinase:fructose 2,6-bisphosphatase.

A previous chemical modification study [Kitamura et al. (1989) J. Biol. Chem. 264, 6344-6438] has shown that N-bromoacetylethanolamine phosphate labeled specifically Cys107 of rat liver Fru 6-P,2-kinase:Fru 2.6-Pase and the corresponding Cys of the bovine heart enzyme, leading to inactivation of kinase activity. Since Fru 6-P provided protection against the inactivation, this region of the enzyme was thought to be a Fru 6-P binding site of the kinase enzyme. To examine this possibility, oligonucleotide-directed mutagenesis has been used to alter several residues in expressed rat testis Fru 6-P,2-kinase:Fru 2,6-Pase. The change of Lys100, Lys103, and Asp112 caused at most a 2-fold increase in KmF6P and a 2-3-fold increase in KmATP, suggesting that these residues are not involved in the direct binding of Fru 6-P. However, change of Arg102 to Leu and to Lys resulted in a 325x and 22x, respectively, increase in KmF6P, and change of Arg102 to Glu resulted in nearly complete loss of the kinase activity. Change of Cys105 to Ala or Ser increased KmF6P about 10x. The Vmax of all these mutated enzymes except the one that changed Arg102 to Glu (R102E) was increased 10% to 85%. The kinetic parameters of Fru 2,6-Pase were not altered by these changes. R102E formed several polymeric forms of the enzyme, including a tetramer. Both R102E and an additional derivative that substituted Lys for Arg102 (R102K) were slightly more susceptible to guanidine inactivation than the wild-type enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed2001

The predicted ATP-binding domains in the hexose transporter GLUT1 critically affect transporter activity.

The glucose transporter GLUT1 has three short amino acid sequences (domains I-III) with homology to typical ATP-binding domains. GLUT1 is a facilitative transporter, however, and transports its substrates down a concentration gradient without a specific requirement for energy or hydrolysis of ATP. Therefore, we assessed the functional role of the predicted ATP-binding domains in GLUT1 by site-directed mutagenesis and expression in Xenopus oocytes. For each mutant, we determined the level of protein expression and the kinetics of transport under zero-trans influx, zero-trans efflux, and equilibrium exchange conditions. Although all five mutants were expressed at levels similar to that of the wild-type GLUT1, each single amino acid change in domains I or III profoundly affected GLUT1 function. The mutants Gly116-->Ala in domain I and Gly332-->Ala in domain III exhibited only 10-20% of the transport activity of the wild-type GLUT1. The mutants Gly111-->Ala in domain I and Leu336-->Ala in domain III showed altered kinetic properties; neither the apparent Km nor the Vmax for 3-methylglucose transport were increased under equilibrium exchange conditions, and they did not show the expected level of countertransport acceleration. The mutant Lys117-->Arg in domain I showed a marked increase in the apparent Km for 3-methylglucose transport under zero-trans efflux and equilibrium exchange conditions while maintaining countertransport acceleration. These results indicate that the predicted ATP-binding domains I and III in GLUT1 are important components of the region in GLUT1 involved in transport of the substrate and that their integrity is critical for maintaining the activity and kinetic properties of the transporter.

Adenosine Triphosphate↗
PubMed1983

Binding of hexose bisphosphates to muscle phosphofructokinase.

On the basis of kinetic activation assays, the apparent affinity of muscle phosphofructokinase for fructose 2,6-bisphosphate was about 9-fold greater than that for fructose 1,6-bisphosphate, which in turn was about 10 times higher than that for glucose 1,6-bisphosphate. Equilibrium binding experiments showed that both fructose bisphosphates bind to phosphofructokinase with negative cooperativity; the affinity for fructose 2,6-bisphosphate was about 1 order of magnitude greater than the affinity for fructose 1,6-bisphosphate. Binding of fructose 2,6-bisphosphate to phosphofructokinase was antagonized by fructose 1,6-bisphosphate and glucose 1,6-bisphosphate and vice versa. Both fructose bisphosphates promoted aggregation of the enzyme to higher polymers as indicated by sucrose density gradient centrifugation. Other indicators of phosphofructokinase conformation such as thiol reactivity and maximum activation of in vitro phosphorylation by the catalytic subunit of cyclic AMP-dependent protein kinase gave identical results in the presence of fructose 2,6-bisphosphate, fructose 1,6-bisphosphate, or glucose 1,6-bisphosphate, indicating a common conformation is produced by all three ligands. It is concluded that the sugar bisphosphates bind to a single site on the enzyme.

Animals↗
PubMed1985

Effects of bilayer cholesterol on human erythrocyte hexose transport protein activity in synthetic lecithin bilayers.

In this study, we describe the effects of altered bilayer cholesterol content on reconstituted, protein-mediated sugar transport. The system used was the human erythrocyte sugar transporter (band 4.5) reconstituted into the bilayers of large unilamellar vesicles. Vesicle preparations were formed from synthetic lecithins whose bilayer cholesterol content ranged from 0 to 50 mol %. Transport was measured by microturbidimetric analysis over the temperature range of 0-65 degrees C while bilayer physical state was characterized by differential scanning calorimetry. Reconstituted transport activity was irreversibly lost between 62 and 65 degrees C. The Km for reconstituted transport was found to increase only slightly with increasing temperature and was not systematically affected by bilayer cholesterol content. The most striking observation of this study is that over certain critical cholesterol concentrations, as little as a 2.5% change in bilayer cholesterol can result in as much as a 100-fold change in Vmax per reconstituted protein. Our findings run counter to the view that increasing bilayer cholesterol content monotonically transforms a membrane into a state of "intermediate fluidity". Abrupt, cholesterol-induced bilayer reorganizations occurring at 15-20 and 30 mol % bilayer cholesterol are markedly reflected in altered sugar transport rates. Increasing the cholesterol content of crystalline distearoyllecithin bilayers inhibits the activity of the reconstituted transporter. It is apparent from these studies that bilayer "fluidity" is neither the sole nor a major determinant of the Indeed, we find the effect of cholesterol on transport activity is independent of its ability to fluidize membranes.

Blood Glucose↗
PubMed1982

Solution structure of 5-keto-D-fructose: relevance to the specificity of hexose kinases.

5-Keto-D-fructose (5KF) is isolated from cultures of Gluconobacter cerinus growing on D-fructose as the sole carbon source. 5KF is a substrate for hexokinase, fructokinase, and several polyol dehydrogenases. 1H and 13C nuclear magnetic resonance studies show that 5KF exists in different forms in anhydrous dimethyl-d6 sulfoxide and D2O. In dimethyl-d6 sulfoxide, 5KF exists as a spirane dimer with linked furanose and pyranose rings, similar to the structure reported for crystalline 5KF [Hassen, L., Hordvik, A., & Hove, R. (1976) J. Chem. Soc., Chem. Commun., 572-. In D2O, 5KF exists predominantly (greater than 95%) in a beta-pyranose form with the 5-keto group hydrated to form a gem-diol. 13C--1H coupling patterns, 13C relaxation measurements, and 13C deuterium-induced differential isotope shifts confirm this structure of 5KF. The phosphorylation of 5KF by fructokinase can be accounted for by an approximately 2% proportion of the beta-furanose form in solution at 25 degrees C. Both the beta-pyranose and beta-furanose forms of 5KF are proposed to be substrates for yeast hexokinase.

Deuterium↗