D-glucose Binding Research Articles

Sugar binding of sodium–glucose cotransporters analyzed by voltage-clamp fluorometry

Sugar absorption is crucial for life and relies on glucose transporters, including sodium-glucose co-transporters (SGLTs). Although the structure of SGLTs has been resolved, the substrate selectivity of SGLTs across diverse isoforms has not been determined owing to the complex substrate-recognition processes and limited analysis methods. Therefore, this study used voltage-clamp fluorometry (VCF) to explore the substrate-binding affinities of human SGLT1 in Xenopus oocytes. VCF analysis revealed high-affinity binding of D-glucose and D-galactose, which are known transported substrates. D-fructose, which is not a transported substrate, also bound to SGLT1, suggesting potential recognition despite the lack of transport activity. VCF analysis using the T287N mutant of the substrate-binding pocket, which has reduced D-glucose transport capacity, showed that its D-galactose-binding affinity exceeded its D-glucose-binding affinity. This suggests that the change in the VCF signal was due to substrate binding to the binding pocket. Both D-fructose and L-sorbose showed similar binding affinities, indicating that SGLT1 preferentially binds to pyranose-form sugars, including D-fructopyranose. Electrophysiological analysis confirmed that D-fructose binding did not affect the SGLT1 transport function. The significance of the VCF assay lies in its ability to measure sugar-protein interactions in living cells, thereby bridging the gap between structural analyses and functional characterizations of sugar transporters. Our findings also provide insights into SGLT substrate selectivity and the potential for developing medicines with reduced side effects by targeting non-glucose sugars with low bioreactivity.

Substrate analog interaction with MCR-1 offers insight into the rising threat of the plasmid-mediated transferable colistin resistance.

Colistin is considered a last-resort antibiotic against most gram-negative bacteria. Recent discoveries of a plasmid-mediated, transferable mobilized colistin-resistance gene ( mcr-1) on all continents have heralded the imminent emergence of pan-drug-resistant superbacteria. The inner-membrane protein MCR-1 can catalyze the transfer of phosphoethanolamine (PEA) to lipid A, resulting in colistin resistance. However, little is known about the mechanism, and few drugs exist to address this issue. We present crystal structures revealing the MCR-1 catalytic domain (cMCR-1) as a monozinc metalloprotein with ethanolamine (ETA) and d-glucose, respectively, thus highlighting 2 possible substrate-binding pockets in the MCR-1-catalyzed PEA transfer reaction. Mutation of the residues involved in ETA and d-glucose binding impairs colistin resistance in recombinant Escherichia coli containing full-length MCR-1. Partial analogs of the substrate are used for cocrystallization with cMCR-1, providing valuable information about the family of PEA transferases. One of the analogs, ETA, causes clear inhibition of polymyxin B resistance, highlighting its potential for drug development. These data demonstrate the crucial role of the PEA- and lipid A-binding pockets and provide novel insights into the structure-based mechanisms, important drug-target hot spots, and a drug template for further drug development to combat the urgent, rising threat of MCR-1-mediated antibiotic resistance.-Wei, P., Song, G., Shi, M., Zhou, Y., Liu, Y., Lei, J., Chen, P., Yin, L. Substrate analog interaction with MCR-1 offers insight into the rising threat of the plasmid-mediated transferable colistin resistance.

The Tp0684 (MglB-2) Lipoprotein of Treponema pallidum: A Glucose-Binding Protein with Divergent Topology.

Treponema pallidum, the bacterium that causes syphilis, is an obligate human parasite. As such, it must acquire energy, in the form of carbon sources, from the host. There is ample evidence that the principal source of energy for this spirochete is D-glucose acquired from its environment, likely via an ABC transporter. Further, there is genetic evidence of a D-glucose chemotaxis system in T. pallidum. Both of these processes may be dependent on a single lipidated chemoreceptor: Tp0684, also called TpMglB-2 for its sequence homology to MglB of Escherichia coli. To broaden our understanding of this potentially vital protein, we determined a 2.05-Å X-ray crystal structure of a soluble form of the recombinant protein. Like its namesake, TpMglB-2 adopts a bilobed fold that is similar to that of the ligand-binding proteins (LBPs) of other ABC transporters. However, the protein has an unusual, circularly permuted topology. This feature prompted a series of biophysical studies that examined whether the protein’s topological distinctiveness affected its putative chemoreceptor functions. Differential scanning fluorimetry and isothermal titration calorimetry were used to confirm that the protein bound D-glucose in a cleft between its two lobes. Additionally, analytical ultracentrifugation was employed to reveal that D-glucose binding is accompanied by a significant conformational change. TpMglB-2 thus appears to be fully functional in vitro, and given the probable central importance of the protein to T. pallidum’s physiology, our results have implications for the viability and pathogenicity of this obligate human pathogen.

Detection of N-(1-deoxy-D-fructos-1-yl) Fumonisins B₂ and B₃ in Corn by High-Resolution LC-Orbitrap MS.

The existence of glucose conjugates of fumonisin B2 (FB2) and fumonisin B3 (FB3) in corn powder was confirmed for the first time. These “bound-fumonisins” (FB2 and FB3 bound to glucose) were identified as N-(1-deoxy-d-fructos-1-yl) fumonisin B2 (NDfrc-FB2) and N-(1-deoxy-d-fructos-1-yl) fumonisin B3 (NDfrc-FB3) respectively, based on the accurate mass measurements of characteristic ions and fragmentation patterns using high-resolution liquid chromatography-Orbitrap mass spectrometry (LC-Orbitrap MS) analysis. Treatment on NDfrc-FB2 and NDfrc-FB3 with the o-phthalaldehyde (OPA) reagent also supported that d-glucose binding to FB2 and FB3 molecules occurred to their primary amine residues.

Surface Enhanced Raman Spectroscopy Detection of Biomolecules Using EBL Fabricated Nanostructured Substrates

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules is reported. The entire sequence of relevant experimental steps is described, involving the fabrication of nanostructured substrates using electron beam lithography, immobilization of biomolecules on the substrates, and their characterization utilizing surface-enhanced Raman spectroscopy (SERS). Three different designs of nano-biological systems are employed, including protein A, glucose binding protein, and a dopamine binding DNA aptamer. In the latter two cases, the binding of respective ligands, D-glucose and dopamine, is also included. The three kinds of biomolecules are immobilized on nanostructured substrates by different methods, and the results of SERS imaging are reported. The capabilities of SERS to detect vibrational modes from surface-immobilized proteins, as well as to capture the protein-ligand and aptamer-ligand binding are demonstrated. The results also illustrate the influence of the surface nanostructure geometry, biomolecules immobilization strategy, Raman activity of the molecules and presence or absence of the ligand binding on the SERS spectra acquired.

Surface enhanced Raman spectroscopy detection of biomolecules using EBL fabricated nanostructured substrates.

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules is reported. The entire sequence of relevant experimental steps is described, involving the fabrication of nanostructured substrates using electron beam lithography, immobilization of biomolecules on the substrates, and their characterization utilizing surface-enhanced Raman spectroscopy (SERS). Three different designs of nano-biological systems are employed, including protein A, glucose binding protein, and a dopamine binding DNA aptamer. In the latter two cases, the binding of respective ligands, D-glucose and dopamine, is also included. The three kinds of biomolecules are immobilized on nanostructured substrates by different methods, and the results of SERS imaging are reported. The capabilities of SERS to detect vibrational modes from surface-immobilized proteins, as well as to capture the protein-ligand and aptamer-ligand binding are demonstrated. The results also illustrate the influence of the surface nanostructure geometry, biomolecules immobilization strategy, Raman activity of the molecules and presence or absence of the ligand binding on the SERS spectra acquired.

Tryptophan residue of the D-galactose/D-glucose-binding protein from E. Coli localized in its active center does not contribute to the change in intrinsic fluorescence upon glucose binding.

Changes of the characteristics of intrinsic tryptophan fluorescence of the wild type of D-galactose/D-glucose-binding protein from Escherichia coli (GGBPwt) induced by D-glucose binding were examined by the intrinsic UV-fluorescence of proteins, circular dyhroism in the near-UV region, and acrylamide-induced fluorescence quenching. The analysis of the different characteristics of GGBPwt and its mutant form GGBP-W183A together with the analysis of the microenvironment of tryptophan residues of GGBPwt revealed that Trp 183, which is directly involved in sugar binding, has the least influence on the provoked by D-glucose blue shift and increase in the intensity of protein intrinsic fluorescence in comparison with other tryptophan residues of GGBP.

The initial noncovalent binding of glucose to human hemoglobin in nonenzymatic glycation

Mechanisms for nonenzymatic protein glycation have been extensively studied albeit with an emphasis at the later stages that gives rise to advanced glycation end products. No detailed investigation of the initial, noncovalent binding of d-glucose to human hemoglobin A (HbA) exists in the literature. Although anionic molecules 2,3-bisphosphoglycerate (BPG), inorganic phosphate (Pi) and HCO3(-) have been implicated in the latter stages of glycation, their involvement at the initial binding of glucose to HbA has not yet been assessed. Results from this computational study involving crystal structures of HbA predict that the transient, ring-opened glucose isomer, assumed to be critical in the later stages of glycation, is not directly involved in initial binding to the β-chain of HbA. All the five structures of glucose generated upon mutorotation will undergo reversible, competitive and slow binding at multiple amino acid residues. The ring-opened structure is most likely generated from previously bound pyranoses that undergo mutarotation while bound. BPG, Pi and HCO3(-) also reversibly bind to HbA with similar energies as glucose isomers (~3-5 kcal/mol) and share common binding sites with glucose isomers. However, there was modest amino acid residue selectivity for binding of certain anionic molecules (1-3 regions) but limited selectivity for glucose structures (≥ 7 regions). The clinical difference between average blood glucose and predicted HbA1c, and the presence of unstable HbA-glucose complexes may be more fully explained by initial noncovalent binding interactions and different concentrations of BPG, Pi and HCO3(-) in serum vs. erythrocytes.

Ortho-Substituted fluorescent aryl monoboronic acid displays physiological binding of d-glucose

An ortho-fluorinated phenylboronic acid incorporated into the BODIPY-fluorophore, sensor 1, exhibits a significant response in emission intensity upon binding of d-glucose and d-fructose. This sensor displays a desired binding strength toward d-glucose at the physiological level, that is Kd values between 10 and 20mM. This binding strength of sensor 1 has been shown to be independent of the employed buffer, that is, saline buffer, 50mM phosphate buffer, and 50mM phosphate buffer in 52.3 w/w% MeOH. The d-glucose binding strength and the fluorescence response of a BODIPY-based ortho-methylated phenylboronic acid, sensor 2, show a significantly decreased d-fructose binding affinity. This sensor also exhibits buffer-dependent binding strength and fluorescence response upon binding of d-glucose and d-fructose.

Identification of a Catalytic Base for Sugar Oxidation in the Pyranose 2‐Oxidase Reaction

Pyranose 2-oxidase (P2O) catalyzes the oxidation of aldopyranoses to form 2-keto sugars and H(2)O(2) . In this study, the mechanistic role of the conserved residues His548 and Asn593 in P2O was investigated by using site-directed mutagenesis, transient kinetics, and pH-dependence studies. As single mutants of H548 resulted in mixed populations of noncovalently bound and covalently linked FAD, double mutants containing H167A were constructed, in which the covalent histidyl-FAD linkage was removed in addition to having the H548 mutation. Single mutants H548A, H548N, H548S, H548D and double mutants (with H167A) could not be reduced by D-glucose. For the H167A/H548R mutant, the flavin could be reduced by D-glucose with the reduction rate constant about 220 times lower than that of the H167A mutant. The pH-dependence studies of H167A/H548R indicated that the rate constant of flavin reduction increased about 360-fold upon a pH rise corresponding to pK(a) >10.1, whereas the reactions of the wild-type and H167A mutant enzymes were pH independent. Therefore, the data suggest that a pK(a) value of >10.1 in the mutant enzyme is associated with the Arg548 residue, and that this residue must be unprotonated to efficiently catalyze flavin reduction. The data imply that for the wild-type P2O, the conserved His548 should be unprotonated in the pH range studied. The unprotonated His548 can act as a general base to abstract the 2-hydroxyl proton of D-glucose and initiate hydride transfer from the substrate to the flavin. Studies of the single mutant N593H showed that the flavin reduction rate constant was 114 times lower than that of the wild-type enzyme and was pH independent, while the K(d) for D-glucose binding was 19 times greater.

Hexose Transporter GLUT1 Harbors Several Distinct Regulatory Binding Sites for Flavones and Tyrphostins

The facilitative hexose transporter GLUT1 activity is blocked by tyrosine kinase inhibitors that include natural products such as flavones and isoflavones and synthetic compounds such as tyrphostins, molecules that are structurally unrelated to the transported substrates [Vera, et al. (2001) Biochemistry, 40, 777-790]. Here we analyzed the interaction of GLUT1 with quercetin (a flavone), genistein (an isoflavone), and tyrphostin A47 and B46 to evaluate if they share one common or have several binding sites on the protein. Kinetic assays showed that genistein, quercetin, and tyrphostin B46 behave as competitive inhibitors of equilibrium exchange and zero-trans uptake transport and noncompetitive inhibitors of net sugar exit out of human red cells, suggesting that they interact with the external surface of the GLUT1 molecule. In contrast, tyrphostin A47 was a competitive inhibitor of equilibrium exchange and zero-trans exit transport and a noncompetitive inhibitor of net sugar entry into red cells, suggesting that it interacts with the cytoplasmic surface of the transporter. Genistein protected GLUT1 against iodide-elicited fluorescence quenching and also decreased the affinity of d-glucose for its external binding site, while quercetin and tyrphostins B46 and A47 promoted fluorescence quenching and did not affect the external d-glucose binding site. These findings are explained by a carrier that presents at least three binding sites for tyrosine kinase inhibitors, in which (i) genistein interacts with the transporter in a conformation that binds glucose on the external surface (outward-facing conformation), in a site which overlaps with the external binding site for d-glucose, (ii) quercetin and tyrphostin B46 interact with the GLUT1 conformation which binds glucose by the internal side of the membrane (inward-facing conformation), but to a site accessible from the external surface of the protein, and (iii) the binding site for tyrphostin A47 is accessible from the inner surface of GLUT1 by binding to the inward-facing conformation of the transporter. These data provide groundwork for a molecular understanding of how the tyrosine kinase inhibitors directly affect glucose transport in animal cells.

In vitro galactation of human serum albumin: Analysis of the protein’s galactation sites by mass spectrometry

The posttranslational modification of proteins by sugars has been demonstrated in diabetes and classical galactosemia. In diabetes, the glycation process occurs as a result of d-glucose nonenzymatically reacting with proteins such as albumin and hemoglobin, used today as important tools to monitor the efficiency of dietary control and therapy during treatment of diabetes. In classical galactosemia, d-galactose contributes to the formation of glycated proteins as well, suggesting that, akin to diabetes with glucated proteins, the monitoring of galactated proteins may facilitate management of patients with galactosemia. The objectives of this study were (i) to galactate human serum albumin (HSA) in vitro; (ii) to determine, by a sodium borohydride-dependent mass peptide mapping method, the galactation sites in HSA; and (iii) to compare HSA’s galactation sites with the protein’s reported glucation sites. Treatment of galactated HSA with sodium borohydride stabilized the condensed sugars on the protein and yielded discrete fragmentation patterns by tandem mass spectrometry, allowing reliable identification of HSA’s galactation sites. Liquid chromatography/electrospray ionization/mass spectrometry, in combination with tandem mass spectrometry, revealed that the principal sites of galactation in HSA were the ε-amino groups of lysine residues 12, 233, 281/276, 414, and 525. Lysyl residues 12, 233, 276, and 525 were previously reported as privileged sites for the nonenzymatic binding of d-glucose with HSA.

A Conserved Active-site Threonine Is Important for Both Sugar and Flavin Oxidations of Pyranose 2-Oxidase

Pyranose 2-oxidase (P2O) catalyzes the oxidation by O(2) of d-glucose and several aldopyranoses to yield the 2-ketoaldoses and H(2)O(2). Based on crystal structures, in one rotamer conformation, the threonine hydroxyl of Thr(169) forms H-bonds to the flavin-N5/O4 locus, whereas, in a different rotamer, it may interact with either sugar or other parts of the P2O.sugar complex. Transient kinetics of wild-type (WT) and Thr(169) --> S/N/G/A replacement variants show that D-Glc binds to T169S, T169N, and WT with the same K(d) (45-47 mm), and the hydride transfer rate constants (k(red)) are similar (15.3-9.7 s(-1) at 4 degrees C). k(red) of T169G with D-glucose (0.7 s(-1), 4 degrees C) is significantly less than that of WT but not as severely affected as in T169A (k(red) of 0.03 s(-1) at 25 degrees C). Transient kinetics of WT and mutants using d-galactose show that P2O binds d-galactose with a one-step binding process, different from binding of d-glucose. In T169S, T169N, and T169G, the overall turnover with d-Gal is faster than that of WT due to an increase of k(red). In the crystal structure of T169S, Ser(169) O gamma assumes a position identical to that of O gamma 1 in Thr(169); in T169G, solvent molecules may be able to rescue H-bonding. Our data suggest that a competent reductive half-reaction requires a side chain at position 169 that is able to form an H-bond within the ES complex. During the oxidative half-reaction, all mutants failed to stabilize a C4a-hydroperoxyflavin intermediate, thus suggesting that the precise position and geometry of the Thr(169) side chain are required for intermediate stabilization.

Defining topological features of membrane proteins by nanoelectrospray ionisation mass spectrometry

The D-galactose-H(+) symport protein, GalP, of Escherichia coli is the bacterial homologue of the human glucose transport protein, GLUT1. Here we demonstrate that mass spectrometry can be used to map modification by covalently bound reagents, and also to detect structural changes in the GalP protein that occur upon substrate binding. The small thiol-group-specific reagent N-ethylmaleimide (NEM) was used to modify the cysteine residues in GalP(His)(6) both alone and in the presence of D-glucose, a known substrate. Employing a mixture of proteolysis and thermal degradation methods, the three cysteine residues were found to undergo sequential reactions with NEM, with Cys374 being modified first, followed by Cys389 and finally Cys19, thus indicating their different accessibilities within the three-dimensional structure of the protein. Prior binding of the substrate D-glucose to the protein protected Cys19 and Cys374 against NEM modification, but not Cys389. Cys374 had been expected to be shielded by D-glucose binding while Cys389 had been expected to be unaffected, consistent with their proposed respective locations in the vicinity of, and distant from, the sugar binding site. However, the inaccessibility of Cys19 was unexpected and suggests a structural change in the protein promoted by D-glucose binding which changes the proximity of Cys19 with respect to the D-glucose-binding site.

Structure and stability of D-galactose/D-glucose-binding protein. The role of D-glucose binding and Ca ion depletion

The effects of guanidine hydrochloride (GdnHCl) on the structure and stability of the D-galactose/D-glucose-binding protein from Escherichia coli (GGBP) and its complex with D-glucose (GGBP/Glc) were investigated by intrinsic protein fluorescence and far-UV circular dichroism (CD). The role of calcium in the stability of the protein structure was also studied. It was shown that the processes of GGBP and GGBP/Glc unfolding induced by GdnHCl followed one-step reversible denaturation mechanism. The obtained data showed that the binding of glucose to GGBP resulted in an increase of the protein stability towards the actions of the GdnHCl which made protein unfolding more cooperative. The stabilities of GGBP alone, GGBP in the presence of glucose, GGBP-depleted calcium (GGBP-Ca), and GGBP/Glc-depleted calcium (GGBP/Glc-Ca) were characterized by difference of Gibbs free energies.

Structure and stability of D-galactose/D-glucose-binding protein. The role of D-glucose binding and Ca ion depletion

The effects of guanidine hydrochloride (GdnHCl) on the structure and stability of the D-galactose/D-glucose-binding protein fromEscherichia coli(GGBP) and its complex with D-glucose (GGBP/Glc) were investigated by intrinsic protein fluorescence and far-UV circular dichroism (CD). The role of calcium in the stability of the protein structure was also studied. It was shown that the processes of GGBP and GGBP/Glc unfolding induced by GdnHCl followed one-step reversible denaturation mechanism. The obtained data showed that the binding of glucose to GGBP resulted in an increase of the protein stability towards the actions of the GdnHCl which made protein unfolding more cooperative. The stabilities of GGBP alone, GGBP in the presence of glucose, GGBP-depleted calcium (GGBP-Ca), and GGBP/Glc-depleted calcium (GGBP/Glc-Ca) were characterized by difference of Gibbs free energies.

Analysis of Glucose Transporter Topology and Structural Dynamics

Homology modeling and scanning cysteine mutagenesis studies suggest that the human glucose transport protein GLUT1 and its distant bacterial homologs LacY and GlpT share similar structures. We tested this hypothesis by mapping the accessibility of purified, reconstituted human erythrocyte GLUT1 to aqueous probes. GLUT1 contains 35 potential tryptic cleavage sites. Fourteen of 16 lysine residues and 18 of 19 arginine residues were accessible to trypsin. GLUT1 lysine residues were modified by isothiocyanates and N-hydroxysuccinimide (NHS) esters in a substrate-dependent manner. Twelve lysine residues were accessible to sulfo-NHS-LC-biotin. GLUT1 trypsinization released full-length transmembrane helix 1, cytoplasmic loop 6-7, and the long cytoplasmic C terminus from membranes. Trypsin-digested GLUT1 retained cytochalasin B and d-glucose binding capacity and released full-length transmembrane helix 8 upon cytochalasin B (but not D-glucose) binding. Transmembrane helix 8 release did not abrogate cytochalasin B binding. GLUT1 was extensively proteolyzed by alpha-chymotrypsin, which cuts putative pore-forming amphipathic alpha-helices 1, 2, 4, 7, 8, 10, and 11 at multiple sites to release transmembrane peptide fragments into the aqueous solvent. Putative scaffolding membrane helices 3, 6, 9, and 12 are strongly hydrophobic, resistant to alpha-chymotrypsin, and retained by the membrane bilayer. These observations provide experimental support for the proposed GLUT1 architecture; indicate that the proposed topology of membrane helices 5, 6, and 12 requires adjustment; and suggest that the metastable conformations of transmembrane helices 1 and 8 within the GLUT1 scaffold destabilize a sugar translocation intermediate.

Sugar binding to recombinant wild-type and mutant glucokinase monitored by kinetic measurement and tryptophan fluorescence

Tryptophan fluorescence was used to study GK (glucokinase), an enzyme that plays a prominent role in glucose homoeostasis which, when inactivated or activated by mutations, causes diabetes mellitus or hypoglycaemia in humans. GK has three tryptophan residues, and binding of D-glucose increases their fluorescence. To assess the contribution of individual tryptophan residues to this effect, we generated GST-GK [GK conjugated to GST (glutathione transferase)] and also pure GK with one, two or three of the tryptophan residues of GK replaced with other amino acids (i.e. W99C, W99R, W167A, W167F, W257F, W99R/W167F, W99R/W257F, W167F/W257F and W99R/W167F/W257F). Enzyme kinetics, binding constants for glucose and several other sugars and fluorescence quantum yields (varphi) were determined and compared with those of wild-type GK retaining its three tryptophan residues. Replacement of all three tryptophan residues resulted in an enzyme that retained all characteristic features of GK, thereby demonstrating the unique usefulness of tryptophan fluorescence as an indicator of GK conformation. Curves of glucose binding to wild-type and mutant GK or GST-GK were hyperbolic, whereas catalysis of wild-type and most mutants exhibited co-operativity with D-glucose. Binding studies showed the following order of affinities for the enzyme variants: N-acetyl-D-glucosamine>D-glucose>D-mannose>D-mannoheptulose>2-deoxy-D-glucose>>L-glucose. GK activators increased sugar binding of most enzymes, but not of the mutants Y214A/V452A and C252Y. Contributions to the fluorescence increase from Trp(99) and Trp(167) were large compared with that from Trp(257) and are probably based on distinct mechanisms. The average quantum efficiency of tryptophan fluorescence in the basal and glucose-bound state was modified by activating (Y214A/V452A) or inactivating (C213R and C252Y) mutations and was interpreted as a manifestation of distinct conformational states.

Relative substrate affinities of wild-type and mutant forms of theEscherichia colisugar transporter GalP determined by solid-state NMR

Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is used for the first time to examine the relative substrate-binding affinities of mutant forms of the Escherichia coli sugar transporter GalP in membrane preparations. The SSNMR method of (13)C cross-polarization magic-angle spinning (CP-MAS) is applied to five site-specific mutants (W56F, W239F, R316W, T336Y and W434F), which have a range of different sugar-transport activities compared to the wild-type protein. It is shown that binding of the substrate D-glucose can be detected independently of sugar transport activity using SSNMR, and that the NMR peak intensities for uniformly (13)C-labelled glucose are consistent with wild-type GalP and the mutants having different affinities for the substrate. The W239F and W434F mutants showed binding affinities similar to that of the wild-type protein, whereas the affinity of glucose-binding to the W56F mutant was reduced. The R316W mutant showed no detectable binding; this position corresponds to the second basic residue in the highly conserved (R/K)XGR(R/K) motif in the major facilitator superfamily of transport proteins and to a mutation in human GLUT1 found in individuals with GLUT1-deficiency syndrome. The T336Y mutant also showed no detectable binding; this mutation is likely to have perturbed helix structure or packing to an extent that conformational changes in the protein are hindered. The effects of the mutations on substrate-binding are discussed with reference to the putative positions of the residues in a 3D homology model of GalP based on the X-ray crystal structure of the E. coli glycerol-3-phosphate transporter GlpT.

Substrate Specificity of Sugar Transport by Rabbit SGLT1: Single-Molecule Atomic Force Microscopy versus Transport Studies

In the apical membrane of epithelial cells from the small intestine and the kidney, the high-affinity Na+/d-glucose cotransporter SGLT1 plays a crucial role in selective sugar absorption and reabsorption. How sugars are selected at the molecular level is, however, poorly understood. Here atomic force microscopy (AFM) was employed to investigate the substrate specificity of rbSGLT1 on the single-molecule level, while competitive-uptake assays with isotope-labeled sugars were performed in the study of the stereospecificity of the overall transport. rbSGLT1-transfected Chinese hamster ovary (CHO) cells were used for both approaches. Evidence of binding of d-glucose to the extracellular surface of rbSGLT1 could be obtained using AFM tips carrying 1-thio-d-glucose coupled at the C1 position to a PEG linker via a vinylsulfon group. Competition experiments with monosaccharides in solution revealed the following selectivity ranking of binding: 2-deoxy-d-glucose >or= 6-deoxy-d-glucose > d-glucose > d-galactose >or= alpha-methyl glucoside; 3-deoxy-d-glucose, d-xylose, and l-glucose did not measurably affect binding. These results were different from those of competitive alpha-methyl glucoside transport assays, where the ranking of inhibition was as follows: d-glucose > d-galactose > 6-deoxy-d-glucose; no uptake inhibition by d-xylose, 3-deoxy-d-glucose, 2-deoxy-d-glucose, or l-glucose was observed. Taken together, these results suggest that the substrate specificity of SGLT1 is determined by different recognition sites: one possibly located at the surface of the transporter and others located close to or within the translocation pathway.