Fluctuation-driven directional flow in biochemical cycle: further study of electric activation of Na,K pumps

Silencing of ATP1A1 attenuates cell membrane disruption by nanosecond electric pulses

This study explored the role of the Na/K-ATPase (NKA) in membrane permeabilization induced by nanosecond electric pulses. Using CRISPR/Cas9 and shRNA, we silenced the ATP1A1 gene, which encodes α1 NKA subunit in U937 human monocytes. Silencing reduced the rate and the cumulative uptake of YoPro-1 dye after electroporation by 300-ns, 7-10 kV/cm pulses, while ouabain, a specific NKA inhibitor, enhanced YoPro-1 entry. We conclude that the α1 subunit supports the electropermeabilized membrane state, by forming or stabilizing electropores or by hindering repair mechanisms, and this role is independent of NKA's ion pump function.

Measurement of the low-frequency charge noise of bacteria

Bacteria meticulously regulate their intracellular ion concentrations and create ionic concentration gradients across the bacterial membrane. These ionic concentration gradients provide free energy for many cellular processes and are maintained by transmembrane transport. Given the physical dimensions of a bacterium and the stochasticity in transmembrane transport, intracellular ion concentrations and hence the charge state of a bacterium are bound to fluctuate. Here we investigate the charge noise of hundreds of nonmotile bacteria by combining electrical measurement techniques from condensed matter physics with microfluidics. In our experiments, bacteria in a microchannel generate charge density fluctuations in the embedding electrolyte due to random influx and efflux of ions. Detected as electrical resistance noise, these charge density fluctuations display a power spectral density proportional to 1/f^{2} for frequencies 0.05Hz≤f≤1Hz. Fits to a simple noise model suggest that the steady-state charge of a bacterium fluctuates by ±1.30×10^{6}e(e≈1.60×10^{-19}C), indicating that bacterial ion homeostasis is highly dynamic and dominated by strong charge noise. The rms charge noise can then be used to estimate the fluctuations in the membrane potential; however, the estimates are unreliable due to our limited understanding of the intracellular concentration gradients.

Reducing ischemic kidney injury through application of a synchronization modulation electric field to maintain Na+/K+-ATPase functions

Renal ischemia-reperfusion injury is an important contributor to the development of delayed graft function after transplantation, which is associated with higher rejection rates and poorer long-term outcomes. One of the earliest impairments during ischemia is Na⁺/K⁺-ATPase (Na/K pump) dysfunction due to insufficient ATP supply, resulting in subsequent cellular damage. Therefore, strategies that preserve ATP or maintain Na/K pump function may limit the extent of renal injury during ischemia-reperfusion. Here, we applied a synchronization modulation electric field to activate Na/K pumps, thereby maintaining cellular functions under ATP-insufficient conditions. We tested the effectiveness of this technique in two models of ischemic renal injury: an in situ renal ischemia-reperfusion injury model (predominantly warm ischemia) and a kidney transplantation model (predominantly cold ischemia). Application of the synchronization modulation electric field to a renal ischemia-reperfusion injury mouse model preserved Na/K pump activity, thereby reducing kidney injury, as reflected by 40% lower plasma creatinine (1.17 ± 0.03 mg/dl) in the electric field-treated group as compared to the untreated control group (1.89 ± 0.06 mg/dl). In a mouse kidney transplantation model, renal graft function was improved by more than 50% with the application of the synchronization modulation electric field according to glomerular filtration rate measurements (85.40 ± 12.18 μl/min in the untreated group versus 142.80 ± 11.65 μl/min in the electric field-treated group). This technique for preserving Na/K pump function may have therapeutic potential not only for ischemic kidney injury but also for other diseases associated with Na/K pump dysfunction due to inadequate ATP supply.

Biochemical and biomolecular effects induced by a static magnetic field in Saccharomyces cerevisiae: Evidence for oxidative stress

Exposure to static magnetic fields (SMF) can cause changes in microorganism metabolism altering key subcellular functions. The purpose of this study was to investigate whether an applied SMF could induce biological effects on growth of Saccharomyces cerevisiae, and then to probe biochemical and bio-molecular responses. We found a decrease in growth and viability under SMF (250mT) after 6h with a significant decrease in colony forming units followed by an increase between 6 h and 9 h. Moreover, measurements of antioxidant enzyme activities (catalase, superoxide dismutase, glutathione peroxidase) demonstrated a particular profile suggesting oxidative stress. For instance, SOD and catalase activities increased in magnetized cultures after 9 h compared with unexposed samples. However, SMF exposure caused a decrease in glutathione peroxidase activity. Finally, SMF caused an increase in MDA levels as well as the content of protein carbonyl groups after 6 and 9 h of exposure.

Some immunological responses of common carp (Cyprinus carpio) fingerling to acute extremely low-frequency electromagnetic fields (50 Hz)

Increasing of anthropogenic electromagnetic fields in aquatic environments has been recently become the core of attention. In this study, the effect of extremely low-frequency electromagnetic fields (50 Hz) on immune status and metabolic markers of common carp fingerling was assessed. The fish were exposed to extremely low-frequency electromagnetic fields at four intensities of 0.1, 0.5, 1, and 2 mT only once for 2 h; then, they were reared for 60 days. Results showed that the levels of aspartate aminotransferase (AST) and alanine transaminase (ALT) and alkaline phosphatase (ALP) were increased with an increase in the electromagnetic field intensity on 15 and 60 days post exposure. A significant increase was obtained in these enzyme levels in all the tested intensities compared to the control one (p < 0.05), with a maximum value measured in 2-mT trail. Conversely, with an increasing in the electromagnetic intensity, the levels of C3, C4, and lysozyme were reduced in all the treated groups in comparison with the control group (p < 0.05). The results suggested a significant impact of electromagnetic on fish immunophysiological functions. Therefore, it is required to have serious attention in aquatic ecosystems.

Nanoscale Ion Pump Derived from a Biological Water Channel

Biological molecular machines perform the work of supporting life at the smallest of scales, including the work of shuttling ions across cell boundaries and against chemical gradients. Systems of artificial channels at the nanoscale can likewise control ionic concentration by way of ionic current rectification, species selectivity, and voltage gating mechanisms. Here, we theoretically show that a voltage-gated, ion species-selective, and rectifying ion channel can be built using the components of a biological water channel aquaporin. Through all-atom molecular dynamics simulations, we show that the ionic conductance of a truncated aquaporin channel nonlinearly increases with the bias magnitude, depends on the channel's orientation, and is highly cation specific but only for one polarity of the transmembrane bias. Further, we show that such an unusually complex response of the channel to transmembrane bias arises from mechanical motion of a positively charged gate that blocks cation transport. By combining two truncated aquaporins, we demonstrate a molecular system that pumps ions against their chemical gradients when subject to an alternating transmembrane bias. Our work sets the stage for future biomimicry efforts directed toward reproducing the function of biological ion pumps using synthetic components.

Forces from the Portal Govern the Late-Stage DNA Transport in a Viral DNA Packaging Nanomotor

In the Phi29 bacteriophage, the DNA packaging nanomotor packs its double-stranded DNA genome into the virus capsid. At the late stage of DNA packaging, the negatively charged genome is increasingly compacted at a higher density in the capsid with a higher internal pressure. During the process, two Donnan effects, osmotic pressure and Donnan equilibrium potentials, are significantly amplified, which, in turn, affect the channel activity of the portal protein, GP10, embedded in the semipermeable capsid shell. In the research, planar lipid bilayer experiments were used to study the channel activities of the viral protein. The Donnan effect on the conformational changes of the viral protein was discovered, indicating GP10 may not be a static channel at the late stage of DNA packaging. Due to the conformational changes, GP10 may generate electrostatic forces that govern the DNA transport. For the section of the genome DNA that remains outside of the connector channel, a strong repulsive force from the viral protein would be generated against the DNA entry; however, for the section of the genome DNA within the channel, the portal protein would become a Brownian motor, which adopts the flash Brownian ratchet mechanism to pump the DNA against the increasingly built-up internal pressure (up to 20 atm) in the capsid. Therefore, the DNA transport in the nanoscale viral channel at the late stage of DNA packaging could be a consequence of Brownian movement of the genomic DNA, which would be rectified and harnessed by the forces from the interior wall of the viral channel under the influence of the Donnan effect.

Alternating electric fields combined with activated carbon for disinfection of Gram negative and Gram positive bacteria in fluidized bed electrode system

Strong electric fields for disinfection of wastewaters have been employed already for several decades. An innovative approach combining low strength (7 V/cm) alternating electric fields with a granular activated carbon fluidized bed electrode (FBE) for disinfection was presented recently. For disinfection performance of FBE several pure microbial cultures were tested: Bacillus subtilis, Bacillus subtilis subsp. subtilis, Enterococcus faecalis as representatives from Gram positive bacteria and Erwinia carotovora, Pseudomonas luteola, Pseudomonas fluorescens and Escherichia coli YMc10 as representatives from Gram negative bacteria. The alternating electric field amplitude and shape were kept constant. Only the effect of alternating electric field frequency on disinfection performance was investigated. From the bacteria tested, the Gram negative strains were more susceptible and the Gram positive microorganisms were more resistant to FBE disinfection. The collected data indicate that the efficiency of disinfection is frequency and strain dependent. During 6 h of disinfection, the decrease above 2 Log units was achieved with P. luteola and E. coli at 10 kHz and at dual frequency shift keying (FSK) modulated signal with frequencies of 10 kHz and 140 kHz. FBE technology appears to offer a new way for selective bacterial disinfection, however further optimizations are needed on treatment duration, and energy input, to improve effectiveness.

Synchronization modulation increases transepithelial potentials in MDCK monolayers through Na/K pumps

Transepithelial potential (TEP) is the voltage across a polarized epithelium. In epithelia that have active transport functions, the force for transmembrane flux of an ion is dictated by the electrochemical gradient in which TEP plays an essential role. In epithelial injury, disruption of the epithelial barrier collapses the TEP at the wound edge, resulting in the establishment of an endogenous wound electric field (∼100 mV/mm) that is directed towards the center of the wound. This endogenous electric field is implicated to enhance wound healing by guiding cell migration. We thus seek techniques to enhance the TEP, which may increase the wound electric fields and enhance wound healing. We report a novel technique, termed synchronization modulation (SM) using a train of electric pulses to synchronize the Na/K pump activity, and then modulating the pumping cycles to increase the efficiency of the Na/K pumps. Kidney epithelial monolayers (MDCK cells) maintain a stable TEP and transepithelial resistance (TER). SM significantly increased TEP over four fold. Either ouabain or digoxin, which block Na/K pump, abolished SM-induced TEP increases. In addition to the pump activity, basolateral distribution of Na/K pumps is essential for an increase in TEP. Our study for the first time developed an electrical approach to significantly increase the TEP. This technique targeting the Na/K pump may be used to modulate TEP, and may have implication in wound healing and in diseases where TEP needs to be modulated.

Na,K-ATPase as A Brownian Motor: Electric Field-InducedConformational Fluctuation Leads to Uphill Pumping of Cation inthe Absence of ATP

Na,K-ATPase uses chemical bond energy of ATP to pump K(+) into, andNa(+) out of a cell. Both are uphill transports. During the catalyticcycle the enzyme alternates between two conformational states, E(1) andE(2). This communication describes an experiment, which employs electricfield to drive oscillation or fluctuation of enzyme conformation betweenthe E(1) and the E(2) states. It is shown that the field-inducedconformational oscillation or fluctuation leads to uphill pumping of thecation by the enzyme without consumption of ATP. Biochemical specificityof the catalysis is preserved. Data indicate that Na,K-ATPase can harvestenergy from the applied electric field to perform chemical work, and aratchet mechanism is inherent in this energy transduction process. ATheory of Electroconformational Coupling (TEC) that embodies essentialfeatures of the Brownian Ratchet successfully simulates the field-frequencyand field-amplitude optima and other features of the ion pumping activity.A four-state TEC motor can achieve high efficiency of the energytransduction, asymptotically reaching 100% under the optimal condition.Pumping by ion rectification fails to reach high efficiency. The TECconcept is also mused to understand other biological motors and engines.

Allosteric model of an ion pump

We present a simple model of a free-energy transducer made of allosterically coupled two ratchet subsystems. Each of the subsystems transports particles from one particle reservoir to another. The coupling of the subsystems imposes correlated transitions of the potential profiles of the two subsystems. As a result, a downhill flux in one subsystem with higher chemical-potential difference drives an uphill flux in the other subsystem with lower chemical-potential difference. The direction of the driven flux inverts depending on the direction of the driving flux. The ratio between the fluxes conveyed by the two subsystems is variable and nonstoichiometric. By selecting appropriate parameters, the maximum ratio of the driven flux to driving flux and maximum free-energy transducing efficiency reaches some 90 and 40%, respectively. At a stalled state, the driven flux vanishes while the driving flux remains finite. The allosteric model enables explicit analysis of the timing between binding-unbinding of particles and transitions of potential profile. The behavior of the model is similar to but different from that of the alternate access model, which is a biochemical model for active transport proteins. Our model works also as a regulatory system. We suggest that the correlated transitions of the subsystems (subunits or domains) through allosteric interaction are the origin of the diverse functions of the protein machineries.

Rectification in a stochastically driven three-junction SQUID rocking ratchet

A three-junction SQUID forms a rocking ratchet for the phase differences across the Josephson junctions. In this Letter we investigate its capability to rectify voltages under stochastic drive currents with a white frequency spectrum up to a cutoff frequency f_{cut}. Experimentally, we accessed the adiabatic regime and find very good agreement with theory. Numerically, we also investigated large values of f_{cut} and find, in agreement with the laws of thermodynamics, that the response disappears, as f_{cut} becomes much larger than the characteristic frequency scales of the system.

Effects of low-frequency magnetic fields on the viability of yeast Saccharomyces cerevisiae

A 50 Hz magnetic field effect on the growth of yeasts Saccharomyces cerevisae was studied. The cylindrical coil induced magnetic fields with inductions up to 10 mT. Duration of exposure varied up to 24 min. Exposure took place at laboratory temperature (24-26 degrees C) and the air ventilator maintained the temperature at the place of the sample. We measured the growth curves of yeasts in broth and we calculated the number of CFU (colony forming units) on solid soil. We found that magnetic field decreases the number of yeasts, and slowed down their growth. The result is similar to the experiments with bacteria E. coli, S. aureus and L. adecarboxylata. It seems that the magnetic fields kill a part of yeasts and the bigger part of them survives and continues in their growth.

Design principles for Brownian molecular machines: how to swim in molasses and walk in a hurricane

Protein molecular motors-perfected over the course of millions of years of evolution-play an essential role in moving and assembling biological structures. Recently chemists have been able to synthesize molecules that emulate in part the remarkable capabilities of these biomolecular motors (for extensive reviews see the recent papers: E. R. Kay, D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed., 2006, 46, 72-191; W. R. Browne and B. L. Feringa, Nat. Nanotechnol., 2006, 1, 25-35; M. N. Chatterjee, E. R. Kay and D. A. Leigh, J. Am. Chem. Soc., 2006, 128, 4058-4073; G. S. Kottas, L. I. Clarke, D. Horinek and J. Michl, Chem. Rev., 2005, 105, 1281-1376; M. A. Garcia-Garibay, Proc. Natl. Acad. Sci., U. S. A., 2005, 102, 10771-10776)). Like their biological counterparts, many of these synthetic machines function in an environment where viscous forces dominate inertia-to move they must "swim in molasses". Further, the thermal noise power exchanged reversibly between the motor and its environment is many orders of magnitude greater than the power provided by the chemical fuel to drive directed motion. One might think that moving in a specific direction would be as difficult as walking in a hurricane. Yet biomolecular motors (and increasingly, synthetic motors) move and accomplish their function with almost deterministic precision. In this Perspective we will investigate the physical principles that govern nanoscale systems at the single molecule level and how these principles can be useful in designing synthetic molecular machines.

A Markovian engine for a biological energy transducer: the catalytic wheel

The molecular machines in biological cells are made of proteins, DNAs and other classes of molecules. The structures of these molecules are characteristically "soft", highly flexible, and yet their interactions with other molecules or ions are specific and selective. This chapter discusses a prevalent form, the catalytic wheel, or the energy transducer of cells, examines its mechanism of action, and extracts from it a set of simple but general rules for understanding the energetics of the biomolecular devices. These rules should also benefit design of manmade nanometer scale machines such as rotary motors or track-guided linear transporters. We will focus on an electric work that, by matching system dynamics and then enhancing the conformational fluctuation of one or several driver proteins, converts stochastic input of energy into rotation or locomotion of a receptor protein. The spatial (or barrier) and temporal symmetry breakings required for selected driver/receptor combinations are examined. This electric ratchet consists of a core engine that follows the Markovian dynamic, alleviates difficulties encountered in rigid mechanical model, and tailors to the soft-matter characteristics of the biomolecules.

Synchronization of Na/K pump molecules by a train of squared pulses

We experimentally studied the Na/K pump currents evoked by a train of squared pulses whose pulse-duration is about the time course of Na-extrusion at physiological conditions. The magnitude of the measured pump current can be as much as three-fold of that induced by the traditional single pulse measurement. The increase in the pump current is directly dependent on the number of pre-pulses. The larger the number of the pre-pulses is, the higher the current magnitude can be obtained. At a particular number of pre-pulses, the pump current becomes saturated. These results suggest that a large number of pre-pulses may synchronize the pump molecules to work at the same pace. As a result, the pump molecules may extrude Na ions at the same time corresponding to the stimulation pulses, and pump in K ions at the same time during the pulse intervals. Therefore, the measured pump current is three-fold of that measured by a single pulse where the outward and inward pump currents are canceled each other.

Rotary motor powered by stochastic uncorrelated dipoles

It is demonstrated that the stochastic back-and-forth vibrations of uncorrelated dipoles may lead to rotation of their ambient dipoles. This peculiar phenomenon is clarified by considering spatial and temporal symmetry breakings. The former asymmetry is the result of the multiple biased Hamiltonian vector fields, which is a ratchet effect, and the latter, of the time sequence specified by a metastable state. Since this driving mechanism is simpler than that of F0F1ATPase, it could benefit the design of nanometer scale rotary devices.

Mechanisms for interaction between RF fields and biological tissue

Interaction of radiofrequency (RF) fields with biological tissue can involve either electric or magnetic fields. Many interaction mechanisms have been considered, both thermal and nonthermal, but it has not been established that any of these could result in adverse health effects at radiation levels below guidelines. The principles underlying most of these mechanisms have been well reviewed. The aim of the present study is to give a qualitative discussion of some of the more recently published work.

Electroendocytosis: exposure of cells to pulsed low electric fields enhances adsorption and uptake of macromolecules

This study demonstrates alteration of cell surface, leading to enhanced adsorption of macromolecules (bovine serum albumin (BSA), dextran, and DNA), after the exposure of cells to unipolar pulsed low electric fields (LEF). Modification of the adsorptive properties of the cell membrane also stems from the observation of LEF-induced cell-cell aggregation. Analysis of the adsorption isotherms of BSA-fluorescein isothiocyanate (FITC) to the surface of COS 5-7 cells reveals that the stimulated adsorption can be attributed to LEF-induced increase in the capacity of both specific and nonspecific binding. The enhanced adsorption was consequently followed by increased uptake. At 20 V/cm the maximal binding and subsequent uptake of BSA-FITC attached to specific sites are 6.5- and 7.4-fold higher than in controls, respectively. The nonspecific LEF-induced binding and uptake of BSA are 34- and 5.2-fold higher than in controls. LEF-enhanced adsorption is a temperature-independent process, whereas LEF-induced uptake is a temperature-dependent one that is abolished at 4 degrees C. The stimulation of adsorption and uptake is reversible, revealing similar decay kinetics at room temperature. It is suggested that electrophoretic segregation of charged components in the outer leaflet of the cell membrane is responsible for both enhanced adsorption and stimulated uptake via changes of the membrane elastic properties that enhance budding and fission processes.

Comparison of the low-frequency magnetic field effects on bacteria Escherichia coli, Leclercia adecarboxylata and Staphylococcus aureus

This work studies biological effects of low-frequency electromagnetic fields. We have exposed three different bacterial strains-Escherichia coli, Leclercia adecarboxylata and Staphylococcus aureus to the magnetic field (t<30 min, B(m)=10 mT, f=50 Hz) in order to compare their viability (number of colony-forming units (CFU)). We have measured the dependence of CFU on time of exposure and on the value of the magnetic field induction B(m). Viability decreases with longer exposure time and/or higher induction B(m) for all strains, but the quantity of the effect is strain-dependent. The highest decrease of the viability and the biggest magnetic field effect was observed with E. coli. The smallest magnetic field effect appears for S. aureus. From the measurement of the growth dynamics we have concluded that the decrease of the CFU starts immediately after the magnetic field was switched on.

Stochastic resonance in a biological motor under complex fluctuations

The dynamics division approach proposed in this work enables us to handle dynamical equations with complex fluctuations. A Brownian motor with cyclic conformational changes is analyzed to understand effects of noise on its signal transduction, and on condition in which stochastic resonance may take place. The result reproduces several features of the experimental data on the electric activation of ion pumping by Na, K-ATPase.

Adiabatic pumping mechanism for ion motive ATPases

An ion motive ATPase is a membrane protein that pumps ions across the membrane at the expense of the chemical energy of adenosine triphosphate (ATP) hydrolysis. Here we describe how an external electric field, by inducing transitions between several protein configurations, can also power this pump. The underlying mechanism may be very similar to that of a recently constructed adiabatic electron pump [Science 283, 1905 (1999)]].

Cellular target of weak magnetic fields: ionic conduction along actin filaments of microvilli

The interaction of weak electromagnetic fields (EMF) with living cells is a most important but still unresolved biophysical problem. For this interaction, thermal and other types of noise appear to cause severe restrictions in the action of weak signals on relevant components of the cell. A recently presented general concept of regulation of ion and substrate pathways through microvilli provides a possible theoretical basis for the comprehension of physiological effects of even extremely low magnetic fields. The actin-based core of microfilaments in microvilli is proposed to represent a cellular interaction site for magnetic fields. Both the central role of F-actin in Ca2+ signaling and its polyelectrolyte nature eliciting specific ion conduction properties render the microvillar actin filament bundle an ideal interaction site for magnetic and electric fields. Ion channels at the tip of microvilli are connected with the cytoplasm by a bundle of microfilaments forming a diffusion barrier system. Because of its polyelectrolyte nature, the microfilament core of microvilli allows Ca2+ entry into the cytoplasm via nonlinear cable-like cation conduction through arrays of condensed ion clouds. The interaction of ion clouds with periodically applied EMFs and field-induced cation pumping through the cascade of potential barriers on the F-actin polyelectrolyte follows well-known physical principles of ion-magnetic field (MF) interaction and signal discrimination as described by the stochastic resonance and Brownian motor hypotheses. The proposed interaction mechanism is in accord with our present knowledge about Ca2+ signaling as the biological main target of MFs and the postulated extreme sensitivity for coherent excitation by very low field energies within specific amplitude and frequency windows. Microvillar F-actin bundles shielded by a lipid membrane appear to function like electronic integration devices for signal-to-noise enhancement; the influence of coherent signals on cation transduction is amplified, whereas that of random noise is reduced.

Ratchet models using driving forces generated by deterministic chaotic maps

This study investigates how ratchets perform under driving forces generated by the circle, baker, and logistic maps with varying driving frequencies. The markedly different unidirectional net transports induced by distinct maps and frequencies are clarified by vector field analysis of the ratchet equations. Analysis results indicate that both the deterministic property of the driving forces and the asymmetric effect due to the ratchet potential impact the transport. Moreover, the driving frequency determines which factor suppresses the other one and dominates the ratchet transport.

Activation of signal-transduction mechanisms may underlie the therapeutic effects of an applied electric field

Successful treatment of various medical complaints with an applied electric field has been reported over the years. The identities of the cellular mechanisms that are influenced by this type of treatment and facilitate the positive effects, remain elusive. A study of many in vitro and in vivo reports revealed that the beneficial effects can be attributed to the activation of membrane proteins, and specifically proteins involved in signal-transduction mechanisms. Not only may the proteins be affected but it is now well established that enhanced Ca(2+)influx, observed to follow electric stimulation of cells, also contributes to many calcium-dependent cellular processes which can be linked to the therapeutic effects discussed in this paper. An hypothesis of the physical changes caused by an applied, relatively small (10(3)to 10(4)V m(-1)range), electric field with low to moderate frequency (below 150 Hz), is postulated.

Constructive role of noise in signal transmissions by biomembrane proteins

We discuss new examples of the constructive role of environmental fluctuations in biophysical processes, namely quantitative enhancement and qualitative sharpening of the outgoing signal in the intercellular signal transduction, e.g., in the synaptic links. An experimental check in a chemical flow reactor is suggested.

The role of thermal activation in motion and force generation by molecular motors

The currently accepted mechanism for ATP-driven motion of kinesin is called the hand-over-hand model, where some chemical transition during the ATP hydrolysis cycle stretches a spring, and motion and force production result from the subsequent relaxation. It is essential in this mechanism for the moving head of kinesin to dissociate, while the other head remains firmly attached to the microtubule. Here we propose an alternative Brownian motor model where the action of ATP modulates the interaction potential between kinesin and the microtubule rather than a spring internal to the kinesin molecule alone. In this model neither head need dissociate (which predicts that under some circumstances a single-headed kinesin can display processive motion) and the transitions by which the motor moves are best described as thermally activated steps. This model is consistent with a wide range of experimental data on the force-velocity curves, the one ATP to one-step stoichiometry observed at small load, and the stochastic properties of the stepping.

Effects of nonequilibrium fluctuations on ionic transport through biomembranes

We investigate the effects of nonequilibrium fluctuations on ionic transport through ion channels in membranes using the concept of localized ratchet. Due to the localization, the ionic population in the binding site can be enhanced or suppressed depending upon ionic potential and its fluctuations, affecting the gating kinetics of the channel. The localized dichotomic fluctuations of ionic potential are shown to give rise to a current reversal differing from the results of periodic ratchets. It is also found that strong correlations between binding energy and membrane potential fluctuations induce resonancelike behaviors in ionic current as the fluctuating rate varies.

Overview: The constructive role of noise in fluctuation driven transport and stochastic resonance

Random noise is typically thought of as the enemy of order rather than as a constructive influence. Recent work has shown however that under certain circumstances, noise and Brownian motion can facilitate transmission of information via a mechanism know as stochastic resonance, and help systems use chemical energy and nonequilibrium fluctuations to drive directed motion via fluctuation driven transport. In this focus issue we have collected several articles that capture the flavor of these developing fields and point the way to new directions for research. (c) 1998 American Institute of Physics.

Barrier fluctuations and stochastic resonance in membrane transport

The role of barrier fluctuations in membrane enzymatic processes, in particular in the active transport of ions through cell membranes, is examined. For enzymes embedded in the cell membrane the role of the barrier height (activation energy) is played by the membrane electric potential. This barrier height can be modulated either by internal fluctuations or by external electrical fields, either random or periodic. Existing experimental data on active transport of Na(+) and Rb(+) in human erythrocytes (catalyzed by Na(+)-K(+)-ATPase) can be interpreted as evidence of stochastic resonance between the external ac field and the fluctuations of the membrane potential. The obtained results suggest that the significant part of these fluctuations is supplied by the stimulated action of neighbor voltage-gated ionic channels. This supports the idea that intrinsic noise plays a constructive role in one of most important and most frequent biophysical processs, viz. ion transmission through cell membranes. Means of further experimental verification of this conjecture are proposed. (c) 1998 American Institute of Physics.