Time-resolved charge movements in the sarcoplasmatic reticulum Ca-ATPase

A method to measure hydrolytic activity of adenosinetriphosphatases (ATPases)

The detection of small amounts (nanomoles) of inorganic phosphate has a great interest in biochemistry. In particular, phosphate detection is useful to evaluate the rate of hydrolysis of phosphatases, that are enzymes able to remove phosphate from their substrate by hydrolytic cleavage. The hydrolysis rate is correlated to enzyme activity, an extremely important functional parameter. Among phosphatases there are the cation transporting adenosinetriphosphatases (ATPases), that produce inorganic phosphate by cleavage of the γ-phosphate of ATP. These membrane transporters have many fundamental physiological roles and are emerging as potential drug targets. ATPase hydrolytic activity is measured to test enzyme functionality, but it also provides useful information on possible inhibitory effects of molecules that interfere with the hydrolytic process. We have optimized a molybdenum-based protocol that makes use of potassium antimony (III) oxide tartrate (originally employed for phosphate detection in environmental analysis) to allow its use with phosphatase enzymes. In particular, the method was successfully applied to native and recombinant ATPases to demonstrate its reliability, validity, sensitivity and versatility. Our method introduces significant improvements to well-established experimental assays, which are currently employed for ATPase activity measurements. Therefore, it may be valuable in biochemical and biomedical investigations of ATPase enzymes, in combination with more specific tests, as well as in high throughput drug screening.

Kinetic and mesoscopic non-equilibrium description of the Ca(2+) pump: a comparison

We analyse the operation of the Ca(2+)-ATPase ion pump using a kinetic cycle diagram. Using the methodology of Hill, we obtain the cycle fluxes, entropy production and efficiency of the pump. We compare these results with a mesoscopic non-equilibrium description of the pump and show that the kinetic and mesoscopic pictures are in accordance with each other. This gives further support to the mesoscopic theory, which is less restricted and also can include the heat flux as a variable. We also show how motors can be characterised in terms of unidirectional backward fluxes. We proceed to show how the mesoscopic approach can be used to identify fast and slow steps of the model in terms of activation energies, and how this can be used to simplify the kinetic diagram.

Kinetics of luminal proton binding to the SR Ca-ATPase

An open membrane preparation containing SR Ca-ATPase was prepared from sarcoplasmic-reticulum vesicles to study the ion binding kinetics in the P-E(2) conformation. Because Ca(2+) and H(+) binding are electrogenic reactions, fluorescent styryl dyes could be used to determine changes in the binding site occupation in equilibrium titration experiments and time-resolved relaxation processes triggered by a pH jump. By photo release from caged proton the pH of the electrolyte could be decreased in a step of 0.1 pH units by a single ultraviolet-laser flash. Analysis of the pH-jump induced relaxation process in the P-E(2) conformation showed that three Ca-ATPase-specific processes could be identified, fast H(+) binding (τ < 100 μs) and pH-insensitive conformational relaxations after the release of the Ca(2+) ion (τ ∼160 ms), and a slow process (τ ∼3.4 s) whose origin could not be unambiguously revealed. The Ca(2+)-binding affinity in the P-E(2) conformation was reduced with increasing pH, a behavior that can be explained by a reversible transition of the empty P-E(2) state to an inactivated state of the ion pump. All findings are interpreted in the framework of the Post-Albers pump cycle introduced previously, supplemented by an additional transition to an inhibited state of the ion pump.

Electrogenic plasma membrane H+-ATPase activity using voltage sensitive dyes

Fast responding voltage sensitive dyes, RH421 and di-4-ASPBS, were used to study the electrogenic properties of plant plasma membrane proton pumps on sealed plasma membrane vesicles extracted by two-phase partitioning from Beta vulgaris and Avena sativa cv Swan root material. Fluorescence spectroscopy in the presence of the dye RH421 (10.8 nM) was sufficiently sensitive to detect electrogenic activity of the extracted plant vesicles. The dye detection system could detect inhibition of electrogenic activity of vesicles by vanadate (75 μM) and stimulation by nigericin (0.5 μM). The newly developed dye di-4-ASPBS was less sensitive to detecting the electrogenic proton pump activity. This study represents an important innovation in plant biophysics as this class of fast responding voltage sensitive dyes have never to our knowledge been used to study electrogenic proton pump activity derived from plant membranes and represents a novel approach for carrying out such studies.

Energy dissipation in slipping biological pumps

We describe active transport in slipping biological pumps, using mesoscopic nonequilibrium thermodynamics. The pump operation is characterised by its stochastic nature and energy dissipation. We show how heating as well as cooling effects can be associated with pump operation. We use as an example the well studied active transport of Ca2+ across a biological membrane by means of its ATPase, and use published data to find values for the transport coefficients of the pump under various conditions. Most of the transport coefficients of the pump, including those that relate ATP hydrolysis or synthesis to thermal effects, are estimated. This can give a quantitative description of thermogenesis. We show by calculation that all of these coupling coefficients are significant.

Coefficients for active transport and thermogenesis of Ca2+-ATPase isoforms

Coefficients for active transport of ions and heat in vesicles with Ca(2+)-ATPase from sarcoplasmic reticulum are defined in terms of a newly proposed thermodynamic theory and calculated using experiments reported in the literature. The coefficients characterize in a quantitative manner different performances of the enzyme isoforms. Four enzyme isoforms are examined, namely from white and red muscle tissue, from blood platelets, and from brown adipose mitochondria. The results indicate that the isoforms have a somewhat specialized function. White muscle tissue and brown adipose tissue have the same active transport coefficient ratio, but the activity level of the enzyme in white muscle is higher than in brown adipose tissue. The thermogenesis ratio is high in both white muscle and brown adipose tissue, in agreement with a specific role in nonshivering thermogenesis. Other isoforms do not have this ability to generate heat. A calcium-dependence of the coefficients is found, which can be understood as being in accordance with the role of this ion as a messenger in muscle contraction as well as in thermogenesis. The investigation points to new experiments related to structure as well as to function of the isoforms.

Evidence for domain motion in proteins affecting global diffusion properties: a nuclear magnetic resonance study

The rotational diffusion of proteins is an important hydrodynamic property. Compact protein structures were found previously to exhibit hydration layer viscosity, etaloc, higher than the viscosity of bulk water, eta. This implies an apparent activation energy for rotational diffusion higher than the activation energy of water viscosity, Eeta=15.4+/-0.3 kJ/mol. In this study we examine etaloc of internally mobile proteins using 15N spin relaxation methods. We also examine the activation enthalpy, DeltaH#, and activation entropy, DeltaS#, for rotational diffusion. Of particular relevance are internally mobile ligand-free forms and compact ligand-bound forms of multidomain proteins. Adenylate kinase (AKeco) and Ca2+-calmodulin (Ca2+-CaM) are typical examples. For AKeco (Ca2+-CaM) we find that DeltaH# is 14.5+/-0.5 (15.7+/-0.4) kJ/mol. For the complex of AKeco with the inhibitor AP5A (the complex of Ca2+-CaM with the peptide smMLCKp), we find that DeltaH# is 18.1+/-0.7 (18.2+/-0.5) kJ/mol. The internally mobile outer surface protein A has DeltaH#=12.6+/-0.8 kJ/mol, and the compact protein Staphylococcal nuclease has DeltaH#=18.8+/-0.6 kJ/mol. For the internally mobile and compact proteins studied, <|DeltaS(|> equals 62+/-7 J/(mol K) and 44+/-5 J/(mol K), respectively. The fact is that etaloc>eta (DeltaH#>Eeta) for compact proteins was ascribed previously to electrostatic interactions between surface sites and water rigidifying the hydration layer. We find herein that obliteration of these interactions by domain motion leads to etaloc approximately eta, DeltaH# approximately Eeta, and large activation entropy for internally mobile protein structures.

The measurable heat flux that accompanies active transport by Ca2+-ATPase

We present a new mesoscopic basis which can be used to derive flux equations for the forward and reverse mode of operation of ion-pumps. We obtain a description of the fluxes far from global equilibrium. An asymmetric set of transport coefficients is obtained, by assuming that the chemical reaction as well as the ion transports are activated, and that the enzyme has a temperature independent of the activation coordinates. Close to global equilibrium, the description reduces to the well known one from non-equilibrium thermodynamics with a symmetric set of transport coefficients. We show how the measurable heat flux and the heat production under isothermal conditions, as well as thermogenesis, can be defined. Thermogenesis is defined via the onset of the chemical reaction or ion transports by a temperature drop. A prescription has been given for how to determine transport coefficients on the mesocopic level, using the macroscopic coefficient obtained from measurements, the activation enthalpy, and a proper probability distribution. The method may give new impetus to a long-standing unsolved transport problem in biophysics.

Properties of the SR Ca-ATPase in an Open Microsomal Membrane Preparation

SR vesicles isolated from rabbit muscle were treated by a SDS incubation and subsequent dialysis to obtain open membrane fragments that allow a direct access to the luminal membrane surface and especially to the ion-binding sites in the P-E(2) conformation of the Ca-ATPase. The open membrane fragments showed about 80% of the enzyme activity in the untreated membranes. Pump function was investigated by using electrochromic styryl dyes. The kinetic properties of cytoplasmic ion binding showed no significant differences between the Ca-ATPases in SR vesicles and in membrane fragments. From pH-dependent Ca(2+) binding it could be deduced that due to the SDS treatment the density of negatively charged lipid was increased by one elementary charge per 12 lipid molecules. Major differences between Ca-ATPase from SR vesicles and membrane fragments were the respective fluorescence amplitudes. This effect is, however, produced by dye-lipid interaction and not by pump function. It was demonstrated that time-resolved kinetics may be study by the use of caged compounds such as caged ATP or caged calcium also in the case of the membrane fragments.

Charge transfer in P-type ATPases investigated on planar membranes

Planar lipid bilayers, e.g., black lipid membranes (BLM) and solid supported membranes (SSM), have been employed to investigate charge movements during the reaction cycle of P-type ATPases. The BLM/SSM method allows a direct measurement of the electrical currents generated by the cation transporter following chemical activation by a substrate concentration jump. The electrical current transients provides information about the reaction mechanism of the enzyme. In particular, the BLM/SSM technique allows identification of electrogenic steps which in turn may be used to localize ion translocation during the reaction cycle of the pump. In addition, using the high time resolution of the technique, especially when rapid activation via caged ATP is employed, rate constants of electrogenic and electroneutral steps can be determined. In the present review, we will discuss the main results obtained by the BLM and SSM methods and how they have contributed to unravel the transport mechanism of P-type ATPases.

Electrogenic steps of the SR Ca-ATPase enzymatic cycle and the effect of curcumin

Sarcoplasmic reticulum (SR) vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps in the presence of calcium ions. The resulting capacitive current transients are compared with those calculated on the basis of the enzymatic cycle of the calcium pump. This comparison provides information on the kinetics of the E(2)-E(1) conformational change and on its pH dependence. The alteration in the current transients following ATP concentration jumps in the presence of curcumin is examined. In particular, curcumin decreases the rate of slippage of the Ca-ATPase, and at concentrations above 10 microM reduces calcium transport by this pump.

Kinetics of proton binding to the sarcoplasmic reticulum Ca-ATPase in the E1 state

A new caged proton, 2-methoxy-5-nitrophenyl sulfate, was synthesized and used in time-resolved pH jump experiments to study proton binding in the sarcoplasmic reticulum Ca-ATPase. The major advantage of this compound is that it does not produce significant artifacts in experiments in which the fluorescent styryl dye 2BITC is used to monitor ion movements in the Ca pump. Two rate-limiting processes were resolved and their dependence on pH, Ca(2+) concentration, and temperature investigated. The faster process showed a relaxation time between 4 and 8 ms independent on pH and Ca(2+) concentration, and the time constant of the slower process varied between 31 ms (0 Ca(2+)) and 100 ms (100 microM Ca(2+)). A consistent mechanism to explain the results was derived in agreement with previous studies and the generally accepted Post-Albers scheme of the pump cycle. This mechanism requires that under physiological conditions the ion-binding sites are always occupied and two protons and a Ca(2+) ion replace each other. In the absence of ATP at low pH a nonphysiological state can be induced in which up to four protons bind to the Ca pump in the E(1) conformation. So far it could not be verified whether these additional protons bind to amino acid side chains or are coordinated as hydronium ions.

Activation Energy of Catalysis-Related Domain Motion in E. coli Adenylate Kinase

Adenylate kinase from E. coli (AKeco), folded into domains CORE, AMPbd, and LID, catalyzes the reaction AMP + ATP <--> 2ADP. Previous X-ray crystallography and optical solution methods showed that the domains AMPbd and LID, and the conserved P-loop, execute large-amplitude catalysis-related motions. We used (15)N NMR spin relaxation methods to find that the simplified model-free (MF) analysis does not, whereas our general Slowly Relaxing Local Structure analysis does, detect catalytic domain motion. SRLS set for the first time the correlation time for domain motion at tau(L)perpendicular = 8.2 ns, to be compared with tau(m) = 15.1 ns for global tumbling. These results were obtained at 303 K. Herein we conduct a temperature-dependent investigation of tau(L)perpendicular and tau(m) in the range of 288-310 K. We found that the activation energy for global tumbling is Ea = 16.9 +/- 0.5 kJ/mol, the hydrodynamic volume of hydrated AKeco is 65.6 +/- 2.1 nm3, its radius is 2.50 +/- 0.03 nm, and the number of hydration layers is 1.77. The average tau(L)perpendicular value decreases from 11 ns at 288 K to 4 ns at 310 K, with activation energies of 29.7 +/- 3.3, 32.1 +/- 4.3, and 30.4 +/- 4.3 kJ/mol for the domains AMPbd and LID, and the catalytic P-loop, respectively. These values are two-to-three times smaller than typical activation energies of enzymatic reactions. Hence kinase catalysis appears not to be controlled by domain motion in the ligand-free enzyme. However, the latter process clearly facilitates important mechanical aspects such as steric recognition and capturing of the AMP and ATP substrates, their proper positioning for phosphorylation, and the release of the ADP product.

Kinetics of Ca2+ binding to the SR Ca-ATPase in the E1 state

The time-resolved kinetics of Ca2+ binding to the SR Ca-ATPase in the E1 state was investigated by Ca(2+)-concentration jump experiments. Ca2+ was released by an ultraviolet-light flash from caged calcium, and charge movements in the membrane domain of the ion pumps were detected by the fluorescent styryl dye 2BITC. The partial reaction (H3E1 <-->) E1 <--> CaE1 <--> Ca2E1 can be characterized by two time constants, tau1 and tau2, both of which are not significantly Ca(2+)-concentration-dependent and only weakly pH-dependent at pH < 7.5. Both time constants differ by a factor of approximately 50 (4.7 vs. 200 ms). The weak substrate-dependence indicates that the rate-limiting process is not related to Ca2+ migration through the access channel and ion binding to the binding sites but to conformational rearrangements preceding the ion movements. The high activation energy obtained for both processes, 42.3 kJ mol(-1) and 60.3 kJ mol(-1) at pH 7.2, support this concept. Transient binding of Ca ions to the loop L67 and a movement of the Ca-loaded loop are discussed as a mechanism that facilitates the entrance of both Ca ions into the access channel to the ion-binding sites.

Time-resolved charge translocation by sarcoplasmic reticulum Ca-ATPase measured on a solid supported membrane

Sarcoplasmic reticulum vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps both in the absence and in the presence of K(+) ions and at different pH values. Ca(2+) concentration jumps in the absence of ATP were also carried out. The resulting capacitive current transients were analyzed together with the charge under the transients. The relaxation time constants of the current transients were interpreted on the basis of an equivalent circuit. The current transient after ATP concentration jumps and the charge after Ca(2+) concentration jumps in the absence of ATP exhibit almost the same dependence upon the Ca(2+) concentration, with a half-saturating value of approximately 1.5 microM. The pH dependence of the charge after Ca(2+) translocation demonstrates the occurrence of one H(+) per one Ca(2+) countertransport at pH 7 by direct charge-transfer measurements. The presence of K(+) decreases the magnitude of the current transients without altering their shape; this decrease is explained by K(+) binding to the cytoplasmic side of the pump in the E(1) conformation and being released to the same side during the E(1)-E(2) transition.