Receptor binding of glucagon and adenosine 3',5'-monophosphate accumulation in isolated rat fat cells.

The membrane bound coupling factor of photophosphorylation is studied after pretreatment of broken chloroplasts with the bifunctional N,N-orthophenyldimaleimide under energization of the thylakoid membrane by mild flashing light. The proton conduction of the membrane is monitored both via the electrochromic absorption changes and via selective pH-indicating dyes. It is found that the coupling factor, after interaction with N,N-orthophenyldimaleimide during the preillumination period, shortcircuits one of the two protons pumped inside after excitation of chloroplasts with one short flash of light. In contrast to the low proton conductivity of the unperturbed thylakoid membrane (relaxation time for a proton gradient greater than 5s), this extra proton channel leads to a partial relaxation of a proton gradient within a few ms. Although limited to only one proton per electron, this extra proton conducting pathway is not otherwise specific. It operates with protons resulting from both Photosystem I and Photosystem II activity. In addition it operates with protons already present in the internal phase before firing of the exciting light flash. These effects are prevented by the presence of ATP (but not GTP) during the preillumination period. It is suggested that the modified coupling factor is gated open by the light induced electric field across the thylakoid membrane while self closing after passage of one proton per activated coupling factor.     

Mathematical modeling of electron and protein transport, coupled with ATP synthesis in chloroplasts

A mathematical model of electron and proton transport in chloroplasts of higher plants was developed, which takes into account the lateral heterogeneity of the lamellar system. Based on the results of numerical experiments, lateral profiles of pH in the thylakoid lumen and in the narrow gap between grana thylakoids under different metabolic conditions (in the state of photosynthetic control and under photophosphorylation conditions) were simulated. Lateral profiles of pH in the thylakoid lumen and in the intrathylakoid gap were simulated for different values of the proton diffusion coefficient and stroma pH. The model demonstrated that there might be two mechanisms of regulation of electron and proton transport in chloroplasts: (1) the slowing down of noncyclic electron transport due to a decrease in the intrathylakoid pH, and (2) the retardation of plastoquinone reduction due to slow diffusion of protons inside the narrow gap between the thylakoids of grana.     

Effects of Diffusion and Topological Factors on the Efficiency of Energy Coupling in Chloroplasts with Heterogeneous Partitioning of Protein Complexes in Thylakoids of Grana and Stroma. A Mathematical Model

In this work, we studied theoretically the effects of diffusion restrictions and topological factors that could influence the efficiency of energy coupling in the heterogeneous lamellar system of higher plant chloroplasts. Our computations are based on a mathematical model for electron and proton transport in chloroplasts coupled to ATP synthesis in chloroplasts that takes into account the nonuniform distribution of electron transport and ATP synthase complexes in the thylakoids of grana and stroma. Numerical experiments allowed the lateral profiles of pH in the thylakoid lumen and in the narrow gap between grana thylakoids to be simulated under different metabolic conditions (in the state of photosynthetic control and under conditions of photophosphorylation). This model also provided an opportunity to simulate the effects of steric constraints (the extent of appression of thylakoids in grana) on the rates of non-cyclic electron transport and ATP synthesis. This model demonstrated that there might be two mechanisms of regulation of electron and proton transport in chloroplasts: 1) slowing down of non-cyclic electron transport due to a decrease in the intra-thylakoid pH, and 2) retardation of plastoquinone reduction due to slow diffusion of protons inside the narrow gap between the thylakoids of grana. Numerical experiments for model systems that differ with respect to the arrangement of thylakoids in grana allowed the effects of osmolarity on the photophosphorylation rate in chloroplasts to be explained.     

Electron and proton transport in chloroplasts taking into account lateral heterogeneity of thylakoids. Mathematical model]

A mathematical model of a chloroplast was constructed, which takes into account the inhomogeneous distribution of complexes of photosystems I and II between granal and intergranal thylakoids. The structural and functional complexes of photosystems I and II, which are localized in intergranal and granal thylakoids, respectively, and the b/f complex, which is uniformly distributed in thylakoid membranes, are assumed to be immobile. The interactions between spatially distant electron transport complexes are provided by plastoquinone and plastocyanine, which diffuse in the thylakoid membrane and intrathylakoid space, respectively. The main stages of proton transport associated with the functioning of photosystem II and oxidation-reduction transformations of plastoquinone are considered. The model takes into account the interactions of protons with membrane-bound buffer groups, the lateral diffusion of hydrogen ions in the intrathylakoid space and in the lumen between adjacent granal thylakoids, and the transmembrane proton transport associated with the function of ATP synthase and passive leakage of protons from thylakoids outside. The numerical integration of two systems of differential equations describing the behavior of some variables in two different regions: granal and intergranal thylakoids was performed. The model describes adequately the kinetics of processes being studied and predicts the occurrence of inhomogeneous lateral profiles of proton potentials and redox state of electron carriers. Modeling the electron and proton transport with allowance for the topological features of chloroplasts (lateral heterogeneity of thylakoids) is important for correct interpretation of "power-flux" interactions and the experimentally measured kinetic parameters averaged over the entire spatially inhomogeneous thylakoid system.     

Influence of photoinhibition on electron transport and photophosphorylation of isolated chloroplasts

The effects of a photoinhibition treatment (PIT) on electron transport and photophosphorylation reactions were measured in chloroplasts isolated from triazine-resistant and susceptible Chenopodium album plants grown under high and low irradiance. Electron transport dependent on photosystem I (PSI) alone was much less affected by PIT than that dependent on both photosystem II (PSII) and PSI. There was a smaller difference in susceptibility to PIT between the photophosphorylation activitity dependent on PSI alone and that dependent on both PSII and PSI. Because in all cases photophosphorylation activity decreased faster upon PIT than the rate of electron transport, we conclude that photoinhibition causes a gradual uncoupling of electron transport with phosphorylation. Since the extent of the light-induced proton gradient across the thylakoid membrane decreased upon PIT, it is suggested that photoinhibiton causes a proton leakiness of the membrane. We have found no significant differences to PIT of the various reactions measured in chloroplasts isolated from triazine-resistant and susceptible plants. We have also not observed any significant differences to PIT of the photophosphorylation reactions in chloroplasts of plants grown under low irradiance, compared with those grown under high irradiance. However, the electron transport reactions in chloroplasts from plants grown under low irradiance appeared to be somewhat less sensitive to PIT than those grown under high irradiance.     

Damage to the chloroplast H+-ATPase complex by low concentrations of glutaraldehyde

The energy-dependent processes coupled to electron transport were studied in isolated pea chloroplasts treated with low concentrations (1-5 mM) of glutaraldehyde (GA) in the dark and in the light sufficient to cause energization of the membrane. After GA treatment the chloroplasts exhibited a strong suppression of cyclic and non-cyclic phosphorylation, coupled (+ADP+Pi) electron transport and diminution of the light-activated Mg2+-ATPase activity. The rate of basal electron transport was unaffected. The GA-treated chloroplasts were found to retain the capacity to form the osmotic component of the transmembrane potential. These data and the results of the effect of florizine and ATP on electron transport suggest that the effect of GA on energy transduction processes associated with photophosphorylation may consist in its action on reversible H+-ATPase. In light-adapted samples treated with GA the data characterizing the formation of a high energy state (rate of photophosphorylation, steady-state level of photo-induced quenching of atebrin fluorescence and its dark recovery; photo-induced absorbance changes at 520 nm; rate of the slow phase of delayed fluorescence increment) appear to be changed to a greater extent as compared to the dark-adapted samples. The observed changes may arise from a greater conductivity of thylakoid membranes due to fixation of the H+-ATPase proton channel in the "open" state.     

Gated proton conduction via the coupling factor of photophosphorylation modified by N,N-orthophenyldimaleimide

The membrane bound coupling factor of photophosphorylation is studied after pretreatment of broken chloroplasts with the bifunctional N,N-orthophenyldimaleimide under energization of the thylakoid membrane by mild flashing light. The proton conduction of the membrane is monitored both via the electrochromic absorption changes and via selective pH-indicating dyes. It is found that the coupling factor, after interaction with N,N-orthophenyldimaleimide during the preillumination period, shortcircuits one of the two protons pumped inside after excitation of chloroplasts with one short flash of light. In contrast to the low proton conductivity of the unperturbed thylakoid membrane (relaxation time for a proton gradient > 5s), this extra proton channel leads to a partial relaxation of a proton gradient within a few ms. Although limited to only one proton per electron, this extra proton conducting pathway is not otherwise specific. It operates with protons resulting from both Photosystem I and Photosystem II activity. In addition it operates with protons already present in the internal phase before firing of the exciting light flash. These effects are prevented by the presence of ATP (but not GTP) during the preillumination period. It is suggested that the modified coupling factor is gated open by the light induced electric field across the thylakoid membrane while self closing after passage of one proton per activated coupling factor.     

Energy-dependent changes in the conformation of the epsilon subunit of the chloroplast ATP synthase

Rabbit antiserum raised against the isolated native epsilon subunit of the chloroplast coupling factor 1 activated the ATPase activity of coupling factor 1 in solution by removing the epsilon subunit. Incubation of thylakoid membranes with the antiserum in the dark had no effect on photophosphorylation or on the dithiothreitol-induced Mg2+-ATPase activity. Incubation with the antiserum during illumination, however, strongly inhibited both activities and caused the membranes to become leaky to protons. The results indicate that the formation of a proton gradient across the thylakoid membrane induces a change in conformation of the epsilon subunit of the ATP synthase such that it becomes susceptible to attack and removal by the antibodies. This change may be a part of the mechanism that results in energy-dependent activation of the ATP synthase.     

The role of different thylakoid glycolipids in the function of reconstituted chloroplast ATP synthase

ATPase activity of CF₀CF₁ from spinach chloroplasts is specifically stimulated by chloroplast lipids (Pick, U., Gounaris, K., Admon, A. and Barber, J. (1984) Biochim. Biophys. Acta 765, 12–20). The association of CF₀-CF₁ with isolated lipids and their mixtures has been examined by analyzing the stimulation of ATPase and ATP-P_i exchange activities, by binding studies and by measurement of proton conductance of reconstituted proteoliposomes. Monogalactosyldiacylglycerol is the only chloroplast lipid which by itself activates ATP hydrolysis. A mild saturation of the fatty acids of the lipid partially inhibits the activation. CF₀-CF₁ has a higher binding capacity for monogalactosyldiacylglycerol (1.5 mg/mg protein) than for other thylakoid glycolipids. However, ATPase activation is not correlated with the amount of bound lipid but rather with its type. For the same amount of bound lipid, monogalactosyldiacylglycerol best activates ATP hydrolysis, while the acidic lipids phosphatidylglycerol and sulphoquinovosyldiacylglycerol inhibit ATPase activity. Optimal activation of ATP-P_i exchange requires, in addition to monogalactosyldiacylglycerol, digalactosyldiacylglycerol and sulphoquinovosyldiacylglycerol at a ratio of 6:3:1, respectively. Correlations between proton conductance, ATP-P_i exchange and uncoupler stimulation of ATPase activity indicate that sulphoquinovosyldiacylglycerol reduces the permeability of the proteoliposomes to protons. The results suggest that: (a) association of CF₀-CF₁ with polyunsaturated monogalactosyldiacylglycerol greatly stimulates ATPase activity; (b) reconstitution of coupled CF₀-CF₁ proteoliposomes requires a careful balance of the natural glycolipids of thylakoid membranes in similar proportions to their occurrence in chloroplasts, and (c) sulphoquinovosyldiacylglycerol may control the permeability of chloroplast membranes to protons.     

Functional and topological aspects of pH-dependent regulation of electron and proton transport in chloroplasts in silico

In this work, we summarize results of computer simulation of electron and proton transport processes coupled to ATP synthesis in chloroplasts performed within the frames of a mathematical model developed as a system of differential equations for concentrations of electron carriers and hydrogen ion inside and outside the granal and stromal thylakoids. The model takes into account topological peculiarities and lateral heterogeneity of the chloroplast lamellar system. This allowed us to analyze the influence of restricted diffusion of protons inside small compartments of a chloroplast (e.g., in the narrow inter-thylakoid gap) on electron transport processes. The model adequately describes two modes of pH-dependent feedback control of electron transport associated with: (i) the acidification of the thylakoid lumen, which causes the slowing down of plastoquinol oxidation and stimulates an increase in dissipation of excess energy in PS2, and (ii) the alkalization of stroma, inducing the activation of the BBC (Bassham-Benson-Calvin) cycle and intensified consumption of ATP and NADPH. The influence of ATP on electron transport is mediated by modulation of the thylakoid membrane conductivity to protons through the ATP synthase complexes. We also analyze the contribution of alternative electron transport pathways to the maintenance of optimal balance between the energy donating and energy consuming stages of the light-induced photosynthetic processes.     

Effect of pyridine homologues on proton flux through the CF0 · CF1 complex and photophosphorylation in chloroplasts

At concentrations below 1 mM, hydrophobic pyridine homologues decrease the rate of photophosphorylation and light-stimulated hydrolysis of ATP and light-activated exchange of the tightly bound nucleotides in chloroplasts, but increase the rate of the Hill reaction. Unlike uncoupling agents, the presence of the organic base at such low concentrations decreases the rate of light-dependent leakage and has no effect on the efficiency of two-stage photophosphorylation in broken chloroplasts. By assuming that the organic base is bound to independent equivalent sites in the thylakoid membrane, a simple expression can be derived which relates the observed rates of photophosphorylation and light-stimulated hydrolysis of ATP quantitatively to the concentration of the organic base in solution and gives dissociation equilibrium constants which are on the order of the relative hydrophobicities of the pyridine homologues. A possible mechanistic model for the CF₀ · CF₁ complex is proposed which could serve as the basis for a unified interpretation of the kinetics of proton translocation in illuminated chloroplasts, the steady-state rate of photophosphorylation, the light-stimulated ATPase activity, and the light-activated exchange of tightly bound adenine nucleotides.Abbreviations AMPPNP adenylylimidodiphosphate - Chl chlorophyll - CF₀ · CF₁ the coupling factor complex of chloroplasts - DCCD N,N-dicyclohexylcarbodiimide - DTT dithiothreitol - FCCP carbonylcyanide-p-trifluoromethoxyphenylhydrazone - TCA trichloroacetic acid - Tricine N-tris-(hydroxymethyl)methylglycine     

Binding of 2'(3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-diphosphate opens the pathway for protons through the chloroplast ATPase complex

The effect of 2'(3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-diphosphate (TNP-ADP) on photophosphorylation and on the proton conductivity of the thylakoid membrane has been investigated. The results show that TNP-ADP is a potent competitive inhibitor of photophosphorylation (Ki = 1-2 microM). Moreover, in the absence of ADP and Pi, TNP-ADP accelerates basal electron transport of chloroplasts. Addition of ADP, which promotes release of the analogue from CF1, completely reverses this effect of TNP-ADP; likewise Pi alone reverses stimulation of electron transport by TNP-ADP. Dicyclohexylcarbodiimide treatment, which is known to close CF0 to H+, completely abolishes the effect of TNP-ADP. The measurements of the alkalization of the medium and the acidification of the thylakoid lumen following single turnover flashes showed that binding of TNP-ADP to CF1 increased membrane permeability for H+. Further results suggest that binding of TNP-ADP to the catalytic site of CF1 opens the CF0-CF1 complex for H+. Since ADP, as well as Pi alone, reverses the effect, it is concluded that TNP-ADP induces a conformation of the CF0-CF1 complex similar to the one triggered by simultaneous binding of ADP plus Pi. This may be achieved by interaction of the TNP residue with the Pi binding site. Thus it seems that the status of the catalytic site(s) in CF1 can be transmitted to the CF0 part to control proton flux through the ATPase complex in an economically reasonable way.     

Effect of Chlorophyll-Protein Complex I Deficiency on the Physiological Character of a Chlamydomonas reinhardtii Mutant

In the mutant CC-1047 of Chlamydomonas reinhardtii, LDS-PAGE showed that the chlorophyll-protein complex I (CPI) is almost absent. The mutant could not grow in a culture medium without organic carbon source while the wild type (WT) C. reinhardtii grew quickly. When an organic carbon source was added into the culture medium, the mutant grew almost as well as WT. The rate of photosystem 1 (PS1) electron transport (DCPIPMV) and the rate of whole chain electron transport (H₂OMV) of chloroplasts of the CC-1047 mutant were both lower than those of WT. The photophosphorylation activity, photosynthetic O₂ evolution rate, and rate of NADP⁺ photoreduction of CC-1047 were also much lower than the activities of WT. There were some differences in ATPase activity between the mutant and WT. Two different activation ways were used to activate the latent ATPase using methanol and dithiothreitol (DTT) as activation substrate. More methanol and DTT were required for the mutant than WT to obtain the maximum activity. Thus the photosynthetic apparatus could not operate normally when CPI was absent because of the abnormal PS1 electron transport. Meanwhile, the other adjacent complexes of the thylakoid membrane, for example, ATP synthase complex, were slightly affected.     

Photosynthetic control, “energy-dependent” quenching of chlorophyll fluorescence and photophosphorylation under influence of tertiary amines

The effects of the tertiary amines tetracaine, brucine and dibucaine on photophosphorylation and control of photosynthetic electron transport in isolated chloroplasts of Spinacia oleracea were investigated. Tertiary amines inhibited photophosphorylation while the related electron transport decreased to the rates, observed under non-phosphorylating conditions. Light induced quenching of 9-aminoacridine fluorescence and uptake of ¹⁴C-labelled methylamine in the thylakoid lumen declined in parallel with photophosphorylation, indicating a decline of the transthylakoid proton gradient. In the presence of ionophoric uncouplers such as nigericin, no effect of tertiary amines on electron transport was seen in a range of concentration where photophosphorylation was inhibited. Under the influence of the tertiary amines tested, pH-dependent feed-back control of photosystem II, as indicated by energy-dependent quenching of chlorophyll fluorescence, was unaffected or even increased in a range of concentration where 9-aminoacridine fluorescence quenching and photophosphorylation were inhibited. The data are discussed with respect to a possible involvement of localized proton flow pathways in energy coupling and feed-back control of electron transport.Abbreviations 9-AA 9-aminoacridine - J _e flux of photosynthetic electron transport - PC photosynthetic control - pH₁ H⁺ concentration in the thylakoid lumen - pmf proton motive force - _P potential quantum yield of photochemistry of photosystem II (with open reaction centers) - Q _A primary quinone-type electron acceptor of photosystem II - q _Q photochemical quenching of chlorophyll fluorescence - q _E energy-dependent quenching of chlorophyll fluorescence - q _AA light-induced quenching of 9-amino-acridine fluorescence     

Plant Cell Membranes as Biochemical Targets of the Phytotoxin Helminthosporol

Helminthosporol is one of the natural sesquiterpenoid toxins isolated and identified in the culture medium of the phytopathogenic ascomycete fungus Cochliobolus sativus. The effect of this phytotoxin was investigated on enzymatic activities, electron and ion transport in mitochondria, chloroplasts, and microsomes of plant. The results indicate that helminthosporol drastically affects the membrane permeability of these organelles to protons and subtrate anions, inhibiting the mitochondrial oxidative phosphorylation, the photophosphorylation in chloroplasts, and the proton pumping across the cell plasma membrane. The 1,3--glucan synthase activity, involved in defense mechanisms of plant cells against stress and damage, e.g., during pathogen attack, was also strongly inhibited by the toxin.     

叶绿体结构状态与光化学活性的关系

叶绿体被膜阻碍以Fecy为受体的电子传递,而对以DCIP为受体的电子传递无妨。被膜完整度越高则P/O值也越高。类囊体膜结构的完整度高,则电子传递速率和P/O值也比膜结构受到破损时高。类囊体膜结构的完整度对保持PS Ⅱ活性是必要的,随着完整度的降低,PS Ⅱ在电子传递中所占比重相应减少。     

Inhibition by triphenyltin chloride of a tightly-bound membrane component involved in photophosphorylation.

At very low concentrations (less than 1 muM) triphenyltin chloride inhibits ATP formation and coupled electron transport in isolated spinach chloroplasts. Basal (-Pi) and uncoupled electron transport are not affected by triphenyltin. The membrane-bount ATP in equilibrium Pi exchange and Mg2+-dependent ATPase activities of chloroplasts are also completely sensitive to triphenyltin, although the Ca2+-dependent and Mg2+-dependent ATPase activities of the isolated coupling factor protein are insensitive to triphenyltin. The light-driven proton pump in chloroplasts is stimulated (up to 60%) by low levels of triphenyltin. Indeed, the amount of triphenyltin necessary to inhibit ATP formation or stimulate proton uptake is dependent upon the amount of chloroplasts present in the reaction mixture, with an apparent stoichiometry of 2-2.5 triphenyltin molecules/100 chlorophyll molecules at 50% inhibition of ATP formation and half-maximal stimulation of proton uptake. Chloroplasts partially stripped of coupling factor by an EDTA was are no longer able to accumulate protons in the light. However, low levels of triphenyltin can effectively restore this ability. The amount of triphenyltin required for the restoration of net proton uptake is also dependent upon the amount of chloroplasts, with a stoichiometry of 4-5 triphenyltin molecules/100 chlorophyll molecules at 50% reconstitution. On the basis of this and other evidence it is concluded that triphenyltin chloride inhibits phosphorylation.Atp in equilibrium Pi exchange and membrane-bound ATPase activities in chloroplasts by specifically blocking the transport of protons through a membrane-bound carrier or channel located in a hydrophobic region of the membrane at or near the functional binding site for the coupling factor.     

Photosynthesis in Salt-Adapted Heterotrophic Tobacco Cells and Regenerated Plants

Tobacco (Nicotiana tabacum L.) cells growing heterotrophically in the light on supplied sucrose (S0) have previously been adapted to grow in 428 mM NaCl (S25). Among the changes occurring in salinity-adapted cell cultures are (a) elevated levels of chlorophyll compared to unadapted cells; (b) decreased levels of starch; (c) alterations in chloroplast ultrastructure, including loss of starch grains, increased thylakoid membrane structure, and the presence of plastoglobules; and (d) increased rates of O2 evolution, CO2 fixation, and photophosphorylation relative to S0 cells. These latter changes apparently derive from the fact that thylakoid membranes in S25 cells contain higher levels of photosystem I- and II-associated proteins as well as thylakoid ATPase components. S25 chloroplasts contain immunologically detectable levels of ribulose-1,5-bisphosphate carboxylase/oxygenase, whereas S0 completely lack the enzyme. These changes taken together suggest that even in the presence of sucrose, S25 cells have acquired a significant degree of salt-tolerant photosynthetic competence. This salt-tolerant photoysynthetic capability manifests itself in plants backcrossed with normal plants for three generations. These plants contain chloroplasts that demonstrate in vitro more salt-tolerant CO2 fixation, O2 evolution, and photophosphorylation than do backcross progeny of plants regenerated from S0 cultures.     

Quercetin interaction with the chloroplast ATPase complex

1. Quercetin, a flavonoid which acts as an energy transfer inhibitor in photophosphorylation is shown to inhibit the P-ATP exchange activity of membrane-bound CF1 and the ATPase activity of isolated CF1. Quercetin, affects also the proton uptake in chloroplasts in a manner similar to that of dicyclohexylcarbodiimide. 2. The light-dependent proton uptake in EDTA-treated chloroplasts is stimulated by quercetin. In untreated chloroplasts quercetin has a dual effect: it enhances at pH above 7.5 while at lower pH values it decreases the extent of H+ uptake. Similar effects were obtained with dicyclohexylcarbodiimide. 3. Like quercetin, dicyclohexylcarbodiimide was also found to inhibit the ATPase activity of isolated CF1. 4. Quercetin inhibits uncoupled electron transport induced by either EDTA-treatment of chloroplasts or by addition of uncouplers. Quercetin restores H+ uptake in both types of uncoupled chloroplasts. 5. The mode of action of quercetin and dicyclohexylcarbodiimide in photophosphorylation is discussed, and interaction with both CF1 and F0 is suggested.     

Lipid molecular motion and enzyme activity in sarcoplasmic reticulum membrane.

In biochemically active sarcoplasmic reticulum vesicles (SR) the physical state of the membrane lipids was studied by high angle x-ray diffraction and proton nuclear magnetic resonance (NMR) at 220 MHz, and related to thermal effects observed in SR functional parameters. It is shown by high angle x-ray diffraction that even at temperatures as low as 1 degree C nearly all the SR lipid hydrocarbon chains are in a disordered conformation and only a very small part (less than 3%) are in rigid crystalline order. Consistent with this observation, the NMR data indicate that the majority of SR phospholipid molecules are in a state of restricted anisotropic motion having no apparent crystalline order at temperatures as low as 5 degrees C. At this temperature most of the resonance signal is contained in a broad feature-less line of 700-Hz half-width. On the other hand, as the temperature is raised, high-resolution NMR signals, representing groups with highly isotropic motion, begin to grow in intensity. It is estimated that by 35 degrees C 90-100% of the phosphatidylcholine N-methyl protons and 35% of the hydrocarbon-chain protons give high-resolution signals. Concurrent studies on functional parameters reveal thermal effects giving rise to nonlinear Arrhenius plots for the rates of calcium transport and calcium activated ATPase. The thermal effects observed on functional parameters and on the character of phospholipid molecular motion exhibit a parallel behavior, suggesting a relationship between enzyme activity and the physical state of the membrane lipids.