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At least 19 recordsLinked to original sources

PubMed1987

The mannose permease of Escherichia coli consists of three different proteins. Amino acid sequence and function in sugar transport, sugar phosphorylation, and penetration of phage lambda DNA.

The mannose permease of the bacterial phosphotransferase system mediates sugar transport across the cytoplasmic membrane concomitant with sugar phosphorylation. It also functions as a receptor for bacterial chemotaxis and is required for infection of the cell by bacteriophage lambda where it most likely functions as a pore for penetration of lambda DNA. The permease consists of three different subunits, IIIMan, II-PMan, and II-MMan, which are encoded in a single transcriptional unit ptsLPM. The complete amino acid sequence of the subunits is deduced from the nucleotide sequence. IIIMan (35 kDa) is a hydrophilic protein which is transiently phosphorylated and most likely contains the active site for sugar phosphorylation. II-PMan (28 kDa) is very hydrophobic; II-MMan (31 kDa) is moderately hydrophobic. Both are integral membrane proteins and most likely form the transmembrane channel. All three subunits are required for sugar transport and phosphorylation; II-PMan and II-MMan alone are sufficient for penetration of lambda DNA. Truncated forms of II-MMan and II-PMan are described that mediate lambda DNA penetration but have no apparent sugar transport activity. Residual sugar phosphorylation activity is found with the truncated form of II-PMan. No obvious homologies at the level of amino acid sequence could be detected with other bacterial transport proteins.

Amino Acid Sequence↗

PubMed1999

Structure and function of facilitative sugar transporters.

Sugar transporters from one group of the major facilitator superfamily of membrane transporters. A conserved common central pore structure lies at the heart of these transporters and diverse functionality is brought about by alterations to this pore or regions associated with it. Recent mutagenesis studies of sugar transporters within the framework of tenable models for the distantly related lactose permease argue that this model is a good paradigm for other members of the major facilitator superfamily.

Amino Acid Sequence↗

PubMed1996

Molecular cloning, immunochemical localization to the vacuole, and expression in transgenic yeast and tobacco of a putative sugar transporter from sugar beet.

Several plant genes have been cloned that encode members of the sugar transporter subgroup of the major facilitator superfamily of transporters. Here we report the cloning, expression, and membrane localization of one of these porters found in sugar beet (Beta vulgaris L.). This clone, cDNA-1, codes for a protein with 490 amino acids and an estimated molecular mass of 54 kD. The predicted membrane topology and sequence homology suggest that cDNA-1 is a member of the sugar transporter family. RNA gel blot analysis revealed that this putative sugar transporter is expressed in all vegetative tissues and expression increases with development in leaves. DNA gel blot analysis indicated that multiple gene copies exist for this putative sugar transporter in the sugar beet genome. Antibodies directed against small peptides representing the N- and C-terminal domains of the cDNA1 protein identified a 40-kD polypeptide in microsomes isolated from cDNA-1-transformed yeast (Saccharomyces cerevisiae). Moreover, the same protein was identified in sugar beet and transgenic tobacco (Nicotaina tobacum L.) membrane fractions. Detailed analysis of the transporter's distribution across linear sucrose gradients and flotation centrifugations showed that it co-migrates with tonoplast membrane markers. We conclude that this carrier is located on the tonoplast membrane and that it may mediate sugar partitioning between the vacuole and cytoplasmic compartments.

Amino Acid Sequence↗

PubMed1999

The human erythrocyte sugar transporter presents two sugar import sites.

The human erythrocyte sugar transporter presents sugar import (e2) and sugar export (e1) sites simultaneously. This study asks whether the sugar transporter exposes only one or multiple import sites. We approached this question by analysis of cytochalasin B binding to the human erythrocyte sugar export site in the presence of sugars that bind to the sugar import site. Extracellular maltose does not enter human erythrocytes. High concentrations of maltose (1-100 mM) inhibit cytochalasin B binding to human red cells. Low concentrations (25-500 microM) increase the level of erythrocyte cytochalasin B binding. Maltose modulation of cytochalasin B binding is mediated by altered affinity of sugar export sites for cytochalasin B. Similar results are obtained with other cell-impermeant inhibitors of sugar uptake. Extracellular D-glucose (a transported sugar) stimulates cytochalasin B binding at low D-glucose concentrations (10-250 microM), but this effect is lost at higher concentrations. Intracellular D-glucose inhibits cytochalasin B binding. Low concentrations of extracellular maltose and other nontransported inhibitors stimulate 3-O-methylglucose uptake in erythrocytes. Higher sugar concentrations (1-100 mM) inhibit transport. These data support the hypothesis that the erythrocyte sugar transporter presents two sugar import sites and at least one sugar export site. This conclusion is consistent with the proposed oligomeric structure of the sugar transporter, a complex of four GluT1 proteins in which each subunit presents a translocation pathway.

3-O-Methylglucose↗

PubMed1985

The potential dependence of the intestinal Na+-dependent sugar transporter.

The unidirectional influx of the lipophilic cation tetraphenylphosphonium (TPP+) into isolated intestinal epithelial cells exhibits a marked dependence on the membrane potential (delta psi) maintained by or experimentally imposed on these cells. By taking advantage of this fact, we have described a "crossover" procedure that allows the relative permeability of a cation and anion pair to be determined. Measurements of such relative permeabilities permits diffusion potentials of defined magnitude to be imposed across the plasma membrane of ATP-depleted cells. This in turn allows description of the relationship between [14C]TPP+ influx and delta psi. We have determined that the flux-potential relationship is that predicted by the Goldman flux equation. Using this relationship as a calibration tool for delta psi, we then determined the quantitative relationship between membrane potential and the Na+-dependent influx of an actively transported sugar, alpha-methylglucoside (alpha-MG). The influx of [14C]alpha-MG also shows an exponential dependence on delta psi although it is more sharply potential dependent than that shown by TPP+. The specific relationship is consistent with that expected for a system with 2:1 Na+ stoichiometry which obeys the potential dependence predicted by Eyring rate theory with a single energy barrier occurring near the midpoint of the membrane. Over the range of potentials from +33 to -61 mV, we find no evidence for a minimum or threshold potential necessary to support transport and no evidence for an optimal potential that can maximize sugar transport. The data raise the possibility for using either [14C]TPP+ or [14C]alpha-MG influx as the basis for a new noninvasive procedure for measurement of delta psi.

Animals↗

PubMed1975

Evidence for two asymmetric conformational states in the human erythrocyte sugar-transport system.

6-O-methyl-, 6-O-propyl-, 6-O-pentyl- and 6-O-benzyl-D-galactose, and 6-O-methyl-, 6-O-propyl- and 6-O-pentyl-D-glucose inhibit the glucose-transport system of the human erythrocyte when added to the external medium. Penetration of 6-O-methyl-D-galactose is inhibited by D-glucose, suggesting that it is transported by the glucose-transport system, but the longer-chain 6-O-alkyl-D-galactoses penetrate by a slower D-glucose-insensitive route at rates proportional to their olive oil/water partition coefficients. 6-O-n-Propyl-D-glucose and 6-O-n-propyl-D-galactose do not significantly inhibit L-sorbose entry or D-glucose exit when present only on the inside of the cells whereas propyl-beta-D-glucopyranoside, which also penetrates the membrane slowly by a glucose-insensitive route, only inhibits L-sorbose entry or D-glucose exit when present inside the cells, and not when on the outside. The 6-O-alkyl-D-galactoses, like the other nontransported C-4 and C-6 derivatives, maltose and 4,6-O-ethylidene-D-glucose, protect against fluorodinitrobenzene inactivation, whereas propyl beta-D-glucopyranoside stimulates the inactivation. Of the transported sugars tested, those modified at C-1, C-2 and C-3 enhance fluorodinitrobenzene inactivation, where those modified at C-4 and C-6 do not, but are inert or protect against inactivation. An asymmetric mechanism is proposed with two conformational states in which the sugar binds to the transport system so that C-4 and C-6 are in contact with the solvent on the outside and C-1 is in contact with the solvent on the inside of the cell. It is suggested that fluorodinitrobenzene reacts with the form of the transport system that binds sugars at the inner side of the membrane. An Appendix describes the theoretical basis of the experimental methods used for the determination of kinetic constants for non-permeating inhibitors.

Binding Sites↗

PubMed1994

Differential glycosylation of the glucose transporter coincides with enhanced sugar transport in respiration deficient cells.

Sugar transport regulation was characterized in terms of the expression and subcellular distribution of the glucose transporter (GT) in the V79 hamster fibroblast cell line and in a respiration deficient mutant (G14) of the V79 cell line. Comparison of GT content in V79 and G14 cells and cell fractions revealed that the 3-fold elevation in basal sugar transport observed in the G14 cell line did not coincide with any significant difference in either the whole cell or plasma membrane GT content when compared to the V79 parental cell line. Determination of delta-antibody binding to intact cell monolayers supported the finding that the two cell lines demonstrate equivalent plasma membrane GT content. Further, D-glucose inhibitable cytochalasin B binding to total cell membranes indicates that additional, unrecognized GT isoforms do not occur in either cell line. A higher average molecular weight GT was detected in the G14 cell line, and treatment of GT enriched preparations with endoglycosidase F established that the G14 cell line exhibits a hyperglycosylated form of the same core GT protein expressed in V79. These results suggest that an enhancement of the intrinsic activity of the GT expressed in G14 is responsible for its increased sugar transport capabilities and this may be related to differences in GT post-translational processing.

Animals↗

PubMed2004

[Sugar transport, metabolism, accumulation and their regulation in fruits].

Photosynthates transported into fruits are mainly in the form of sucrose in most fruit tree species; but sorbitol takes the place of sucrose in woody Rosaceae plants. The transport of sugars across the plasma membrane from apoplastic space into cells is mediated by sugar transporters. The fact that gene expression of sugar transporters is upregulated just before and during sugar accumulation suggests the participation of sugar transporters in sugar accumulation of fruit. The sucrose-metabolizing enzymes participate in four futile cycles that involve sugar transport between cytosol, vacuole, amyloplast and apoplast. The increase in SS (sucrose synthase) and SPS (sucrose phosphate synthase) activities and mRNA levels during maturation parallels the increase in sugar accumulation indicates that the sucrose-metabolizing enzymes have important roles on sugar accumulation in fruits. The prerequisite for rapid accumulation of sugar in fruit is restriction of hexose catabolism and promotion of its synthesis. In woody Rosaceae plants, the fact that sucrose metabolism is also quite active in fruit suggests that sorbitol and sucrose probably play similar roles in fruit development. Sugars as signal molecules regulate the expression of genes involved in sugar transport and metabolism. Sugar transport, metabolism and accumulation are also regulated by natural environmental factors and cultural practices. The increase in sugar content of tomato fruit in acid invertase gene antisense-inhibited plants provides promising prospect of genetic engineering as a potential effective technique in regulation of sugar accumulation in fruits. Thus, the sugar content of fruit is determined by both intrinsic and extrinsic factors. The future research works will be focused on elucidating the mechanism of sugar signal and other intrinsic signals as well as extrinsic signals including nutrients, plant hormones and physical factors on sugar transport, metabolism and accumulation and the interrelationship among them.

Biological Transport↗

PubMed1990

3,5,3'-tri-iodothyronine enhances sugar transport in rat thymocytes by increasing the intrinsic activity of the plasma membrane sugar transporter.

We have shown that 3,5,3'-tri-iodothyronine (T3) produces a prompt increase in sugar transport in rat thymocytes by increasing the maximal velocity without changing the Michaelis-Menten constant of the plasma membrane sugar transport system. To elucidate further the mechanism of this effect, we have now assessed the influence of T3 on the number and affinity of sugar transporters in thymocytes, measured as the sugar (2-deoxyglucose; dGlc)-displaceable binding of cytochalasin B. Cytochalasin B inhibited in a dose-related manner the uptake of dGlc by rat thymocytes with inhibition constant values of 0.19 and 0.22 mumol/l in the presence and absence of T3 respectively. Binding of cytochalasin B by the sugar-displaceable sites was rapid and saturable, demonstrating a single class of sites having an apparent dissociation constant of 0.33 +/- 0.02 (S.D.) mumol/l and maximal binding capacity of 3.73 +/- 0.48 pmol/20 x 10(6) cells (11.2 +/- 1.4 x 10(4) sites/thymocyte). In the rat thymocyte, sugar transporters were found to be located in two major subcellular pools, the plasma membrane and microsomes, the latter being about twice the size of the former. In these subcellular compartments, as well as in the intact cell, binding of [3H]cytochalasin B by the sugar-displaceable sites constituted about 40% of total cytochalasin B binding. 3,5,3'-Tri-iodothyronine in concentrations that stimulated uptake of dGlc by thymocytes had no effect on [3H]cytochalasin B binding (total and sugar-displaceable) in the intact cell and in the plasma membrane and microsomal compartments, nor did it influence the affinity and number of sugar transporters.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2005

Identification and characterization of human Golgi nucleotide sugar transporter SLC35D2, a novel member of the SLC35 nucleotide sugar transporter family.

We report the molecular cloning of SLC35D2, a novel member of the SLC35 nucleotide sugar transporter family. The gene SLC35D2 maps to chromosome 9q22.33. SLC35D2 cDNA codes for a hydrophobic protein consisting of 337 amino acid residues with 10 putative transmembrane helices. Northern blot analysis revealed the SLC35D2 mRNA as a single major band corresponding to 2.0 kb in length. SLC35D2 was localized in the Golgi membrane and exhibited around 50% similarity with three nucleotide sugar transporters: human SLC35D1 (UDP-glucuronic acid/UDP-N-acetylgalactosamine transporter), fruitfly fringe connection (frc) transporter, and nematode SQV-7 transporter, the latter two being involved in developmental and organogenetic processes. Heterologous expression of SLC35D2 protein in yeast indicated that UDP-N-acetylglucosamine is a candidate for the substrate(s) of the transporter. The sequence similarity, subcellular localization, and transporting substrate suggest that SLC35D2 is a good candidate for the ortholog of frc transporter, which is involved in the Notch signaling system by providing the fringe N-acetylglucosaminyltransferase with the substrate. We also describe the identification and categorization of the human SLC35 gene family.

Amino Acid Sequence↗

PubMedDate not supplied

Conventional transport assays underestimate sugar transport rates in human red cells.

The time course of protein-mediated 3-O-methylglucose uptake by human red cells and by red cell ghosts containing or lacking 4 mM MgATP was measured at ice temperature and sub-saturating sugar levels by conventional sampling procedures and at 20 degrees C by use of a quench-flow apparatus. The temporal resolution of the quench-flow apparatus (as fast as 5-ms sample times) was confirmed by analysis of alkaline hydrolysis of dinitrophenolacetate. Red cell sugar uptake at 4 degrees C is consistent with two processes [fast (tau = 120 s) and slow (tau = 1100 s)] that occur in series. Intracellular ATP increases the size and the rate of equilibration of the fast compartment and slows the rate of filling of the slow compartment. Red cell ghost volume and protein content are unaffected by lysis/resealing in the presence of ATP. Uptake at 20 degrees C is also consistent with two processes [fast (tau = 10 ms) and slow (tau = 15 s)] that occur in series. ATP increases the size of both compartments and the rate of filling of the small compartment at 20 degrees C. Preliminary estimates indicate that the sugar uptake capacity of human red cells at 20 degrees C is underestimated by as much as 8-fold by measuring sugar uptake over 2 s vs. 26 ms. We discuss the implications of multiphasic sugar uptake in the context of models for protein-mediated sugar transport.

3-O-Methylglucose↗

PubMed2005

The glycolytic genes pfk and pyk from Lactobacillus casei are induced by sugars transported by the phosphoenolpyruvate:sugar phosphotransferase system and repressed by CcpA.

In Lactobacillus casei BL23, phosphofructokinase activity was higher in cells utilizing sugars transported by the phosphoenolpyruvate:sugar phosphotransferase system (PTS). The phosphofructokinase gene (pfk) was cloned from L. casei and shown to be clustered with the gene encoding pyruvate kinase (pyk). pfk and pyk genes are cotranscribed and induced upon growth on sugars transported by the PTS. Contrarily to the model proposed for Lactococcus lactis, where the global catabolite regulator protein (CcpA) is involved in PTS-induced transcription of pfk and pyk, a ccpA mutation resulted in a slight increase in pfk-pyk expression in L. casei. This weak regulation was evidenced by CcpA binding to a region of the pfk-pyk promoter which contained two cre sequences significantly deviated from the consensus. The PTS induction of pfk-pyk seems to be counteracted by the CcpA-mediated repression. Our results suggest that the need to accommodate the levels of pfk-pyk mRNA to the availability of sugars is fulfilled in L. casei by a PTS/CcpA-mediated signal transduction different from L. lactis.

Amino Acid Sequence↗

PubMed2001

Sucrose transporters in two members of the Scrophulariaceae with different types of transport sugar.

In order to study differences between sugar transport in oligosaccharide-translocating and sucrose-translocating species, two members of the Scrophulariaceae, Asarina barclaiana Pennell and Alonsoa meridionalis O. Kuntze, were analysed regarding minor-vein anatomy, sugar concentrations in leaves and phloem sap, and expression of sucrose transporters. The minor veins of Asarina barclaiana possess mainly transfer cells and modified intermediary cells and those of Alonsoa meridionalis have intermediary cells and ordinary companion cells. Phloem sap from these plants was collected by the laser-aphid-stylet technique. The main carbon transport forms in Asarina were sucrose and in Alonsoa raffinose and stachyose. The sum of the carbohydrate concentrations in the phloem sap of both species was as high as that in apoplastic phloem loaders. In Asarina the ratio of the sucrose concentration in the phloem to that in the cytosol of source cells was about 35 and the corresponding ratio in Alonsoa was about two. Sucrose transporter cDNAs were isolated from leaves of both species. By means of semi-quantitative reverse transcription-polymerase chain reaction, sucrose transporter mRNA was detected in different organs and also in the phloem sap. This is the first time that sucrose transporters have been found in oligosaccharide-translocating species and that the mRNA of these sucrose transporters has been localized directly in the phloem sap. Taken together, our observations indicate that Asarina is an apoplastic phloem loader, while the results for Alonsoa are ambiguous: some properties are typical of the symplastic phloem-loading mechanism, but probably a sucrose transporter is involved in loading and/or retrieval of sucrose into the phloem.

Amino Acid Sequence↗

PubMed1987

Comparison of different dietary sugars as inducers of intestinal sugar transporters.

Intestinal sugar transport increases with dietary carbohydrate levels, but the specific regulatory signals involved have been little studied. Hence we compared rations containing one of five sugars [D-glucose, D-galactose, 3-O-methyl-D-glucose (3-O-MG), D-fructose, and maltose] in their effects on brush-border uptake of five transported solutes (D-glucose, D-galactose, 3-O-MG, D-fructose, and L-proline) by everted sleeves of mouse small intestine. As confirmed by transepithelial potential difference (PD) measurements, there is a distinct fructose transporter that does not evoke a PD, along with one or more aldohexose transporters that do evoke a PD. Galactose and 3-O-MG rations cause a twofold increase in feeding rates, mucosal hyperplasia, and hence nonspecific increases in uptake per unit length of intestine for all transported solutes. Dietary fructose is by far the best specific inducer of the fructose transporter. The five dietary sugars are of fairly similar potency as specific inducers of aldohexose transport, but dietary galactose and fructose may be slightly more potent than glucose. Regulatory signals need not be transported substrates, or vice versa, and need not be metabolizable. Variation in uptake ratios of pairs of aldohexoses with ration and intestinal position suggest multiple aldohexose transporters of overlapping specificity, with different relative activities at different positions and with different susceptibilities to induction by different dietary sugars.

3-O-Methylglucose↗

PubMed1983

Inhibition by the antimicrobial agent chlorhexidine of acid production and sugar transport in oral streptococcal bacteria.

Oral streptococci transport sugars via the phosphoenolpyruvate-phosphotransferase (PEP-PTS) system. In a specific assay of this system, low concentrations of chlorhexidine abolished the activity of the glucose and sucrose PTS in batch-grown cells of Streptococcus mutans Ingbritt and B13, Strep. sanguis NCTC 7865, Strep. mitis ATCC 903, Strep. milleri NCTC 10709 and Strep. salivarius NCTC 8606. Intact cells and cells made permeable to the assay reagents with toluene were used. Toluenized cells were more sensitive to chlorhexidine than intact cells (0.09 and 0.25 mM, respectively). This PTS-inhibitory concentration of chlorhexidine reduced acid production from glucose in pH fall experiments to values higher than are obtained solely from endogenous metabolism. The effect of chlorhexidine on rates of acid production was determined at pH 7.0 using cells washed with either 135 mM NaCl or 135 mM KCl. In general, faster rates of acid production from the metabolism of glucose and sucrose were obtained with potassium-treated cells. Addition of the PTS-inhibitory concentration of chlorhexidine markedly reduced or totally abolished acid production by NaCl-treated cells; a greater residual-activity was detected in the same cells washed with KCl (except with Strep. mutans B13 and Strep. mitis ATCC 903). The PTS-inhibitory concentration of chlorhexidine had little or no effect on the viability of cells. The results confirm the existence of sugar uptake systems in oral streptococci additional to the PTS and provide an explanation for the additive anti-caries effect of mouth-rinses containing both fluoride and chlorhexidine.

Acids↗

PubMed2006

Independent and simultaneous translocation of two substrates by a nucleotide sugar transporter.

Nucleotide sugar transporters play an essential role in protein and lipid glycosylation, and mutations can result in developmental phenotypes. We have characterized a transporter of UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine encoded by the Caenorhabditis elegans gene C03H5.2. Surprisingly, translocation of these substrates occurs in an independent and simultaneous manner that is neither a competitive nor a symport transport. Incubations of Golgi apparatus vesicles of Saccharomyces cerevisiae expressing C03H5.2 protein with these nucleotide sugars labeled with (3)H and (14)C in their sugars showed that both substrates enter the lumen to the same extent, whether or not they are incubated alone or in the presence of a 10-fold excess of the other nucleotide sugar. Vesicles containing a deletion mutant of the C03H5.2 protein transport UDP-N-acetylglucosamine at rates comparable with that of wild-type transporter, whereas transport of UDP-N-acetylgalactosamine was decreased by 85-90%, resulting in an asymmetrical loss of substrate transport.

Animals↗

PubMed1982

Sugar transport regulation in avian red blood cells: role of Ca2+ in the stimulatory effects of anoxia, adrenaline, and ascorbic acid.

Membrane transport of sugar and Ca2+ was studied in pigeon erythrocytes by measuring the cell to medium distribution of 3-O-[14C]methyl-D-glucose and 45Ca. We have found that stimulation of sugar transport by anoxia, adrenaline, or ascorbic acid was not dependent on external Ca2+, nor was it additive to the stimulatory effect of the calcium ionophore A23187. Stimulation by ascorbic acid was dependent on concentration and time. The slow basal 45Ca efflux was greatly accelerated by A23187, and this was further increased by adrenaline. A metabolic substrate mixture consisting of adenine, inosine, and fumarate (AIF) did not alter 45Ca efflux, except for antagonizing the effect of adrenaline in the presence of A23187. Sugar transport, whether basal or stimulated by adrenaline or ascorbic acid, was significantly decreased by AIF, independently of external Ca2+. Stimulation by A23187 in the absence of external Ca2+ was also antagonized by AIF. In cells depleted of Ca2+ by treatment with A23187 and EGTA, transport stimulation by adrenaline was abolished. These results suggest that release of Ca2+ from intracellular storage into the cytoplasm plays a role in the stimulation of sugar transport by adrenaline and anoxia and also by A23187 in the absence of external Ca2+. The data provide further indirect support for a calcium-dependent mechanism of sugar transport regulation in nucleated erythrocytes.

Adenine↗

PubMed1987

Equilibrium ligand binding to the human erythrocyte sugar transporter. Evidence for two sugar-binding sites per carrier.

Equilibrium [3H]cytochalasin B binding to class I sites of human red cell membranes (the sugar transporter) was examined in the presence and absence of intracellular or extracellular sugars known to interact with the transport system. D-Glucose, a transported sugar, is without effect on cytochalasin B binding when present in the extracellular medium but is an effective inhibitor of binding when present within the cell. Ethylidene glucose and maltose (reactive but nontransported sugars) inhibit cytochalasin B (CCB) binding when present either outside or inside the red cell. Inhibition by intracellular sugar (Si) is of the simple, linear competitive type. Inhibition by extracellular sugars (So) is more complex; the Kd(app) for cytochalasin B binding increases in a saturable fashion with [So]. These observations are compared with the predictions of the one-site, alternating conformer model and the two-site model for substrate binding to the sugar transporter, X. The experimental results are inconsistent with the one-site model but are explained by a two-site model in which the ternary complexes of So . X . Si or So . X . CCBi exist and where the binding sites for So and Si display negative cooperativity when occupied by nontransported substrate and little or no cooperativity when occupied by the transported species, D-glucose.

Binding Sites↗