Molecular electrometer and binding of cations to phospholipid bilayers

Despite the vast amount of experimental and theoretical studies on the binding affinity of cations -especially the biologically relevant Na+ and Ca2+ - for phospholipid bilayers, there is no consensus in the literature. Here we show that by interpreting changes in the choline headgroup order parameters according to the 'molecular electrometer' concept [Seelig et al., Biochemistry, 1987, 26, 7535], one can directly compare the ion binding affinities between simulations and experiments. Our findings strongly support the view that in contrast to Ca2+ and other multivalent ions, Na+ and other monovalent ions (except Li+) do not specifically bind to phosphatidylcholine lipid bilayers at sub-molar concentrations. However, the Na+ binding affinity was overestimated by several molecular dynamics simulation models, resulting in artificially positively charged bilayers and exaggerated structural effects in the lipid headgroups. While qualitatively correct headgroup order parameter response was observed with Ca2+ binding in all the tested models, no model had sufficient quantitative accuracy to interpret the Ca2+: lipid stoichiometry or the induced atomistic resolution structural changes. All scientific contributions to this open collaboration work were made publicly, using nmrlipids. blogspot.fi as the main communication platform.Peer reviewe

Role of Lipids in Spheroidal High Density Lipoproteins

We study the structure and dynamics of spherical high density lipoprotein (HDL) particles through coarse-grained multi-microsecond molecular dynamics simulations. We simulate both a lipid droplet without the apolipoprotein A-I (apoA-I) and the full HDL particle including two apoA-I molecules surrounding the lipid compartment. The present models are the first ones among computational studies where the size and lipid composition of HDL are realistic, corresponding to human serum HDL. We focus on the role of lipids in HDL structure and dynamics. Particular attention is paid to the assembly of lipids and the influence of lipid-protein interactions on HDL properties. We find that the properties of lipids depend significantly on their location in the particle (core, intermediate region, surface). Unlike the hydrophobic core, the intermediate and surface regions are characterized by prominent conformational lipid order. Yet, not only the conformations but also the dynamics of lipids are found to be distinctly different in the different regions of HDL, highlighting the importance of dynamics in considering the functionalization of HDL. The structure of the lipid droplet close to the HDL-water interface is altered by the presence of apoA-Is, with most prominent changes being observed for cholesterol and polar lipids. For cholesterol, slow trafficking between the surface layer and the regimes underneath is observed. The lipid-protein interactions are strongest for cholesterol, in particular its interaction with hydrophobic residues of apoA-I. Our results reveal that not only hydrophobicity but also conformational entropy of the molecules are the driving forces in the formation of HDL structure. The results provide the first detailed structural model for HDL and its dynamics with and without apoA-I, and indicate how the interplay and competition between entropy and detailed interactions may be used in nanoparticle and drug design through self-assembly

Partial density profiles of lipids and ions

<p>Partial density profiles of lipids and ions calculated using the script available on Zenodo. Each simulated system contains 600 DPPC molecules, 150 mM NaCl and 30 waters/lipid. The ion model is the one described by Roux. The analysis has been performed over the last 40 ns of each 100 ns trajectory. All simulations have been performed with the Slipids force field.</p

Application of molecular modelling and EPR spectroscopy to lipid membranes - a combined approach

Knowledge of molecular interactions, thermodynamics, temperature and system composition effects are crucial for understanding the role that different lipids play in vital life processes in biological membranes. This knowledge is also important for understanding the impact that electromagnetic fields have on the order and mobility of molecules in lipid bilayers. The last decade has seen radical improvement in the molecular modelling of complex molecular and bio-molecular systems including lipid bilayers using Molecular Dynamics (MD) simulation techniques. MD simulations are now much faster and more accurate allowing researchers to predict complex molecularphenomena using actual structures.In this paper we present our recent results on the application of large scale MD simulations to phospholipid bilayers under different composition and conditions.Examples include: both all-atom and coarse-grain large scale MD simulations of binary and ternary compositions of lipid bilayers, modelling of the penetration of gas molecules (O2) in lipid bilayers, the effects of antimicrobial peptides on biological membranes, separation of lipid microdomains as a model for the study of lipid rafts. We also report MD simulations on lipid bilayers doped with structurally different nitroxide spin probes that are employed inexperimental variable temperature EPR spectroscopy. Finally, our recent preliminary results of all-atom MD modelling of the lipid bilayers subjected to microwave electric fields are also presented and discussed

Dynamics of Activation of Lecithin: Cholesterol Acyltransferase by Apolipoprotein A-I

The product of transesterification of phospholipid acyl chains and unesterified cholesterol (UC) by the enzyme lecithin:cholesterol acyltransferase (LCAT) is cholesteryl ester (CE). Activation of LCAT by apolipoprotein (apo) A-I on nascent (discoidal) high-density lipoproteins (HDL) is essential for formation of mature (spheroidal) HDL during the antiatherogenic process of reverse cholesterol transport. Here we report all-atom and coarse-grained (CG) molecular dynamics (MD) simulations of HDL particles that have major implications for mechanisms of I-CAT activation. Both the all-atom and CG simulations provide support for a model in which the helix 5/5 domains or apoA-I create an amphipathic "presentation tunnel" that exposes methyl ends of acyl chains at the bilayer center to solvent. Further, CG simulations show that UC also becomes inserted with high efficiency into the amphipathic presentation tunnel with its hydroxyl moiety (UC-OH) exposed to solvent; these results are consistent with trajectory analyses of the all-atom simulations showing that UC is being concentrated in the vicinity of the presentation tunnel. Finally, consistent with known product inhibition of CE-rich HDL by CE, CG simulations of CE-rich spheroidal HDL indicate partial blockage of the amphipathic presentation tunnel by CE. These results lead us to propose the following working hypothesis. After attachment of LCAT to discoidal HDL, the helix 5/5 domains in apoA-I form amphipathic presentation tunnels for migration of hydrophobic acyl chains and amphipathic UC from the bilayer to the phospholipase A2-like and esterification active sites of LCAT, respectively. This hypothesis is currently being tested by site-directed mutagenesis

L-DOPA and Droxidopa: From Force Field Development to Molecular Docking into Human &beta;2-Adrenergic Receptor

The increasing interest in the molecular mechanism of the binding of different agonists and antagonists to &beta;2-adrenergic receptor (&beta;2AR) inactive and active states has led us to investigate protein&ndash;ligand interactions using molecular docking calculations. To perform this study, the 3.2 &Aring; X-ray crystal structure of the active conformation of human &beta;2AR in the complex with the endogenous agonist adrenaline has been used as a template for investigating the binding of two exogenous catecholamines to this adrenergic receptor. Here, we show the derivation of L-DOPA and Droxidopa OPLS all atom (AA) force field (FF) parameters via quantum mechanical (QM) calculations, molecular dynamics (MD) simulations in aqueous solutions of the two catecholamines and the molecular docking of both ligands into rigid and flexible &beta;2AR models. We observe that both ligands share with adrenaline similar experimentally observed binding anchor sites, which are constituted by Asp113/Asn312 and Ser203/Ser204/Ser207 side chains. Moreover, both L-DOPA and Droxidopa molecules exhibit binding affinities comparable to that predicted for adrenaline, which is in good agreement with previous experimental and computational results. L-DOPA and Droxidopa OPLS AA FFs have also been tested by performing MD simulations of these ligands docked into &beta;2AR proteins embedded in lipid membranes. Both hydrogen bonds and hydrophobic interaction networks observed over the 1 &mu;s MD simulation are comparable with those derived from molecular docking calculations and MD simulations performed with the CHARMM FF

"Sticky" and "Promiscuous", the Yin and Yang of Apolipoprotein A-I Termini in Discoidal High-Density Lipoproteins: A Combined Computational-Experimental Approach

Apolipoprotein (apo) A-I-containing lipoproteins in the form of high-density lipoproteins (HDL) are inversely correlated with atherosclerosis. Because HDL is a soft form of condensed matter easily deformable by thermal fluctuations, the molecular mechanisms for HDL remodeling are not well understood. A promising approach to understanding HDL structure and dynamics is molecular dynamics (MD). In the present study, two computational strategies, MD simulated annealing (MDSA) and MD temperature jump, were combined with experimental particle reconstitution to explore molecular mechanisms for phospholipid- (PL-) rich HDL particle remodeling. The N-terminal domains of full-length apoA-I were shown to be "sticky", acting as a molecular latch largely driven by salt bridges, until, at a critical threshold of particle size, the associated domains released to expose extensive hydrocarbon regions of the PL to solvent. The "sticky" N-termini also associate with other apoA-I domains, perhaps being involved in N-terminal loops suggested by other laboratories. Alternatively, the overlapping helix 10 C-terminal domains of apoA-I were observed to be extremely mobile or "promiscuous", transiently exposing limited hydrocarbon regions of PL. Based upon these models and reconstitution studies, we propose that separation of the N-terminal domains, as particles exceed a critical size, triggers fusion between particles or between particles and membranes, while the C-terminal domains of apoA-I drive the exchange of polar lipids down concentration gradients between particles. This hypothesis has significant biological relevance since lipid exchange and particle remodeling are critically important processes during metabolism of HDL particles at every step in the antiatherogenic process of reverse cholesterol transport