Potassium-39 NMR of K+ interaction with the gramicidin channel and NMR-derived conductance ratios for Na+, K+ and Rb+

Abstract

A potassium-39 NMR study of potassium ion interaction with the gramicidin transmembrane channel in phospholipid bilayers at high ion activity is reported which allows determination of a weak binding constant, Kwb approximately equal to 8.3/M, and an off-rate constant for the weak site, kwoff approximately equal to 2.6 X 10(7)/sec. These values are interpreted with the aid of additional NMR data as the binding constant for formation of the doubly occupied channel state and the rate constant for an ion leaving the doubly occupied state. Considering the singly occupied channel state for the potassium ion to be "electrically silent" at 1 molar ion activity, as with the sodium ion, the single-channel conductance for 100 mV and 30 degrees C calculated to be 29 pS, and using the same approximation with previous NMR results on the sodium and rubidium ions, reasonable conductance ratios were calculated. Further experimental estimates of the other three constants with the experimental location of binding sites and Eyring rate theory to introduce voltage dependence allowed a more complete calculation of the two-site channel. The single-channel conductance for potassium ion is calculated to be 24 pS at 1 M activity and 26 pS at 0.6 M activity, which compares for diphytanoyl phosphatidylcholine membranes to an experimental most probable single-channel conductance of 25 pS and a mean channel conductance of 20 pS. The calculated conductance ratios using NMR-derived constants were gamma (K)/gamma (Na) = 2.0 and gamma (Rb)/gamma (Na) = 4.3.(ABSTRACT TRUNCATED AT 250 WORDS)

Conformational transitions of gramicidin A in phospholipid model membranes. A high-performance liquid chromatography assessment

“…Other laboratories have prepared samples by adding gramicidin either as a dry powder or from organic solution to preformed micelles or vesicles. Such preparations require extensive incubation at elevated temperatures (around 70 °C) to achieve the characteristic spectral features of the gramicidin HH dimer channel state (Urry et al, 1979(Urry et al, ,1985(Urry et al, ,1986Spinsi et al, 1979;Masotti et al, 1980). On the basis of the above observations, it has been suggested that a mixture of gramicidin conformers may be present in different proportions in different reconstitution experiments, the double-helical dimer being a possible intermediate in the membrane insertion process (Wallace, 1986).…”

31P and 1H magnetic resonance spectroscopy of acute alcohol cardiac depression in rats

“…The Na+K+ pump provides the active transport of Na+ out of the cell. Gramicidin-A-like channels have been shown to regulate both Na' and water uptake of the cell (27,28).…”