Involvement of the alpha-subunit N-terminus in the mechanism of the Na+,K+-ATPase

Previous studies have shown that cytoplasmic K+ release and the associated E2 → E1 conformational change of the Na+,K+-ATPase is a major rate-determining step of the enzyme's ion pumping cycle and hence a prime site of acute regulatory intervention. From the ionic strength dependence of the enzyme's distribution between the E2 and E1 states, it has also been found that E2 is stabilized by an electrostatic attraction. Any disruption of this electrostatic attraction would, thus, have profound effects on the rate of ion pumping. The aim of this paper is to identify the location of this interaction. Using enhanced-sampling molecular dynamics simulations with a predicted N-terminal structure added to the X-ray crystal structure of the Na+,K+-ATPase, a previously postulated salt bridge between Lys32 and Glu233 (rat sequence numbering) of the enzyme's α-subunit can be excluded. The residues never approach closely enough to form a salt bridge. In contrast, strong interactions with anionic lipid head groups were seen. To investigate the possibility of a protein-lipid interaction experimentally, the surface charge density of Na+,K+-ATPase-containing membrane fragments was estimated from zeta potential measurements to be 0.019 (± 0.001) C m-2. This is in good agreement with the charge density previously determined to be responsible for stabilization of the E2 state of 0.023 (± 0.009) C m-2 and the membrane charge density estimated here from published electron-microscopic images of 0.018C m-2. The results are, therefore, consistent with an interaction of the Na+,K+-ATPase α-subunit N-terminus with negatively-charged lipid head groups of the neighbouring cytoplasmic membrane surface as the origin of the electrostatic interaction stabilising the E2 state.

Interaction of salt with ether- and ester-linked phospholipid bilayers

A distinguishing feature of Archaeal plasma membranes is that their phospholipids contain ether-links, as opposed to bacterial and eukaryotic plasma membranes where phospholipids primarily contain ester-links. Experiments show that this chemical difference in headgroup-tail linkage does produce distinct differences in model bilayer properties. Here we examine the effects of salt on bilayer structure in the case of an ether-linked lipid bilayer. We use molecular dynamics simulations and compare equilibrium properties of two model lipid bilayers in NaCl salt solution - POPC and its ether-linked analog that we refer to as HOPC. We make the following key observations. The headgroup region of HOPC "adsorbs" fewer ions compared to the headgroup region of POPC. Consistent with this, we note that the Debye screening length in the HOPC system is ∼ 10% shorter than that in the POPC system. Herein, we introduce a protocol to identify the lipid-water interfacial boundary that reproduces the bulk salt distribution consistent with Gouy-Chapman theory. We also note that the HOPC bilayer has excess solvent in the headgroup region when compared to POPC, coinciding with a trough in the electrostatic potential. Waters in this region have longer autocorrelation times and smaller lateral diffusion rates compared to the corresponding region in the POPC bilayer, suggesting that the waters in HOPC are more strongly coordinated to the lipid headgroups. Furthermore, we note that it is this region of tightly coordinated waters in the HOPC system that has a lower density of Na⁺ ions. Based on these observations we conclude that an ether-linked lipid bilayer has a lower binding affinity for Na⁺ compared to an ester-linked lipid bilayer.