Two enzyme active transport in vitro with ph induced asymmetrical functional structures: I. The model and its analytical treatment

A class of systems is characterized by the asymmetrical distribution of a sink and a source reaction, the asymmetry of the global chemical equation (energy liberation) and by an asymmetrical one-wave space profile. These systems belong to the family of primary chemical cells and can deplete and enrich the media they separate. A “ one way ” transport-reaction chain is needed for specific “ real ” active transport. A two enzyme model of this class is described in which the spatial asymmetry is due to a (diffusive) pH gradient; this distribution of “ potential ” enzyme activities is called the “ functional structure ”. Equal potential enzyme activities and absence of reactive back action on local pH are assumed in the “ square model ” version of the pump. Analytical expressions of the enzymatic diffusion reactions are derived for zero and first order kinetics, i.e. in function of substrate concentrations. Tables of equations are presented. The intrinsic properties of the pump are characterized by (dimensionless) transport reaction parameters, (membrane composition); the “ potential ” activity is controlled by the pH gradient; the “ effective ” pumping is also a function of the substrate concentrations on the boundaries.     

Ion channel–transporter interactions

All living cells require membrane proteins that act as conduits for the regulated transport of ions, solutes and other small molecules across the cell membrane. Ion channels provide a pore that permits often rapid, highly selective and tightly regulated movement of ions down their electrochemical gradient. In contrast, active transporters can move moieties up their electrochemical gradient. The secondary active transporters (such as SLC superfamily solute transporters) achieve this by coupling uphill movement of the substrate to downhill movement of another ion, such as sodium. The primary active transporters (including H⁺/K⁺-ATPases and Na⁺/K⁺-ATPases) utilize ATP hydrolysis as an energy source to power uphill transport. It is well known that proteins in each of these classes work in concert with members of the other classes to ensure, for example, ion homeostasis, ion secretion and restoration of ion balance following action potentials. More recently, evidence is emerging of direct physical interaction between true ion channels, and some primary or secondary active transporters. Here, we review the first known members of this new class of macromolecular complexes that we term “chansporters”, explore their biological roles and discuss the pathophysiological consequences of their disruption. We compare functional and/or physical interactions between the ubiquitous KCNQ1 potassium channel and various active transporters, and examine other newly discovered chansporter complexes that suggest we may be seeing the tip of the iceberg in a newly emerging signaling modality.     

"Noise" as a unifying concept in the theory of schizophrenia

The psychological effects of pharmacologically active substances can only be adequately described in terms of the psychological processes upon which they act. Psychopharmacology therefore depends on the development of psychological “models”, i.e. formally stated and testable hypotheses concerning the psychological processes required for the performance of any given task. Information theory is not concerned with the physical nature of events but only with those features which confer specificity upon them. It therefore provides a suitable theoretical language in which to describe interactions between biochemical, neurophysiological and psychological events. (i) The first section of this paper defines the information-theory concept of noise. Starting from first principles with the “noisy channel” and progressing to the “noisy system”, some of its psychophysiological implications are explored. (ii) An “order-memory” task is described which was used for three years to study the thinking of acutely disturbed young adult psychiatric patients, including many with acute schizophrenia. On the basis of a simple model, it is possible to calculate the value of a “critical” noise-level which marks the dividing line between two qualitatively different modes of functioning. (iii) A number of results are either reported or summarized, which show the functional significance of a supra-critical noise-level and its connection with the acute schizophrenic state. (iv) A “control-theory” approach to schizophrenia is outlined, which shows how specific and non-specific hereditary factors could be accommodated in the hypothesis but mainly emphasizes the concept of a mutual struggle for control between parent and child, in which the “loser” overloads the regulatory capacity of the other by “going noisy”. In this way the “loser” escapes from control but is more or less disabled by his own cognitive noise.     

Semiconductor theory of ion transport in thin lipid membranes: I. Potential and field distributions

In the theory as presented in this paper and the following one, we shall attempt to apply the semiconductor principles and methods to the study of ion transport in thin lipid membranes. Detailed formulations are given on the potential energy barriers at the interfaces, voltage drops in the polar and non-polar regions, and potential and field distributions in the diffuse double layer and within a charged membrane. These results will be used mainly as the boundary conditions for the solution of ion flow as to be given in the following paper. The analysis clearly indicates that the ion transport is interface-limited and is profoundly influenced by the presence of surface charges. An explanation of Na⁺ extrusion in nerve membrane is given based on the field distribution analysis. The theory also suggests that the “membrane potential” depends mainly on surface charges but not necessarily on ion permeation through the membrane.     

Active transport mediated by physical processes : Evidence resulting from xenon passage into the red cell

The study of the process of ¹³³Xe accumulation into the erythrocyte leads to the conclusion that monoatomic molecules of this noble gas are actively transported at the level of cell membrane. Hence, it inherently results that the active transport of a substance is possible without its chemical coupling with molecular carriers. The physical phenomena of phase transition stand at the basis of the active transport process.     

A possible model for receptors in excitable membranes

A model is proposed for receptors in excitable membranes based on the following assumptions. The receptor site and the process it excites in the membrane are located close to each other. The change of the electrostatic potential in the neighbourhood of the receptor site on the adsorption of a molecule (or ion) influences a potential dependent process in the membrane, such as ion permeability, rate of enzymatic reactions, ion binding etc. A comment is also made about the connection between measured physiological activity of a molecule and its ?real” physical activity.     

A model of oxidative phosphorylation that accommodates the chemical intermediate, chemiosmotic, localized proton and conformational hypotheses.

A model of oxidative phosphorylation is formulated which accounts for a wide variety of experimental observations of mitochondria in molecular terms.The central feature of the model is the postulated existence of a hypothetical enzymic system called the “proton transfer complex” in the inner membrane that catalyzes group-specific proton transfer reactions between membrane components. Thus, the “proton transfer complex” is assumed to catalyze either the intramembrane proton transfer reactions (e.g., between the electron transfer complex and oligomycin-sensitive ATPase) or the transmembrane proton transfer reactions between the matrix and intracristal spaces. The former appears to be involved in the phosphorylation of matrix ADP and the latter in the active transport of adenine nucleotides and substrates across the inner membrane.     

Secondary active transport

Secondary active transport is defined as the transport of a solute in the direction of its increasing electrochemical potential coupled to the facilitated diffusion of a second solute (usually an ion) in the direction of its decreasing electrochemical potential. The coupling agents are membrane proteins (carriers), each of which catalyzes simultaneously the facilitated diffusion of the driving ion and the active transport of a given solute. The review starts with some considerations on the energetics followed by a presentation of the kinetics of secondary active transport. Examples of information which may be gained by such studies are discussed. In the second part, some examples of secondary transport are given; we also describe the characteristics of the corresponding carriers. The various transport systems presented are: the D-glucose/Na+ symport in brush-border membranes, the lactose/H+ symport in E. coli, the Na+/H+ antiport, the different transport systems in the inner mitochondrial membrane.     

Structured bienzymatical models formed by sequential enzymes bound into artificial supports: Active glucose transport effect

Summary Two different artificial membrane systems bearing two built-in sequential enzymes are studied and compared in this communication.The first is a nonstructured membrane bearing two mixed enzymes: -galactosidase and glucose-oxidase. Its use enables a mathematical model to be formulated describing the heterogeneous phase kinetics of a bienzymatic system. The second is a multi-layer membrane system in which the structural dissymmetry involves a spatial orientation of the reacting metabolites, resulting in active glucose transport.The latter system consists of two active leaflets, the first phosphorylating glucose (hexokinase+ATP), the second dephosphorylating glucose-6 phosphate (phosphatase). On either side of this system, a perm-selective proteic layer allows the passage of glucose but not of glucose-6 phosphate. When positioned between two compartments containing glucose, such a membrane accumulates glucose on its phosphatase side, while degrading ATP.The accumulation of glucose as a function of the initial concentration shows the classical saturation of the transport system. Fructose competes with glucose transport.The chemical balance of these two reactions has the appearance of hydrolysis of ATP. Vectorial catalysis is a result of the dissymmetry in distribution of active sites and can be explained by an oscillatory concentration profile of glucose inside the membrane.The bienzymatic mechanism, a model of which is given here, is valid for any thickness of active layers and applicable to a system where both active sides are part of the same molecule as soon as it forms a uniformly oriented monolayer throughout the membrane structure.     

Molecular genetic and biochemical approaches for defining lipid-dependent membrane protein folding

We provide an overview of lipid-dependent polytopic membrane protein folding and topogenesis. Lipid dependence of this process was determined by employing Escherichia coli cells in which specific lipids can be eliminated, substituted, tightly titrated or controlled temporally during membrane protein synthesis and assembly. The secondary transport protein lactose permease (LacY) was used to establish general principles underlying the molecular basis of lipid-dependent effects on protein domain folding, protein transmembrane domain (TM) orientation, and function. These principles were then extended to several other secondary transport proteins of E. coli. The methods used to follow proper conformational organization of protein domains and the topological organization of protein TMs in whole cells and membranes are described. The proper folding of an extramembrane domain of LacY that is crucial for energy dependent uphill transport function depends on specific lipids acting as non-protein molecular chaperones. Correct TM topogenesis is dependent on charge interactions between the cytoplasmic surface of membrane proteins and a proper balance of the membrane surface net charge defined by the lipid head groups. Short-range interactions between the nascent protein chain and the translocon are necessary but not sufficient for establishment of final topology. After release from the translocon short-range interactions between lipid head groups and the nascent protein chain, partitioning of protein hydrophobic domains into the membrane bilayer, and long-range interactions within the protein thermodynamically drive final membrane protein organization. Given the diversity of membrane lipid compositions throughout nature, it is tempting to speculate that during the course of evolution the physical and chemical properties of proteins and lipids have co-evolved in the context of the lipid environment of membrane systems in which both are mutually dependent on each other for functional organization of proteins. This article is part of a Special Issue entitled: Protein Folding in Membranes.     

Electrostatic plasma turbulence and spitzer longitudinal conductivity in tokamaks

An analysis has revealed that there may be three radically different steady states of a tokamak plasma: (i) discharges in which the electron and ion transport can be described by neoclassical theory; (ii) discharges with the Spitzer longitudinal conductivity, neoclassical ion transport, and “anomalous” electron transport; and (iii) discharges in which the electron transport and ion transport are both “anomalous.” The dimensionless parameters responsible for the occurrence of the three types of discharges are determined. In accordance with the criteria derived for the achievement of different steady states, discharges of the second type are most typical of modern tokamaks and discharges of the third type can occur only if the turbulence is sufficiently strong. Discharges of the first type cannot occur in the range of the working parameters of present-day tokamaks and future tokamak reactors, but they can be ignited in a large class of magnetic confinement systems. The physical reason for the onset of different types of discharges is associated with the fact that turbulent fluctuations play very different roles in the dynamics of the ion and electron components of a finite-size magnetized plasma. The question of the self-consistency between the profiles is considered. A law is derived according to which the thermal diffusivity increases toward the plasma edge.     

Optimization criteria of the mechanism governing the stability of the membrane potential

For small changes in ion concentration within the physiological range the membrane potential transients can be explained in terms of two linear models both for passive and active transport. Using frog sartorius muscle as a suitable model system the ion pump is considered to work within the steepest range of the flux-concentration characteristic. Further for the small perturbations the equations describing passive ion transport can be safely linearized. The conclusion appears inescapable that for the muscle membrane the intracellular ion concentration adjusts itself in some optimal manner to the level of the extracellular ions. The active ion transport represents a control parameter for the membrane potential. The model structure corresponds to a dynamic system, the control processes of which are optimized with respect to a quadratic integral-criterion function. Here, both the performance index of the control sequence in the membrane processes and the energy consumed by the ion fluxes have been considered for small perturbations of Na⁺, K⁺, and Cl^? in the neighbourhood of the physiological working point. As it is, the control system governing the active and passive ion transport processes is essentially optimized with respect to a minimal energy usage. The amount of energy consumed during the transients predicted by the model has been calculated.     

The representation of membrane admittance.

An integral representation for the membrane admittance in terms of its known current response to a voltage step function is presented. It is demonstrated that the frequency-dependent terms in the contribution to the membrane admittance by the ion-selective conductance of the nerve membrane are proportional to the static conductances. The additional information contained in the real and imaginary parts of the membrane admittance should allow the parameters of the ion conductance to be determined. Eventually, these measurements should also give information about the electric dipole displacement currents of the conductance systems themselves, and about the metabolically supported active ion transport currents that maintain the ion concentration gradients.     

Ion homeostasis, channels, and transporters: an update on cellular mechanisms

The steady-state maintenance of highly asymmetric concentrations of the major inorganic cations and anions is a major function of both plasma membranes and the membranes of intracellular organelles. Homeostatic regulation of these ionic gradients is critical for most functions. Due to their charge, the movements of ions across biological membranes necessarily involves facilitation by intrinsic membrane transport proteins. The functional characterization and categorization of membrane transport proteins was a major focus of cell physiological research from the 1950s through the 1980s. On the basis of these functional analyses, ion transport proteins were broadly divided into two classes: channels and carrier-type transporters (which include exchangers, cotransporters, and ATP-driven ion pumps). Beginning in the mid-1980s, these functional analyses of ion transport and homeostasis were complemented by the cloning of genes encoding many ion channels and transporter proteins. Comparison of the predicted primary amino acid sequences and structures of functionally similar ion transport proteins facilitated their grouping within families and superfamilies of structurally related membrane proteins. Postgenomics research in ion transport biology increasingly involves two powerful approaches. One involves elucidation of the molecular structures, at the atomic level in some cases, of model ion transport proteins. The second uses the tools of cell biology to explore the cell-specific function or subcellular localization of ion transport proteins. This review will describe how these approaches have provided new, and sometimes surprising, insights regarding four major questions in current ion transporter research. 1) What are the fundamental differences between ion channels and ion transporters? 2) How does the interaction of an ion transport protein with so-called adapter proteins affect its subcellular localization or regulation by various intracellular signal transduction pathways? 3) How does the specific lipid composition of the local membrane microenvironment modulate the function of an ion transport protein? 4) How can the basic functional properties of a ubiquitously expressed ion transport protein vary depending on the cell type in which it is expressed?     

Comparative molecular biological analysis of membrane transport genes in organisms

Comparative analyses of membrane transport genes revealed many differences in the features of transport homeostasis in eight diverse organisms, ranging from bacteria to animals and plants. In bacteria, membrane-transport systems depend mainly on single genes encoding proteins involved in an ATP-dependent pump and secondary transport proteins that use H⁺ as a co-transport molecule. Animals are especially divergent in their channel genes, and plants have larger numbers of P-type ATPase and secondary active transporters than do other organisms. The secondary transporter genes have diverged evolutionarily in both animals and plants for different co-transporter molecules. Animals use Na⁺ ions for the formation of concentration gradients across plasma membranes, dependent on secondary active transporters and on membrane voltages that in turn are dependent on ion transport regulation systems. Plants use H⁺ ions pooled in vacuoles and the apoplast to transport various substances; these proton gradients are also dependent on secondary active transporters. We also compared the numbers of membrane transporter genes in Arabidopsis and rice. Although many transporter genes are similar in these plants, Arabidopsis has a more diverse array of genes for multi-efflux transport and for response to stress signals, and rice has more secondary transporter genes for carbohydrate and nutrient transport. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Exergy‐Based Population Dynamics

This article discusses whether “sustainability” has a physical meaning in applied thermodynamics. If it has, then it should be possible to derive general principles and rules for devising “sustainable systems.” If not, then other sides of the issue retain their relevance, but thermodynamic laws are not appropriate by themselves to decide whether a system or a scenario is sustainable. Here, we make use of a single axiom: that final consumption (material or immaterial) can be quantified solely in terms of equivalent primary exergy flows. On this basis, we develop a system theory that shows that if “simple” systems are based solely on the exploitation of fossil resources, they cannot be thermodynamically “sustainable.” But as renewable resources are brought into the picture and the system complexity grows, there are thresholds below or beyond which the system exhibits an ability to maintain itself (perhaps through fluctuations), in a self‐preserving (i.e., a sustainable) state. It appears that both complexity and the degree of nonlinearity of the transfer functions of the systems play a major role and—even for some of the simplest cases—lead to nontrivial solutions in phase space. Therefore, even if the examples presented in the article can be considered rather crude approximations to real, complex systems at best, the results show a trend that is worth further consideration.     

Significance of composition and structure of the enterobacterial outer membrane to the transport of desired and undesired substances and its inhibition

The enterobacterial outer membrane forms a bilayer. Its outer monolyer consists of lipopolysaccharides and proteins, its inner monolayer of phospholipids and proteins. It thus represents an efficient penetration barrier against hydrophobic and anionic compounds (such as detergents or hydrophobic antibiotics) and against higher molecular substances (such as proteolytic, lipolytic, and murolylic enzymes). Some of the proteins (“porins”) form channels through the outer membrane through which neutral and cationic hydrophylic compounds up to a molecular weight of about 800 can pass. Besides the porins additional transport systems have been described. They play an important part in providing the bacteria with substances necessary for their growth, i.e., phosphate, iron ions, and others. Organic polycations are able to generate more or less severe disorganizations in the outer membrane through which they can pass the bilayer (“self-promoted pathway”). Some of these polycations represent efficient antibiotics (polymyxin B, nourseothricin). Bacteria are able to protect themselves against the harmful action of these substances by changing the composition of the outer membrane.     

Mechanisms of Photoreceptor Current Generation in Light and Darkness

PENN and Hagins first demonstrated a sustained “dark” voltage along the axis of rat retinal rods which results from membrane current leaving the inner segment and entering the outer segment^1,2. They also found that the response to light (receptor potential) is a reduction in this “dark” voltage which does not alter its spatial distribution. I have examined the retinal “dark” voltage and the light-evoked receptor potential in terms of possible passive and active transport mechanisms of electrogenesis.     

A structural overview of the plasma membrane Na+,K+-ATPase and H+-ATPase ion pumps

Plasma membrane ATPases are primary active transporters of cations that maintain steep concentration gradients. The ion gradients and membrane potentials derived from them form the basis for a range of essential cellular processes, in particular Na(+)-dependent and proton-dependent secondary transport systems that are responsible for uptake and extrusion of metabolites and other ions. The ion gradients are also both directly and indirectly used to control pH homeostasis and to regulate cell volume. The plasma membrane H(+)-ATPase maintains a proton gradient in plants and fungi and the Na(+),K(+)-ATPase maintains a Na(+) and K(+) gradient in animal cells. Structural information provides insight into the function of these two distinct but related P-type pumps.