Gliding edge dislocations in proteins as a mechanism for active ion transport

Mechanism responsible for oligomycin-induced occlusion of Na+ within Na/K-ATPase

The mechanism whereby oligomycin occludes Na+ within Na/K-ATPase was investigated to study Na+ and K+ transport mechanisms. Oligomycin stimulated Na+ binding to Na/K-ATPase but inhibited Na-K and Na-Na exchange. The oligomycin concentration required to stimulate Na+ binding to half-maximal was 4.5 microM, which was close to the concentration that reduced Na-Na and Na-K exchange and ATPase activity to half-maximal, suggesting that Na/K-ATPase possesses an oligomycin binding site responsible for stimulating Na+ binding and reducing ion exchange and ATPase activity. In contrast, neither K+ binding nor K+ transport was affected by oligomycin. Limited tryptic digestion of Na/K-ATPase showed that, unlike Na+, K+, and ouabain, oligomycin treatment did not result in a specific digestion pattern. Oligomycin appeared to inhibit ouabain binding in a noncompetitive manner, whereas it did not affect ATP binding. Na/K-ATPase isoforms with low and high sensitivities to ouabain were equally sensitive to oligomycin. These results suggest that the oligomycin binding site is located on the extracellular side of Na/K-ATPase, at a different position from the ouabain binding site, and this antibiotic did not induce a conformational change of Na/K-ATPase. We propose that oligomycin interacts with the Na+ occlusion site from the extracellular side of Na/K-ATPase, which delays Na+ release to the extracellular side without inducing a conformational change, suggesting that the pathways responsible for Na+ and K+ transport differ.

Model calculations of polarization effects in elastic membrane channels

Molecular channels embedded in biological membranes are extremely well balanced systems, because the dielectric barrier imposed on an ion's self-energy, which would make the membrane practically impenetrable, is locally cancelled by the interaction of the ion with the channel up to a few kT at room temperature. This purpose can serve only highly specialized systems with a certain ligand configuration and appropriate properties. In this article two different models are tested with respect to their polarization properties and discussed in detail, one for ionic polarization and the other for dipolar polarization. The channel-mediated attractive forces between two ions are also investigated in both cases.

Na+, K+-ATPase: relation of conformational transitions to function

In its native environment, Na+, K+-ATPase of the plasma membrane is an oligomer consisting of two or more of each of two major subunits. Na+ and K+ move across the membrane through the channels that exist between the catalytic subunits of this oligomer. Two distinct ligand-induced conformational transitions (one due to the binding of K+ and ATP to the enzyme, and the other resulting from the phosphorylation of the enzyme in the presence of Na+ and ATP) cause changes in the geometries of the intersubunit channels, and provide the necessary energy-linked gating mechanisms for the transmembrane movements of ions against electrochemical gradients.