Energy coupling in secondary active transport

Bacterial Resistance to Antimicrobial Agents

Bacterial pathogens as causative agents of infection constitute an alarming concern in the public health sector. In particular, bacteria with resistance to multiple antimicrobial agents can confound chemotherapeutic efficacy towards infectious diseases. Multidrug-resistant bacteria harbor various molecular and cellular mechanisms for antimicrobial resistance. These antimicrobial resistance mechanisms include active antimicrobial efflux, reduced drug entry into cells of pathogens, enzymatic metabolism of antimicrobial agents to inactive products, biofilm formation, altered drug targets, and protection of antimicrobial targets. These microbial systems represent suitable focuses for investigation to establish the means for their circumvention and to reestablish therapeutic effectiveness. This review briefly summarizes the various antimicrobial resistance mechanisms that are harbored within infectious bacteria.

Rhizosphere pH and cation-anion balance determine the exudation of nitrification inhibitor 3-epi-brachialactone suggesting release via secondary transport

Biological nitrification inhibition (BNI) of Brachiaria humidicola has been attributed to nitrification-inhibiting fusicoccanes, most prominently 3-epi-brachialactone. However, its release mechanism from B. humidicola roots remains elusive. Two hydroponic experiments were performed to investigate the role of rhizosphere pH and nutritional N form in regulating 3-epi-brachialactone release by B. humidicola and verify the underlying release pathway. Low rhizosphere pH and NH₄ ⁺ nutrition promoted 3-epi-brachialactone exudation. However, the substitution of NH₄ ⁺ by K⁺ revealed that the NH₄ ⁺ effect was not founded in a direct physiological response to the N form but was related to the cation-anion balance during nutrient uptake. Release of 3-epi-brachialactone correlated with the transmembrane proton gradient ΔpH and NH₄ ⁺ uptake (R² = 0.92 for high ~6.8 and R² = 0.84 for low ~4.2 trap solution pH). This corroborated the release of 3-epi-brachialactone through secondary transport, with the proton motive force (ΔP) defining transport rates across the plasma membrane. It was concluded that 3-epi-brachialactone release cannot be conceptualized as a regulated response to soil pH or NH₄ ⁺ availability, but merely as the result of associated changes in ΔP.

Functional and Structural Roles of the Major Facilitator Superfamily Bacterial Multidrug Efflux Pumps

Pathogenic microorganisms that are multidrug-resistant can pose severe clinical and public health concerns. In particular, bacterial multidrug efflux transporters of the major facilitator superfamily constitute a notable group of drug resistance mechanisms primarily because multidrug-resistant pathogens can become refractory to antimicrobial agents, thus resulting in potentially untreatable bacterial infections. The major facilitator superfamily is composed of thousands of solute transporters that are related in terms of their phylogenetic relationships, primary amino acid sequences, two- and three-dimensional structures, modes of energization (passive and secondary active), and in their mechanisms of solute and ion translocation across the membrane. The major facilitator superfamily is also composed of numerous families and sub-families of homologous transporters that are conserved across all living taxa, from bacteria to humans. Members of this superfamily share several classes of highly conserved amino acid sequence motifs that play essential mechanistic roles during transport. The structural and functional importance of multidrug efflux pumps that belong to the major facilitator family and that are harbored by Gram-negative and -positive bacterial pathogens are considered here.

The role of chloride ions on the transport of glycine in plasma membrane vesicles from glial cells

The high-affinity transport system for glycine in plasma membrane vesicles from C6 glioma cells is dependent on Na+ and also on the presence of Cl- in the incubation medium. This anion requirement is relatively specific for Cl-, since other anions are also capable of stimulating the glycine transport in the following order of decreasing efficacy: Cl- greater than Br- greater than SCN- congruent to I- greater than NO3- greater than F-. Chloride ions raise the Vmax for transport and, to a lesser extent, act on the Km. The data provided by direct measurements of the coupling of sodium and chloride to the transport of glycine by using a kinetic approach suggest a stoichiometry for the translocation cycle catalyzed by the glycine transporter of two sodium ions and one chloride ion per glycine zwitterion.

Evidence for electrogenic accumulation of mefloquine by malarial parasites

The uptake of mefloquine and chloroquine by Plasmodium chabaudi-infected mouse erythrocytes was measured in the presence and absence of ionophores and uncoupler in order to distinguish between the pH-dependent and pH-independent absorption of these drugs. Nigericin and CCCP (carbonylcyanide m-chlorophenylhydrazone) were used to relax the proton gradients and electrical potentials across the membranes. It was found that 40-60% of the mefloquine uptake, and 90% of the chloroquine uptake, was pH-dependent, the remainder being due to passive binding to cellular constituents. The distribution ratio of the pH-dependent uptake for mefloquine was about three times greater than for chloroquine. According to the lysosomotropic weak base hypothesis in which the neutral forms of weak bases are assumed to equilibrate across membranes, the mefloquine distribution should be smaller than the chloroquine distribution: since mefloquine is singly charged and chloroquine is doubly charged, the chloroquine distribution ratio should vary as the square of the mefloquine ratio. We interpret the greater uptake ratio of mefloquine to be evidence for the involvement of secondary active transport, with drug uptake being coupled to proton outflow by an antiporter protein. It is proposed that the uptake of mefloquine is electrogenic, with the proton gradient and the electrical potential both contributing to the driving force, but that the proton gradient alone is responsible for the chloroquine uptake.

Lysine and arginine transport in the photosynthetic bacterium Chromatium vinosum

The photosynthetic purple sulfur bacterium Chromatium vinosum can take up both arginine and lysine in the light and, to a lesser extent, in the dark. Competitive inhibition experiments suggest the likely presence of two transport systems in this bacterium: One capable of transporting either lysine or arginine and a second capable of transporting arginine but not lysine. Uptake of both amino acids is electrogenic and appears to involve the cotransport of neither protons nor sodium ions. It is suggested that the transport occurs via an electrogenic uniport.

Highly permeant anions and glucose uptake as an alternative for quantitative generation and estimation of membrane potential differences in brush-border membrane vesicles

We have analyzed the combined utilization of highly permeant anions to induce membrane diffusion potentials and glucose uptake to probe the created potentials as a new approach to quantitative generation and estimation of membrane potential differences in vesicle studies. Rabbit jejunal brush-border membrane vesicles were used in our experiments so that membrane potential differences can be calculated from the Goldman-Hodgkin-Katz equation with the relative ion permeabilities recently reported for this preparation (Gunther, R.D., Schell, R.E. and Wright, E.M. (1984) J. Membrane Biol. 78, 119-127) or approximated by the Nernst potential for the anion. Iodide was selected as the highly permeant anion after showing its absence of effect on glucose uptake with equal concentrations of Na+ inside and outside the vesicles and the membrane potential clamped to zero with gramicidin D. Membrane potential was varied by altering the intra- and extravesicular iodide concentrations while keeping isosmolarity and isotonicity constant by chloride replacement. In these conditions, glucose uptake was sensitive and correlated to the expected membrane potentials. Moreover, a linear relationship between the log initial rate of glucose transport and membrane potential differences could be established. This linear relationship was quite insensitive to inside replacement of choline by potassium and to pH variations in the incubation medium, thus showing the reproducibility and the versatility of the method and the adequacy of glucose uptake as a probe for membrane potentials. However, no information can be gained on the stoichiometry of the Na+-glucose transporter as the slope of the straight line depends on both the charge carried by the fully loaded carrier and the point in the electric field at which the transition state of the carrier from cis to trans occurs. This new approach was compared with the more conventional one using valinomycin-induced K+-diffusion potentials and the Nernst potential for potassium as means for creating and estimating membrane potential differences. Both techniques were not equivalent, as linear relationships showing smaller slopes and sensitivity to pH were recorded with the latter. These differences are compatible with a potassium permeability in the presence of valinomycin that is lower than generally assumed, at least when compared to the permeability of the other ions present in the incubation medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Interpretation of steady-state current-voltage curves: consequences and implications of current subtraction in transport studies

A problem often confronted in analyses of charge-carrying transport processes in vivo lies in identifying porter-specific component currents and their dependence on membrane potential. Frequently, current-voltage (I-V)--or more precisely, difference-current-voltage (dI-V)--relations, both for primary and for secondary transport processes, have been extracted from the overall membrane current-voltage profiles by subtracting currents measured before and after experimental manipulations expected to alter the porter characteristics only. This paper examines the consequences of current subtraction within the context of a generalized kinetic carrier model for Class I transport mechanisms (U.-P. Hansen, D. Gradmann, D. Sanders and C.L. Slayman, 1981, J. Membrane Biol. 63:165-190). Attention is focused primarily on dI-V profiles associated with ion-driven secondary transport for which external solute concentrations usually serve as the experimental variable, but precisely analogous results and the same conclusions are indicated in relation to studies of primary electrogenesis. The model comprises a single transport loop linking n (3 or more) discrete states of a carrier 'molecule.' State transitions include one membrane charge-transport step and one solute-binding step. Fundamental properties of dI-V relations are derived analytically for all n-state formulations by analogy to common experimental designs. Additional features are revealed through analysis of a "reduced" 2-state empirical form, and numerical examples, computed using this and a "minimum" 4-state formulation, illustrate dI-V curves under principle limiting conditions. Class I models generate a wide range of dI-V profiles which can accommodate essentially all of the data now extant for primary and secondary transport systems, including difference current relations showing regions of negative slope conductance. The particular features exhibited by the curves depend on the relative magnitudes and orderings of reaction rate constants within the transport loop. Two distinct classes of dI-V curves result which reflect the relative rates of membrane charge transit and carrier recycling steps. Also evident in difference current relations are contributions from 'hidden' carrier states not directly associated with charge translocation in circumstances which can give rise to observations of counterflow or exchange diffusion. Conductance-voltage relations provide a semi-quantitative means to obtaining pairs of empirical rate parameters.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental analysis of ion/solute cotransport by substrate binding and facilitated diffusion

An ion/solute cotransporter can be studied in the absence of a transmembrane gradient of the electrochemical potential of the ion. Inspection of the appropriate equations discloses that basic parameters of the cotransport cycle can be obtained by measuring cosubstrate binding and the initial-velocity kinetics of four modes of facilitated diffusion as a function of the concentration of the cotransported ion. The following information can be derived: estimates of the affinities of both cosubstrates, the number of binary intermediates participating in cotransport (equivalent to determining the order of cosubstrate binding and release), and the rate constants for the reorientation of the binding sites during cotransport. In general, both maximal velocities and half-saturation constants for the facilitated diffusion of one cosubstrate depend upon the concentration of the other. In some cases, the maximal velocities of influx and efflux do not increase monotonically with the concentration of the ion but pass through a maximum and decrease. If direct binding studies are not possible, affinities of the cosubstrates can be estimated from data for equilibrium exchange or countertransport. Also, an approximate description of the time course of the transient accumulation (overshoot) during countertransport is derived. Under certain circumstances, the height of the overshoot is proportional to the concentration of the cotransported ion. Thus, countertransport can be employed as a simple test to establish if a solute is cotransported with a particular ion. This treatment allows many effects noted in galactoside countertransport in Escherichia coli to be explained.

Osmotic effects on bacterial transport and energetics

Paracoccus denitrificans suspended in media containing 20-300 mM NaCl swelled progressively as the salt concentration was decreased. The increase in intracellular water volume was accompanied by an enhancement of respiration and a stimulation of the rates of net potassium and alpha-aminoisobutyric acid accumulation. It is postulated that influx of water and consequent lowering of intracellular solute concentration trigger transport mechanisms which are destined to restore the original ion and metabolite balance. Since a number of transport reactions operate against the electrochemical gradient of their substrates, energy utilization increases. The increased ATP usage and lowering of [ATP] stimulates the activity of the respiratory chain and increases oxygen uptake and energy production.

The static head method for determining the charge stoichiometry of coupled transport systems. Applications to the sodium-coupled D-glucose transporters of the renal proximal tubule

The static head method for determining the charge stoichiometry (the number of moles of charge translocated per mole of substrate) of a coupled transport system is presented. The method involves establishing experimental conditions under which a membrane potential exactly balances the thermodynamic driving force of a known substrate gradient. The charge stoichiometry can then be calculated from thermodynamic principles. In contrast to the usual steady-state method for determining charge stoichiometry in cell suspensions and vesicle preparations, the static head method is applicable to systems which are not capable of maintaining a constant membrane potential over time. The charge stoichiometries of two renal sodium coupled D-glucose transporters previously identified in brush-border membrane vesicle preparations from the outer cortex (early proximal tubule) and outer medulla (late proximal tubule) are determined. The charge stoichiometries of these transporters are in good agreement with their sodium/glucose coupling ratios arguing against the possibility that glucose transport is coupled to ions other than sodium in these membranes.

Stoichiometry of H+/amino acid cotransport in Neurospora crassa revealed by current-voltage analysis

Coupling of ions to the uptake of neutral and basic amino acids via a general amino acid transport system (System II), was studied in a mutant of Neurospora crassa (bat mtr) which lacks other transport systems for these solutes. All amino acids tested--including ones bearing no net charge--elicited rapid membrane depolarization, as expected for ion-coupled transport. (Since amino acid transport in Neurospora is not dependent on extracellular Na+ or K+, the associated ion is presumed to be H+.) Although the 14C-labeled amino acid fluxes through System II are largely independent of the identity of the amino acid, the depolarization caused by basic amino acids (L-lysine and L-ornithine) is 60-70% greater than that for neutral amino acids (e.g. L-leucine). This difference is consistent with a constant H+/amino acid stoichiometry of 2, the extra charge for lysine and ornithine being that on the amino acid itself, so that the charge ratio basic:neutral amino acids is 3:2. When actual membrane charge flow associated with amino acid uptake was compared with measured 14C-labeled amino acid influx, ratios of 2.07 charges/mol L-leucine and 3.40 charges/mol L-lysine were obtained, again in accord with a constant translocation stoichiometry of 2H+/amino acid. The advantages of this electrical method for estimating H+/solute stoichiometry in cotransport are discussed in relation to more familiar methods.

Uptake of adenosine in a marine bacterium is not an active transport process

The uptake and intracellular interconversions of [8-14C]adenosine in a marine bacterium Vibrio harveyi were investigated under varying physiological conditions. The results indicated that in contrast with the current views, translocation of adenosine across the cytoplasmic membrane in Vibrio harveyi was not driven by respiration. The uptake of adenosine was dependent upon its intracellular utilization and was inhibited under conditions preventing its metabolic conversions.

L-aspartate transport in the photosynthetic bacterium Chromatium vinosum

The photosynthetic purple sulfur bacterium Chromatium vinosum appears to contain two active transport systems for L-aspartate. The higher affinity system (S0.5 = 60 microM) appears to be an electrogenic aspartate/H+ symport and the lower affinity system (S0.5 = 220 microM) appears to involve an aspartate/Na+ symport. In addition to a possible role in providing the driving force for aspartate uptake, transmembrane Na+ gradients may also have allosteric effects on aspartate transport in C. vinosum.

Cation-dependent binding of substrate to the folate transport protein of Lactobacillus casei

Lactobacillus casei cells grown in the presence of limiting folate contained large amounts of a membrane-associated binding protein which mediates folate transport. Binding to this protein at 4 degrees C was time and concentration dependent and at low levels (1 to 10 nM) of folate required 60 min to reach a steady state. The apparent dissociation constant (K(d)) for folate was 1.2 nM at pH 7.5 in 100 mM K-phosphate buffer, and it varied by less than twofold when measured over a range of pH values (5.5 to 7.5) or in buffered salt solutions of differing ionic compositions. Conversely, removal of ions and their replacement with isotonic sucrose (pH 7.5) led to a 200-fold reduction in binding affinity for folate. Restoration of the high-affinity state of the binding protein could be achieved by the readdition of various cations to the sucrose medium. K(d) measurements over a range of cation concentrations revealed that a half-maximal restoration of binding affinity was obtained with relatively low levels (10 to 50 muM) of divalent cations (e.g., Ca(2+), Mg(2+), and ethylenediammonium(2+) ions). Monovalent cations (e.g., Na(+), K(+), and Tris(+)) were also effective, but only at concentrations in the millimolar range. The K(d) for folate reached a minimum of 0.6 nM at pH 7.5 in the presence of excess CaCl(2). In cells suspended in sucrose, the affinity of the binding protein for folate increased 20-fold by decreasing the pH from 7.5 to 4.5, indicating that protons can partially fulfill the cation requirement. These results suggest that the folate transport protein of L. casei may contain both a substrate- and cation-binding site and that folate binds with a high affinity only after the cation-binding site has been occupied. The presence of these binding sites would support the hypothesis that folate is transported across the cell membrane via a cation-folate symport mechanism.

Lactose-H+(-OH) transport system of Escherichia coli. Multistate gated pore model based on half-sites stoichiometry for high-affinity substrate binding in a symmetrical dimer

A model is proposed for the D-galactoside-H+(-OH) transporter of Escherichia coli that accounts for essentially all the experimental observations established for this system to date. In this model, the functional unit is postulated to be a dimer (consisting of two copies of lac Y-specified polypeptide) which spans the membrane with a 2-fold symmetry axis in the membrane plane (Lancaster, J.R. (1978) J. Theor. Biol. 75, 35-50). The functional dimer is assumed to possess a single pore flanked by an inner gate (gi) and an outer gate (go) and encompassing two oppositely oriented galactoside binding sites, designated m and mu. When go is open and gi is closed under non-energized conditions, binding site m adopts a configuration defined as State A (i.e., moA) exhibiting high affinity toward Class Ga galactosides (thiodigalactoside, melibiose, alpha-p-nitrophenygalactoside) but low affinity for Class Gb galactosides (lactose, beta-o-nitrophenylgalactoside, beta-isopropylthiogalactoside), whereas binding site mu adopts State B (i.e., muoB) displaying relatively high affinity toward Class Gb galactosides but comparatively low affinity for Class Ga galactosides; further, each moA : muoB dimer contains one thiol group whose reaction with N-ethylmaleimide inactivates the transporter unless blocked by galactoside binding at site moA, while the second homologous thiol of the dimer is unreactive toward thiol reagents. Translocation of the moA : muoB dimer involves closing of go followed by opening of gi, and causes the two thiols (as well as sites m and mu) to interchange roles in a symmetrical fashion: moA : muoB in equilibrium miB : muiA. In the presence of a substantial (negative) transmembrane delta potential of muH+, the m : mu dimer is postulated to undergo an electrogenic protein conformation change to a second form, *(m : mu), in which both sites m and mu possess low affinity toward internal Class Gb substrates; galactoside transport in both m : mu and *(m : mu) is assumed to be coupled to H+-symport (-OH-antiport) with a stoichiometry of approximately 1 : 1. Finally, five characteristic predictions of the half-sites model are outlined for further tests of its validity.

Kinetic analysis of a family of cotransport models

The kinetic properties of a family of cotransport models are studied. The most general model allows the substrate and activator to bind in a random fashion to the transporter (iso random bi-bi mechanism). The other models require an ordered binding sequence (iso ordered bi-bi mechanism) and differ according to the order and symmetry of the binding events at the two membranes faces. In all cases it has been assumed that the translocation of the carrier is the rate-limiting step in the transport process. It is demonstrated that under zero trans, equilibrium exchange and infinite trans experimental conditions the usual kinetic parameters Km and V can be expressed as simple functions of the activator concentration and a minimal set of model dependent constants with well defined kinetic interpretations. Kinetic criteria for distinguishing between the various models are established. The incorporation of the effects of membrane potential into the flux equations is also treated with the aid of certain simplifying assumptions. The usefullness of the concept of 'effective charge' for non mobile carrier mechanisms is emphasized.