Electrogenic Binding of Ions at the Cytoplasmic Side of the Na⁺,K⁺-ATPase

Electrogenic binding of ions from the cytoplasmic side of the Na+,K+-ATPase has been studied by measurements of changes of the membrane capacitance and conductance triggered by a jump of pH or of the sodium-ion concentration in the absence of ATP. The pH jumps were performed in experiments with membrane fragments containing purified Na+,K+-ATPase adsorbed to a bilayer lipid membrane (BLM). Protons were released in a sub-millisecond time range from a photosensitive compound (caged H+) triggered by a UV light flash. The sodium concentration jumps were carried out by a fast solution exchange in experiments with membrane fragments attached to a solid-supported membrane deposited on a gold electrode. The change of the membrane capacitance triggered by the pH jump depended on the sodium-ion concentration. Potassium ions had a similar effect on the capacitance change triggered by a pH jump. The effects of these ions are explained by the their competition with protons in the binding sites on cytoplasmic side of the Na+,K+-ATPase. The approximation of the experimental data by a theoretical model yields the dissociation constants, K, and the cooperativity coefficients, n, of the binding sites for sodium ions (K = 2.7 mM, n = 2) and potassium ions (K = 1.7 mM, n = 2). In the presence of magnesium ions the apparent dissociation constants of sodium increased. A possible reason of the inhibition of sodium-ion binding by magnesium ions can be an electrostatic or conformational effect of magnesium ions bound to a separate site of the Na+,K+-ATPase close to the entrance to the sodium-ion binding sites

Binding of Potassium Ions Inside the Access Channel at the Cytoplasmic Side of Na⁺,K⁺-ATPase

Binding of potassium ions through an access channel from the cytoplasmic side of Na+,K+-ATPase and the effect of pH and magnesium ions on this process have been studied. The studies were carried out by a previously developed method of measuring small increments of the admittance (capacitance and conductivity) of a compound membrane consisting of a bilayer lipid membrane with adsorbed membranes fragments containing Na+,K+-ATPase. The capacitance change of the membrane with the Na+,K+-ATPase was induced abruptly by release of protons from a bound form (caged H+) upon a UV-light flash in the absence of magnesium ions. The change of admittance consisted of an initial fast jump and a slow relaxation to a stationary value within a time of about 1–2 s. The kinetics of the capacitance relaxation depended on pH and the concentration of magnesium and potassium ions. The dependence of the rapid capacitance jump on the potassium concentration corresponded to the predictions of the model developed earlier that describes binding of sodium or potassium ions in competition with protons. The effect of magnesium ions can be explained by the assuming that they bind to the Na+,K+-ATPase and affect binding of potassium ions because of either changes in protein conformation or the creation of an electrostatic field in the access channel on the cytoplasmic side.publishe

Electrostatic Potentials Caused by the Release of Protons from Photoactivated Compound Sodium 2-Methoxy-5-nitrophenyl Sulfate at the Surface of Bilayer Lipid Membrane

Lateral transport and release of protons at the water–membrane interface play crucial roles in cell bioenergetics. Therefore, versatile techniques need to be developed for investigating as well as clarifying the main features of these processes at the molecular level. Here, we experimentally measured the kinetics of binding of protons released from the photoactivated compound sodium 2-methoxy-5-nitrophenyl sulfate (MNPS) at the surface of a bilayer lipid membrane (BLM). We developed a theoretical model of this process describing the damage of MNPS coupled with the release of the protons at the membrane surface, as well as the exchange of MNPS molecules and protons between the membrane and solution. We found that the total change in the boundary potential difference across the membrane, ∆ϕb, is the sum of opposing effects of adsorption of MNPS anions and release of protons at the membrane–water interface. Steady-state change in the ∆ϕb due to protons decreased with the concentration of the buffer and increased with the pH of the solution. The change in the concentration of protons evaluated from measurements of ∆ϕb was close to that in the unstirred water layer near the BLM. This result, as well as rate constants of the proton exchange between the membrane and the bulk solution, indicated that the rate-limiting step of the proton surface to bulk release is the change in the concentration of protons in the unstirred layer. This means that the protons released from MNPS remain in equilibrium between the BLM surface and an adjacent water layer

Influenza virus Matrix Protein M1 preserves its conformation with pH, changing multimerization state at the priming stage due to electrostatics

Influenza A virus matrix protein M1 plays an essential role in the virus lifecycle, but its functional and structural properties are not entirely defined. Here we employed small-angle X-ray scattering, atomic force microscopy and zeta-potential measurements to characterize the overall structure and association behavior of the full-length M1 at different pH conditions. We demonstrate that the protein consists of a globular N-terminal domain and a flexible C-terminal extension. The globular N-terminal domain of M1 monomers appears preserved in the range of pH from 4.0 to 6.8, while the C-terminal domain remains flexible and the tendency to form multimers changes dramatically. We found that the protein multimerization process is reversible, whereby the binding between M1 molecules starts to break around pH 6. A predicted electrostatic model of M1 self-assembly at different pH revealed a good agreement with zeta-potential measurements, allowing one to assess the role of M1 domains in M1-M1 and M1-lipid interactions. Together with the protein sequence analysis, these results provide insights into the mechanism of M1 scaffold formation and the major role of the flexible and disordered C-terminal domain in this process