Examination of substrate-induced conformational changes in the Na+/glucose cotransporter

Na(+)-independent glucose utilization during Mn(2+)-induced contraction in ileal longitudinal smooth muscle

In Ca(2+)- and Na(+)-deficient, isotonic 126 mM K+ medium, addition of 5 mM Mn2+ caused a tension about 2.5 x greater than the tonic response induced by 126 mM K+ medium (Ca2+ 2.5 mM, Na+ 0 mM) in ileal muscle. When glycogen was depleted by incubation in a glucose-free, hypertonic 60 mM K+ medium, addition of 5 mM Mn2+ induced only a very weak tension in Ca(2+)-free, isotonic 126 K+ medium. Phlorizin (10(-3) M), a blocker of Na(+)-coupled glucose cotransporter and ouabain (9 x 10(-5) M), an inhibitor of Na+, K(+)-ATPase, failed to inhibit the tension elicited by 5 mM Mn2+ in a Ca(2+)- and Na(+)-deficient, isotonic 126 mM K+ medium. Mn2+ was accumulated in the intracellular compartment in a Ca(2+)- and Na(+)-deficient, isotonic 126 mM K+ medium. The tissue ATP concentration was significantly reduced in a Na(+)-deficient 126 mM K+ medium. However, it recovered almost completely when 5 mM Mn2+ was added to the isotonic 126 mM K+ medium. These results suggest that the Mn(2+)-induced contraction in depolarized ileal longitudinal muscle in Na(+)-deficient medium may be maintained by a glucose transport which is not dependent on Na+ and insensitive to phlorizin.

Effect of substrates and pH on the intestinal Na+/phosphate cotransporter: evidence for an intervesicular divalent phosphate allosteric regulatory site

Intervesicular divalent phosphate-induced inhibition of the intestinal brush-border membrane Na+/phosphate cotransporter was examined using Na(+)-dependent phosphate uptake, substrate-induced tryptophan fluorescence quenching, and the apparent pKa values for substrate-induced conformational changes. In right-side-out (RSO) reconstituted proteoliposomes, only monovalent phosphate inhibited Na(+)-dependent phosphate uptake in the absence of pre-equilibration. Addition of divalent phosphate to inside-out (ISO) proteoliposomes resulted in 80 +/- 5% inhibition of Na(+)-dependent phosphate uptake in the absence of pre-equilibration. The nature of divalent phosphate-induced inhibition of cotransporter function was examined using cotransporter partial reaction assays based on substrate-induced conformational changes reported as changes in tryptophan fluorescence. Na+ but not K+ induced a quenching of tryptophan fluorescence with a K0.5 of 25 mM and an apparent Hill coefficient of 1.8. Monovalent phosphate (difluorophosphate) induced a further quenching of tryptophan fluorescence with a K0.5 of 53 microM. Divalent phosphate (monofluorophosphate) had no effect on tryptophan fluorescence, but inhibited the difluorophosphate-induced quenching of tryptophan fluorescence. The Na+ to Na++ divalent phosphate (monofluorophosphate) conformation and the Na+ to Na++ monovalent phosphate (difluorophosphate) conformations were compared using tryptophan quench reagents. These transitions had different apparent pKa values and different phenylglyoxal sensitivities consistent with monovalent phosphate and divalent phosphate interacting with the cotransporter at separate sites.

Melibiose permease of Escherichia coli: substrate-induced conformational changes monitored by tryptophan fluorescence spectroscopy

Tryptophan fluorescence spectroscopy has been used to investigate the effects of sugars and coupling cations (H+, Na+, or Li+) on the conformational properties of purified melibiose permease after reconstitution in liposomes. Melibiose permease emission fluorescence is selectively enhanced by sugars, which serve as substrates for the symport reaction, alpha-galactosides producing larger variations (13-17%) than beta-galactosides (7%). Moreover, the sugar-dependent fluorescence increase is specifically potentiated by NaCl and LiCl (5-7 times), which are well-established activators of sugar binding and transport by the permease. The potentiation effect is greater in the presence of LiCl than NaCl. On their own, sodium and lithium ions produce quenching of the fluorescence signal (2%). Evidence suggesting that sugars and cations compete for their respective binding sites is also given. Both the sugar-induced fluorescence variation and the NaCl(or LiCl)-dependent potentiation effect exhibit saturation kinetics. In each ionic condition, the half-maximal fluorescence change is found at a sugar concentration corresponding to the sugar-binding constant. Also, half-maximal potentiation of the fluorescence change by sodium or lithium occurs at a concentration comparable to the activation constant of sugar binding by each ion. The sugar- and ion-dependent fluorescence variations still take place after selective inactivation of the permease substrate translocation capacity by N-ethylmaleimide. Taken together, the data suggest that the changes in permease fluorescence reflect conformational changes occurring upon the formation of ternary sugar/cation/permease complexes.

Kinetic characterization of Na+/D-mannose cotransport in dog kidney: comparison with Na+/D-glucose cotransport

Brush-border membrane vesicles (BBMV) prepared from whole dog kidney cortex, or separately from outer cortex (OC) and outer medulla (OM), were used to study the kinetics and inhibition specificity of Na(+)-dependent D-mannose cotransport. In BBMV from whole cortex the measured parameters for Na+/D-mannose uptake were Km = 0.07 +/- 0.01 mM and Vmax = 4.19 +/- 0.24 nmol/mg protein per min (n = 36). In OC BBMV the Km for Na+/D-mannose was 0.04 mM, Vmax = 3.41 nmol/mg per min. In OM the Km was 0.06 +/- 0.02 mM Vmax = 0.18 nmol/mg per min. Thus only about 5% of Na+/D-mannose activity occurs in OM. Both mannoheptulose (Ki = 5.6 mM) and methyl alpha-D-mannoside (Ki = 0.05 mM) are competitive inhibitors of Na+/D-mannose uptake, but at comparable concentrations have little effect on Na+/D-glucose uptake. Phlorizin is a noncompetitive inhibitor of Na+/D-mannose uptake (Ki = 4.45 microM) but a more potent and competitive inhibitor (Ki = 0.58 microM) of Na+/D-glucose uptake. Phloretin (Ki = 104 microM) is a noncompetitive inhibitor of Na+/D-mannose uptake in BBMV. We conclude that Na+/D-mannose uptake is mediated by a unique high-affinity carrier located in the OC presumably at the luminal surface of the proximal convoluted tubule, with strong specificity requirements for sugars with mannose-like structures (i.e., axial C-2 hydroxyl group). Phlorizin is an inhibitor of both Na+/D-mannose and Na+/D-glucose cotransporters but is approx. 10 times less potent for the Na+/D-mannose system and also has a different mode of inhibition (i.e., noncompetitive vs. competitive). The different phlorizin inhibitory mechanisms on the Na+/D-glucose and Na+/D-mannose cotransporters may be mediated by distinct hydrophobic and sugar binding sites that characterize phlorizin-carrier interaction.

Coupling mechanisms in active transport

In primary and secondary active transport, the mobility and specificity of the carrier are controlled, over the course of the transport reaction, in accordance with a set of 'rules'. The rules are shown to depend on two mechanisms: a substrate--either the driving substrate (a transported ion or ATP) or the driven substrate--may shift a conformational equilibrium or accelerate a rate-limiting conformational change. From an analysis of coupling mechanisms the following conclusions emerge. (i) The ratio of coupled to uncoupled flux, which should be large, cannot be greater than the ratio of substrate dissociation constants in an initial complex and a conformationally altered state. A minimum value for the increased binding force can be estimated from steady-state constants. (ii) In an ordered mechanism, slippage is expected at high concentrations of the substrate adding to the carrier second, while slippage of the first substrate should remain low. (iii) Slippage in coupled transport is minimized if the driven substrate is last on in loading the carrier and last off in unloading, while the reverse order makes the affinity high in loading and low in unloading, as required for efficient transfer from one compartment to another; hence the preferred mechanism may depend on prevailing physiological conditions. (iv) A coupled transport system can be transformed into a facilitated system for one substrate or both if the control of carrier mobility is undermined through modification of the carrier.