Physiological changes in bilayer thickness induced by cholesterol control GPCR rhodopsin function

We monitored the effect on function of the G-protein-coupled receptor (GPCR) rhodopsin from small, stepwise changes in bilayer thickness induced by cholesterol. Over a range of phosphatidylcholine bilayers with hydrophobic thickness from ≈21 Å to 38 Å, the metarhodopsin-I (MI)/metarhodopsin-II (MII) equilibrium was monitored with UV-visible spectroscopy while ordering of hydrocarbon chains was probed by 2H-NMR. Addition of cholesterol shifted equilibrium toward MII for bilayers thinner than the average length of hydrophobic transmembrane helices (27 Å) and to MI for thicker bilayers, while small bilayer thickness changes within the range of the protein hydrophobic thickness drastically up- or downregulated MII formation. The cholesterol-induced shifts toward MII for thinner membranes correlated with the cholesterol-induced increase of bilayer hydrophobic thickness measured by NMR, consistent with continuum elastic modeling. The energetic penalty of adding cholesterol to thick bilayers caused rhodopsin oligomerization and a shift toward MI. In membranes of physiological thickness, changes in bilayer mechanical properties induced by cholesterol potentiated the interplay between bilayer and protein thickness resulting in large swings of the MI-MII equilibrium. In membrane containing cholesterol, elastic deformations near the protein are a dominant energetic contribution to the functional equilibrium of the model GPCR rhodopsin.

How Do Ethanolamine Plasmalogens Contribute to Order and Structure of Neurological Membranes?

Ethanolamine plasmalogen (EtnPLA) is a conical-shaped ether lipid and an essential component of neurological membranes. Low stability against oxidation limits its study in experiments. The concentration of EtnPLA in the bilayer varies depending on cell type and disease progression. Here we report on mixed bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-(1Z-octadecenyl)-2-oleoyl-sn-glycero-3-phosphoethanolamine (C18(Plasm)-18:1PE, PLAPE), an EtnPLA lipid subtype, at mole ratios of 2:1, 1:1, and 1:2. We present X-ray diffuse scattering (XDS) form factors F(qz) from oriented stacks of bilayers, related electron-density profiles, and hydrocarbon chain NMR order parameters. To aid future research on EtnPLA lipids and associated proteins, we have also extended the CHARMM36 all-atom force field to include the PLAPE lipid. The ability of the new force-field parameters to reproduce both X-ray and NMR structural properties of the mixed bilayer is remarkable. Our results indicate a thickening of the bilayer upon incorporation of increasing amounts of PLAPE into mixed bilayers, a reduction of lateral area per molecule, and an increase in lipid tail-ordering. The lateral compressibility modulus (KA) calculated from simulations yielded values for PLAPE similar to POPC.

Permeability of membranes in the liquid ordered and liquid disordered phases

The functional significance of ordered nanodomains (or rafts) in cholesterol rich eukaryotic cell membranes has only begun to be explored. This study exploits the correspondence of cellular rafts and liquid ordered (L_o) phases of three-component lipid bilayers to examine permeability. Molecular dynamics simulations of L_o phase dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and cholesterol show that oxygen and water transit a leaflet through the DOPC and cholesterol rich boundaries of hexagonally packed DPPC microdomains, freely diffuse along the bilayer midplane, and escape the membrane along the boundary regions. Electron paramagnetic resonance experiments provide critical validation: the measured ratio of oxygen concentrations near the midplanes of liquid disordered (L_d) and L_o bilayers of DPPC/DOPC/cholesterol is 1.75 ± 0.35, in very good agreement with 1.3 ± 0.3 obtained from simulation. The results show how cellular rafts can be structurally rigid signaling platforms while remaining nearly as permeable to small molecules as the L_d phase.

Stimulation of α-synuclein amyloid formation by phosphatidylglycerol micellar tubules

α-Synuclein (α-Syn) is a presynaptic protein that is accumulated in its amyloid form in the brains of Parkinson's patients. Although its biological function remains unclear, α-syn has been suggested to bind to synaptic vesicles and facilitate neurotransmitter release. Recently, studies have found that α-syn induces membrane tubulation, highlighting a potential mechanism for α-syn to stabilize highly curved membrane structures which could have both functional and dysfunctional consequences. To understand how membrane remodeling by α-syn affects amyloid formation, we have studied the α-syn aggregation process in the presence of phosphatidylglycerol (PG) micellar tubules, which were the first reported example of membrane tubulation by α-syn. Aggregation kinetics, β-sheet content, and macroscopic protein-lipid structures were observed by Thioflavin T fluorescence, circular dichroism spectroscopy and transmission electron microscopy, respectively. Collectively, the presence of PG micellar tubules formed at a stochiometric (L/P = 1) ratio was found to stimulate α-syn fibril formation. Moreover, transmission electron microscopy and solid-state nuclear magnetic resonance spectroscopy revealed the co-assembly of PG and α-syn into fibril structures. However, isolated micellar tubules do not form fibrils by themselves, suggesting an important role of free α-syn monomers during amyloid formation. In contrast, fibrils did not form in the presence of excess PG lipids (≥L/P = 50), where most of the α-syn molecules are in a membrane-bound α-helical form. Our results provide new mechanistic insights into how membrane tubules modulate α-syn amyloid formation and support a pivotal role of protein-lipid interaction in the dysfunction of α-syn.

The role of membrane curvature elastic stress for function of rhodopsin-like G protein-coupled receptors

The human genome encodes about 800 different G protein-coupled receptors (GPCR). They are key molecules in signal transduction pathways that transmit signals of a variety of ligands such as hormones and neurotransmitters to the cell interior. Upon ligand binding, the receptors undergo structural transitions that either enhance or inhibit transmission of a specific signal to the cell interior. Here we discuss results which indicate that transmission of such signals can be strongly modulated by the composition of the lipid matrix into which GPCR are imbedded. Experimental results have been obtained on rhodopsin, a prototype GPCR whose structure and function is representative for the great majority of GPCR in humans. The data shed light on the importance of curvature elastic stress in the lipid domain for function of GPCR.

Elastic properties of polyunsaturated phosphatidylethanolamines influence rhodopsin function

Membranes with a high content of polyunsaturated phosphatidylethanolamines (PE) facilitate formation of metarhodopsin-II (M(II)), the photointermediate of bovine rhodopsin that activates the G protein transducin. We determined whether M(II)-formation is quantitatively linked to the elastic properties of PEs. Curvature elasticity of monolayers of the polyunsaturated lipids 18 : 0-22 : 6(n - 3)PE, 18 : 0-22 : 5(n)- 6PE and the model lipid 18 : 1(n - 9)-18 : 1,(n- 9)PE were investigated in the inverse hexagonal phase. All three lipids form lipid monolayers with rather low spontaneous radii of curvature of 26-28 angstroms. In membranes, all three PEs generate high negative curvature elastic stress that shifts the equilibrium of MI(I)/M(II) photointermediates of rhodopsin towards M(II) formation.

Contribution of membrane elastic energy to rhodopsin function

We considered the issue of whether shifts in the metarhodopsin I (MI)-metarhodopsin II (MII) equilibrium from lipid composition are fully explicable by differences in bilayer curvature elastic stress. A series of six lipids with known spontaneous radii of monolayer curvature and bending elastic moduli were added at increasing concentrations to the matrix lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and the MI-MII equilibrium measured by flash photolysis followed by recording UV-vis spectra. The average area-per-lipid molecule and the membrane hydrophobic thickness were derived from measurements of the (2)H NMR order parameter profile of the palmitic acid chain in POPC. For the series of ethanolamines with different levels of headgroup methylation, shifts in the MI-MII equilibrium correlated with changes in membrane elastic properties as expressed by the product of spontaneous radius of monolayer curvature, bending elastic modulus, and lateral area per molecule. However, for the entire series of lipids, elastic energy explained the shifts only partially. Additional contributions correlated with the capability of the ethanolamine headgroups to engage in hydrogen bonding with the protein, independent of the state of ethanolamine methylation, with introduction of polyunsaturated sn-2 hydrocarbon chains, and with replacement of the palmitic acid sn-1 chains by oleic acid. The experiments point to the importance of interactions of rhodopsin with particular lipid species in the first layer of lipids surrounding the protein as well as to membrane elastic stress in the lipid-protein domain.

Functional Reconstitution of Rhodopsin into Tubular Lipid Bilayers Supported by Nanoporous Media

We report on a novel reconstitution method for G-protein-coupled receptors (GPCRs) that yields detergent-free, single, tubular membranes in porous anodic aluminum oxide (AAO) filters at concentrations sufficient for structural studies by solid-state NMR. The tubular membranes line the inner surface of pores that traverse the filters, permitting easy removal of detergents during sample preparation as well as delivery of ligands for functional studies. Reconstitution of bovine rhodopsin into AAO filters did not interfere with rhodopsin function. Photoactivation of rhodopsin in AAO pores, monitored by UV-vis spectrophotometry, was indistinguishable from rhodopsin in unsupported unilamellar liposomes. The rhodopsin in AAO pores is G-protein binding competent as shown by a [35S]GTPgammaS binding assay. The lipid-rhodopsin interaction was investigated by 2H NMR on sn-1- or sn-2-chain perdeuterated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phospholine as a matrix lipid. Rhodopsin incorporation increased mosaic spread of bilayer orientations and contributed to spectral density of motions with correlation times in the range of nano- to microseconds, detected as a significant reduction in spin-spin relaxation times. The change in lipid chain order parameters due to interaction with rhodopsin was insignificant.

Evidence for specificity in lipid-rhodopsin interactions

The interaction of bovine rhodopsin with poly- and monounsaturated lipids was studied by (1)H MAS NMR with magnetization transfer from rhodopsin to lipid. Experiments were conducted on bovine rod outer segment (ROS) disks and on recombinant membranes containing lipids with polyunsaturated, docosahexaenoyl (DHA) chains. Poly- and monounsaturated lipids interact specifically with different sites on the rhodopsin surface. Rates of magnetization transfer from protein to DHA are lipid headgroup-dependent and increased in the sequence PC < PS < PE. Boundary lipids are in fast exchange with the lipid matrix on a time scale of milliseconds or shorter. All rhodopsin photointermediates transferred magnetization preferentially to DHA-containing lipids, but highest rates were observed for Meta-III rhodopsin. The experiments show clearly that the surface of rhodopsin has sites for specific interaction with lipids. Current theories of lipid-protein interaction do not account for such surface heterogeneity.

Thermodynamics of the pyruvate kinase reaction and the reversal of glycolysis in heart and skeletal muscle

The effect of temperature, pH, and free [Mg(2+)] on the apparent equilibrium constant of pyruvate kinase (phosphoenol transphosphorylase) (EC ) was investigated. The apparent equilibrium constant, K', for the biochemical reaction P-enolpyruvate + ADP = ATP + Pyr was defined as K' = [ATP][Pyr]/[ADP][P-enolpyruvate], where each reactant represents the sum of all the ionic and metal complexed species in M. The K' at pH 7.0, 1.0 mm free Mg(2+) and I of 0.25 m was 3.89 x 10(4) (n = 8) at 25 degrees C. The standard apparent enthalpy (DeltaH' degrees ) for the biochemical reaction was -4.31 kJmol(-1) in the direction of ATP formation. The corresponding standard apparent entropy (DeltaS' degrees ) was +73.4 J K(-1) mol(-1). The DeltaH degrees and DeltaS degrees values for the reference reaction, P-enolpyruvate(3-) + ADP(3-) + H(+) = ATP(4-) + Pyr(1-), were -6.43 kJmol(-1) and +180 J K(-1) mol(-1), respectively (5 to 38 degrees C). We examined further the mass action ratio in rat heart and skeletal muscle at rest and found that the pyruvate kinase reaction in vivo was close to equilibrium i.e. within a factor of about 3 to 6 of K' in the direction of ATP at the same pH, free [Mg(2+)], and T. We conclude that the pyruvate kinase reaction may be reversed under some conditions in vivo, a finding that challenges the long held dogma that the reaction is displaced far from equilibrium.

Thermodynamics of the arginine kinase reaction

The effect of temperature, pH, free [Mg(2+)], and ionic strength on the apparent equilibrium constant of arginine kinase (EC 2.7.3.3) was determined. At equilibrium, the apparent K' was defined as [see text] where each reactant represents the sum of all the ionic and metal complex species. The K' at pH 7.0, 1.0 mM free [Mg(2+)], and 0. 25 M ionic strength was 29.91 +/- 0.59, 33.44 +/- 0.46, 35.44 +/- 0. 71, 39.64 +/- 0.74, and 45.19 +/- 0.65 (n = 8) at 40, 33, 25, 15, and 5 degrees C, respectively. The standard apparent enthalpy (DeltaH degrees') is -8.19 kJ mol(-1), and the corresponding standard apparent entropy of the reaction (DeltaS degrees') is + 2. 2 J K(-1)mol(-1) in the direction of ATP formation at pH 7.0, free [Mg(2+)] =1.0 mM, ionic strength (I) =0.25 M at 25 degrees C. We further show that the magnitude of transformed Gibbs energy (DeltaG degrees ') of -8.89 kJ mol(-1) is mostly comprised of the enthalpy of the reaction, with 7.4% coming from the entropy TDeltaS degrees' term (+0.66 kJ mol(-1)). Our results are discussed in relation to the thermodynamic properties of its evolutionary successor, creatine kinase.

Adjustment of K' for the creatine kinase, adenylate kinase and ATP hydrolysis equilibria to varying temperature and ionic strength

Comparative physiologists and biochemists working with tissues at varying temperatures and ionic strength are required to adjust apparent equilibrium constants (K') of biochemical reactions to the experimental conditions prior to calculating cytosolic bioenergetic parameters (transformed Gibbs free energy of formation, DeltafG' ATP; cytosolic phosphorylation ratio, [ATP]/[ADP][Pi]; [phosphocreatine]: [orthophosphate] ratio [PCr]/[Pi]) and kinetic parameters (free [ADP], [Pi] and [AMP]). The present study shows how to adjust both K' and the equilibrium constants of reference reactions (Kref) of creatine kinase (ATP: creatine N-phosphotransferase; EC 2.7.3.2), adenylate kinase (ATP:AMP phosphotransferase; EC 2.7.4.3) and adenosinetriphosphatase (ATP phosphohydrolase; EC 3.6.1.3) to temperature and ionic strength. This information, together with our previous study showing how to adjust equilibria to varying pH and pMg, is vital for the quantification of organ and tissue bioenergetics of ectotherms and endotherms under physiological conditions.

Adjustment of K' to varying pH and pMg for the creatine kinase, adenylate kinase and ATP hydrolysis equilibria permitting quantitative bioenergetic assessment

Physiologists and biochemists frequently ignore the importance of adjusting equilibrium constants to the ionic conditions of the cell prior to calculating a number of bioenergetic and kinetic parameters. The present study examines the effect of pH and free magnesium levels (free [Mg2+]) on the apparent equilibrium constants (K') of creatine kinase (ATP: creatine N-phosphotransferase; EC 2.7.3.2), adenylate kinase (ATP:AMP phosphotransferase; EC 2.7.4.3) and adenosinetriphosphatase (ATP phosphohydrolase; EC 3.6.1.3) reactions. We show how K' can be calculated using the equilibrium constant of a specified chemical reaction (Kref) and the appropriate acid-dissociation and Mg(2+)-binding constants at an ionic strength (I) of 0.25 mol l-1 and 38 degrees C. Substituting the experimentally determined intracellular pH and free [Mg2+] into the equation containing a known Kref and two variables, pH and free [Mg2+], enables K' to be calculated at the experimental ionic conditions. Knowledge of K' permits calculation of cytosolic phosphorylation ratio ([ATP]/[ADP][Pi]), cytosolic free [ADP], free [AMP], standard transformed Gibbs energy of formation (delta fG' degrees ATP) and the transformed Gibbs energy of the system (delta fG' ATP) for the biological system. Such information is vital for the quantification of organ and tissue bioenergetics under physiological and pathophysiological conditions.

Effect of temperature on the creatine kinase equilibrium

The effect of temperature on the apparent equilibrium constant of creatine kinase (ATP:creatine N-phosphotransferase (EC 2.7.3.2)) was determined. At equilibrium the apparent K' for the biochemical reaction was defined as [formula: see text] The symbol sigma denotes the sum of all the ionic and metal complex species of the reactant components in M. The K' at pH 7.0, 1.0 mM free Mg2+, and ionic strength of 0.25 M at experimental conditions was 177 +/- 7.0, 217 +/- 11, 255 +/- 10, and 307 +/- 13 (n = 8) at 38, 25, 15, and 5 degrees C, respectively. The standard apparent enthalpy or heat of the reaction at the specified conditions (delta H' degree) was calculated from a van't Hoff plot of log10K' versus 1/T, and found to be -11.93 kJ mol-1 (-2852 cal mol-1) in the direction of ATP formation. The corresponding standard apparent entropy of the reaction (delta S' degree) was +4.70 J K-1 mol-1. The linear function (r2 = 0.99) between log10 K' and 1/K demonstrates that both delta H' degree and delta S' degree are independent of temperature for the creatine kinase reaction, and that delta Cp' degree, the standard apparent heat capacity of products minus reactants in their standard states, is negligible between 5 and 38 degrees C. We further show from our data that the sign and magnitude of the standard apparent Gibbs energy (delta G' degree) of the creatine kinase reaction was comprised mostly of the enthalpy of the reaction, with 11% coming from the entropy T delta S' degree term. The thermodynamic quantities for the following two reference reactions of creatine kinase were also determined. [formula: see text] The delta H degree for Reaction 2 was -16.73 kJ mol-1 (-3998 cal mol-1) and for Reaction 3 was -23.23 kJ mol-1 (-5552 cal mol-1) over the temperature range 5-38 degrees C. The corresponding delta S degree values for the reactions were +110.43 and +83.49 J K-1 mol-1, respectively. Using the delta H' degree of -11.93 kJ mol-1, and one K' value at one temperature, a second K' at a second temperature can be calculated, thus permitting bioenergetic investigations of organs and tissues using the creatine kinase equilibria over the entire physiological temperature range.

Regulation of the purine salvage pathway in rat liver

The regulation of purine metabolism in rat liver has been examined under conditions that alter the flux through the pathway. Rats were given intraperitoneal injections of ethanol, sodium acetate, or sodium phosphate to attain body water concentrations of approximately 70, 20, and 10 mM, respectively. The livers were freeze-clamped after 30 min, and extracts were made for the analysis of metabolites, cofactors, purine bases, and nucleosides; homogenates were made for the measurement of the activities and kinetic parameters of seven enzymes that participate in purine salvage. The values of the equilibrium constants of nine reactions were determined in vitro and compared with the ratios of the reactants measured in liver. The changes in phosphoribosylpyrophosphate (PRPP), a key intermediate in both the de novo and salvage pathways of purine metabolism, were directly correlated with the changes in ribose 5-phosphate (ribose-5-P); ([PRPP] = 1.7[ribose-5-P] - 7.4 mumol/kg). Ribose-5-P concentrations in turn could be predicted from the liver content of fructose 6-phosphate and glyceraldehyde 3-phosphate by calculation from the known equilibria. The maximum velocities in the tissue of the seven enzymes measured were calculated from the measured substrate values in the liver and with consideration of other effectors of enzyme activity. PRPP synthetase was the least active of the enzymes measured, indicating a possible rate-limiting step. The delta G of the enzyme steps differed from equilibrium values by factors ranging from 4 (nucleoside phosphorylase) to 10(5) (PRPP synthetase and purine transferase reactions). The regulation of purine salvage appeared to depend on the levels of PRPP and ribose-5-P.