Apparent dependence of interactions between cytochrome b5 and cytochrome b5 reductase upon translational diffusion in dimyristoyl lecithin liposomes

Structure and Mechanism of a Unique Diiron Center in Mammalian Stearoyl-CoA Desaturase

Stearoyl-CoA desaturase 1 (SCD1) is a membrane-embedded metalloenzyme that catalyzes the formation of a double bond on a saturated acyl-CoA. SCD1 has a diiron center and its proper function requires an electron transport chain composed of NADH (or NADPH), cytochrome b5 reductase (b5R), and cytochrome b5 (cyt b5). Since SCD1 is a key regulator in fat metabolism and is required for survival of cancer cells, there is intense interest in targeting SCD1 for various metabolic diseases and cancers. Crystal structures of human and mouse SCD1 were reported recently; however, both proteins have two zinc ions instead of two iron ions in the catalytic center, and as a result, the enzymes are inactive. Here we report a general approach for incorporating iron into heterologously expressed proteins in HEK293 cells. We produced mouse SCD1 that contains a diiron center and visualized its diiron center by solving its crystal structure to 3.5 Å. We assembled the entire electron transport chain using the purified soluble domains of cyt b5 and b5R, and the purified mouse SCD1, and we showed that three proteins coordinate to produce proper products. These results established an in vitro system that allows precise perturbations of the electron transport chain for the understanding of the catalytic mechanism in SCD1.

Characterization and control of lipid layer fluidity in hybrid bilayer membranes

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the distal lipid layer in hybrid bilayer membranes (HBMs) under water was investigated using vibrational sum frequency spectroscopy (VSFS). VSFS has unique sensitivity to order/disorder transitions in the lipid acyl chains and can determine T(m) for the lipid monolayers in HBMs. We recently reported the observation that T(m) is raised and the transition width is broadened for the overlying phospholipid monolayer in HBM systems formed on densely packed crystalline self-assembled monolayers (SAMs) as compared to that of vesicles in solution. In this report, we establish that T(m) for the lipid layer of HBMs can be controlled by proper choice of the SAM underlayer. The SAM underlayer of the HBM was systematically altered by using an alkane thiol, a saturated thiolipid, a mixed SAM of a saturated lipid-pyridine disulfide, and finally a mixed SAM of an unsaturated lipid-pyridine disulfide. T(m) was measured for two different chain length saturated phosphatidylcholine lipid overlayers on these SAMs. The values obtained show that Tm of the lipid layer of HBMs is sensitive to the composition and/or packing density of molecules in the underlying SAM.

Determination of lipid phase transition temperatures in hybrid bilayer membranes

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the lipid monolayer in hybrid bilayer membranes (HBMs) was investigated using vibrational sum frequency spectroscopy (VSFS). In the gel phase, the acyl chains of the lipid molecules assume an ordered, all-trans configuration, whereas in the LC phase, the acyl chains exhibit a significant number of disordered gauche conformers. VSFS has unique sensitivity to the order/disorder transitions in the acyl chains and was used to determine T(m) for a series of saturated phosphatidylcholine lipids on octadecanethiolate self-assembled monolayers (SAMs). The values obtained for T(m) for all lipids studied are significantly higher than for the corresponding lipids in vesicles in solution. Additionally, the transition widths are broader for the lipids in HBMs. The underlying SAM clearly influences the phase behavior of the overlying lipid monolayer.

Kinetics of bimolecular reactions in model bilayers and biological membranes. A critical review

The quantitative study of the probability of molecular encounters giving rise to a reaction in membranes is a challenging discipline. Model systems, model in the sense that they use model bilayers and model reactants, have been widely used for this purpose, but the methodologies employed for the analysis of the results obtained in experiments, and for experimental design, are so disparate that a concerned experimentalist has difficulty in deciding about the value of each approach. This review intends to examine the several approaches that can be found in the literature showing, when feasible, the weakness, strengths and limits of application of each of them. There is not, so far, a full experimental validation of the most promising theories for the analysis of reactions in two dimensions, what leaves open a large field for new research. The major challenge resides in the time range in which the processes take place, but the possibilities of the existing techniques for these studies are far from exhausted. We review also the attempts of several authors to quantitatively analyze the kinetics of reactions in biological membranes. Especially in this field, the recently developed microspectroscopies enclose a still unexplored potential.

Evaluation of data in terms of two-dimensional random walk model: Interaction between NADH-cytochrome b5 reductase and cytochrome b5

Normally, bimolecular reactions are analyzed in terms of the Smoluchowski theory. However, when one attempts to generalize this analysis to cases where diffusion proceeds in two other than in three dimensions, one soon encounters severe conceptual difficulties. Although kinetic studies of membrane enzymes are generally difficult because the usual kinetic formalism refers to nonaggregated homogenous solutions, a major goal of our research is to define the molecular mechanism(s) by which alterations in membrane-bound substrate contents affect the enzyme activity in the same membrane. For that purpose, a simplified random-walk model was adopted in the present work. The enzyme reaction in the two-dimensional membrane could be calculated theoretically by applying the classical analysis of heat equation. As a result, the theoretical rate equation well accounting experimental findings was derived on the model of the liver microsomal NADH-cytochrome b5 reductase reaction. Furthermore, it was found that the modification of the simple rigid-sphere collision theory by including a term called the steric factor was not necessary in this derived equation.

Differential effects of permeating and nonpermeating solutes on the fatty acid composition of Pseudomonas putida

We examined the effect of reduced water availability on the fatty acid composition of Pseudomonas putida strain mt-2 grown in a defined medium in which the water potential was lowered with the permeating solutes NaCl or polyethylene glycol (PEG) with a molecular weight of 200 (PEG 200) or the nonpermeating solute PEG 8000. Transmission electron microscopy showed that -1.0-MPa PEG 8000-treated cells had convoluted outer membranes, whereas -1.0-MPa NaCl-treated or control cells did not. At the range of water potential (-0.25 to -1.5 MPa) that we examined, reduced water availability imposed by PEG 8000, but not by NaCl or PEG 200, significantly altered the amounts of trans and cis isomers of monounsaturated fatty acids that were present in whole-cell fatty acid extracts. Cells grown in basal medium or under the -0.25-MPa water potential imposed by NaCl or PEG 200 had a higher trans:cis ratio than -0.25-MPa PEG 8000-treated cells. As the water potential was lowered further with PEG 8000 amendments, there was an increase in the amount of trans isomers, resulting in a higher trans:cis ratio. Similar results were observed in cells grown physically separated from PEG 8000, indicating that these changes were not due to PEG toxicity. When cells grown in -1.5-MPa PEG 8000 amendments were exposed to a rapid water potential increase of 1.5 MPa or to a thermodynamically equivalent concentration of the permeating solute, NaCl, there was a decrease in the amount of trans fatty acids with a corresponding increase in the cis isomer. The decrease in the trans/cis ratio following hypoosomotic shock did not occur in the presence of the lipid synthesis inhibitor cerulenin or the growth inhibitors chloramphenicol and rifampicin, which indicates a constitutively operating enzyme system. These results indicate that thermodynamically equivalent concentrations of permeating and nonpermeating solutes have unique effects on membrane fatty acid composition.

Temperature-induced differential kinetic properties between an initial burst and the following steady state in membrane-bound enzymes: studies on lathosterol 5-desaturase

The NADH-dependent lathosterol 5-desaturation reaction that forms 7-dehydrocholesterol is biphasic, an initial burst followed by steady state. The steady-state phase is slower than the burst phase, because the latter diffusion of the lathosterol substrate within the microsomal membrane must occur before the next reaction can take place [Y. Takakuwa, H. Nishino, Y. Ishibe, and T. Ishibashi (1994) J. Biol. Chem. 269, 27889-27893]. In the present study, changes in the structure and function of the membrane were examined by measurement of the Arrhenius activation energy of lathosterol 5-desaturase at various temperatures between 2 and 45 degrees C. At the burst phase, there was a lack of discontinuity in the Arrhenius plots at the presumed phase transition temperature for the microsomal membrane. However, the plots of the activities of the steady state showed breaks at around 17 and 32 degrees C. It was concluded that phospholipid phase transition affects the steady-state phase but not the burst phase. Furthermore, treatment of microsomes with low concentrations of deoxycholate, known to perturb the membrane integrity, resulted in a break of the activation energy of the burst phase. These results have revealed further evidence for our previous model suggesting interaction between the substrate and enzyme within the microsomal membrane via lateral diffusion.

Mechanisms of membrane toxicity of hydrocarbons

Microbial transformations of cyclic hydrocarbons have received much attention during the past three decades. Interest in the degradation of environmental pollutants as well as in applications of microorganisms in the catalysis of chemical reactions has stimulated research in this area. The metabolic pathways of various aromatics, cycloalkanes, and terpenes in different microorganisms have been elucidated, and the genetics of several of these routes have been clarified. The toxicity of these compounds to microorganisms is very important in the microbial degradation of hydrocarbons, but not many researchers have studied the mechanism of this toxic action. In this review, we present general ideas derived from the various reports mentioning toxic effects. Most importantly, lipophilic hydrocarbons accumulate in the membrane lipid bilayer, affecting the structural and functional properties of these membranes. As a result of accumulated hydrocarbon molecules, the membrane loses its integrity, and an increase in permeability to protons and ions has been observed in several instances. Consequently, dissipation of the proton motive force and impairment of intracellular pH homeostasis occur. In addition to the effects of lipophilic compounds on the lipid part of the membrane, proteins embedded in the membrane are affected. The effects on the membrane-embedded proteins probably result to a large extent from changes in the lipid environment; however, direct effects of lipophilic compounds on membrane proteins have also been observed. Finally, the effectiveness of changes in membrane lipid composition, modification of outer membrane lipopolysaccharide, altered cell wall constituents, and active excretion systems in reducing the membrane concentrations of lipophilic compounds is discussed. Also, the adaptations (e.g., increase in lipid ordering, change in lipid/protein ratio) that compensate for the changes in membrane structure are treated.

Mechanisms of membrane toxicity of hydrocarbons

Microbial transformations of cyclic hydrocarbons have received much attention during the past three decades. Interest in the degradation of environmental pollutants as well as in applications of microorganisms in the catalysis of chemical reactions has stimulated research in this area. The metabolic pathways of various aromatics, cycloalkanes, and terpenes in different microorganisms have been elucidated, and the genetics of several of these routes have been clarified. The toxicity of these compounds to microorganisms is very important in the microbial degradation of hydrocarbons, but not many researchers have studied the mechanism of this toxic action. In this review, we present general ideas derived from the various reports mentioning toxic effects. Most importantly, lipophilic hydrocarbons accumulate in the membrane lipid bilayer, affecting the structural and functional properties of these membranes. As a result of accumulated hydrocarbon molecules, the membrane loses its integrity, and an increase in permeability to protons and ions has been observed in several instances. Consequently, dissipation of the proton motive force and impairment of intracellular pH homeostasis occur. In addition to the effects of lipophilic compounds on the lipid part of the membrane, proteins embedded in the membrane are affected. The effects on the membrane-embedded proteins probably result to a large extent from changes in the lipid environment; however, direct effects of lipophilic compounds on membrane proteins have also been observed. Finally, the effectiveness of changes in membrane lipid composition, modification of outer membrane lipopolysaccharide, altered cell wall constituents, and active excretion systems in reducing the membrane concentrations of lipophilic compounds is discussed. Also, the adaptations (e.g., increase in lipid ordering, change in lipid/protein ratio) that compensate for the changes in membrane structure are treated.

Microbial fatty acids and thermal adaptation

The existing literature on the role of fatty acids in microbial temperature adaptation is reviewed. Several modes of change of cellular fatty acids at varying environmental temperatures are shown to exist in yeasts and fungi, Gram-negative bacteria, and bacteria containing iso- and anteiso-branched fatty acids, as well as in a few Gram-positive bacteria. Consequently, the degree of fatty acid unsaturation and cyclization, fatty acid chain length, branching, and cellular fatty acid content increase, decrease, or remain unaltered on lowering the temperature. Moreover, microorganisms seem to be able to change from one mode or alter the cellular fatty acid profile temperature dependently to another on lowering the temperature, as well as even within the same growth temperature range, depending on growth conditions. Therefore, the effect of the temperature on cellular fatty acids appears to be more complicated than known earlier. However, similarities found in the modes of change of cellular fatty acids at varying environmental temperatures in several microorganisms within the above mentioned groups support the existence of a limited amount of common regulatory mechanisms. The models presented enable the prediction of temperature-induced changes occurring in the fatty acids of microorganisms, and enzymatic steps of the fatty acid biosynthesis that possibly are under temperature control.

Lateral diffusion in model membranes is independent of the size of the hydrophobic region of molecules

We have systematically investigated the probe size and shape dependence of lateral diffusion in model dimyristoyl phosphatidylcholine membranes. Linear hydrophobic polymers, which differ in length by an order of magnitude, were used to explore the effect on the lateral diffusion coefficient of hydrodynamic restrictions in the bilayer interior. The polymers employed are isoprenoid alcohols--citronellol, solanesol, and dolichol. Tracer lateral diffusion coefficients were measured by fluorescence photobleaching recovery. Despite the large difference in lengths, the nitrobenzoxadiazole labelled alcohols all diffuse at the rate of lipid self-diffusion (5.0 x 10(-12) m2 s-1, 29 degrees C) in the liquid crystal phase. Companion measurements in isotropic polymer solution, in gel phase lipid membranes and with nonpolar fluorescent polyaromatic hydrocarbons, show a marked dependence of the lateral diffusion coefficient on the probe molecule size. Our results in the liquid crystal phase are in accord with free area theory which asserts that lateral diffusion in the membrane is restricted by the surface-free area. Probe molecules which are significantly longer than the host phospholipid, seven times longer in the case of dolichol, are still restricted in their lateral motion by the surface properties of the bilayer in the liquid crystal phase. Fluorescence quenching experiments indicate that the nitrobenzoxadiazole label does not reside at the aqueous interface, although it must reside in close proximity according to the diffusion measurements.

Bilayer structure and physical dynamics of the cytochrome b5 dimyristoylphosphatidylcholine interaction

Cytochrome b5 is a microsomal membrane protein which provides reducing potential to delta 5-, delta 6-, and delta 9-fatty acid desaturases through its interaction with cytochrome b5 reductase. Low angle x-ray diffraction has been used to determine the structure of an asymmetrically reconstituted cytochrome b5:DMPC model membrane system. Differential scanning calorimetry and fluorescence anisotropy studies were performed to examine the bilayer physical dynamics of this reconstituted system. These latter studies allow us to constrain structural models to those which are consistent with physical dynamics data. Additionally, because the nonpolar peptide secondary structure remains unclear, we tested the sensitivity of our model to different nonpolar peptide domain configurations. In this modeling approach, the nonpolar peptide moiety was arranged in the membrane to meet such chemically determined criteria as protease susceptibility of carboxyl- and amino-termini, tyrosine availability for pH titration and tryptophan 109 location, et cetera. In these studies, we have obtained a reconstituted cytochrome b5:DMPC bilayer structure at approximately 6.3 A resolution and conclude that the nonpolar peptide does not penetrate beyond the bilayer midplane. Structural correlations with calorimetry, fluorescence anisotropy and acyl chain packing data suggest that asymmetric cytochrome b5 incorporation into the bilayer increases acyl chain order. Additionally, we suggest that the heme peptide:bilayer interaction facilitates a discreet heme peptide orientation which would be dependent upon phospholipid headgroup composition.

Lateral diffusion of visual pigments in toad (Bufo marinus) rods and in catfish (Ictalurus punctatus) cones

1. The lateral diffusion coefficient, D, was determined for visual pigments in red rods, green rods and red-sensitive cones with a photon-counting microspectrophotometer. 2. A novel protocol that involved the placement on a photoreceptor of a single micromeasuring/bleaching beam permitted the determination of D of the pigment. 3. Demonstration that D for red rods compared very well with values obtained by other workers using other means validated the protocol. 4. Applied to green rods, the protocol gave a value of D that was about 80% greater than that for red rods. 5. D for cone pigment was found to be slightly less than that of red rod pigment. 6. The dichroic ratio of cones and the average orientation of the chromophore in the cone lamellae were 1.71 and 28.4 deg, respectively. 7. The photosensitivity at lambda max of the red-sensitive cones was found to be about 16% less than that of red rods.

Lateral diffusion of lipid probes in the surface membrane of human platelets. An electron-electron double resonance (ELDOR) study

Electron-electron double resonance (ELDOR) techniques employing [14N], [15N] 16-Doxylstearate spin-label pairs have been used to measure the lateral diffusion constant, D, of lipids in the surface membrane of intact human blood platelets. For freshly prepared platelets, D is 1.0 X 10(-8) cm2/s at 37 degrees C and for platelets stored for 3 d at room temperature under accepted routine blood bank conditions, D is 2.6 X 10(-8) cm2/s at 37 degrees C. This is the first time that D in the surface membrane of platelets is reported. The marked increase in D for stored platelets may be attributed at least partly to loss of cholesterol during storage, suggesting a correlation between lipid lateral diffusion and cholesterol levels in cell membranes.

The eucaryotic aminoacyl-tRNA synthetase complex: suggestions for its structure and function

Aminoacyl-tRNA synthetases from eucaryotic cells generally are isolated as high molecular weight complexes comprised of multiple synthetase activities, and often containing other components as well. A model is proposed for the synthetase complex in which hydrophobic extensions on the proteins serve to maintain them in their high molecular weight form, but are not needed for catalytic activity. The structural similarity of these enzymes to certain membrane-bound proteins, and its implications for synthetase localization and function in vivo, are discussed.

Lateral diffusion of lipids and glycophorin in solid phosphatidylcholine bilayers. The role of structural defects

The lateral mobility of the lipid analog N-4-nitrobenzo-2-oxa-1,3 diazole phosphatidylethanolamine and of the integral protein glycophorin in giant dimyristoylphosphatidylcholine vesicles was studied by the photobleaching technique. Above the temperature of the chain-melting transition (Tm = 23 degrees C), the diffusion coefficient, Dp, of the protein [Dp = (4 +/- 2) X 10(-8) cm2/s at 30 degrees C] was within the experimental errors equal to the corresponding values DL of the lipid analog. In the P beta 1 phase the diffusion of lipid and glycophorin was studied as a function of the probe and the protein concentration. (a) At low lipid-probe content (cL less than 5 mmol/mol of total lipid), approximately 20% of the probe diffuses fast (D approximately equal to 10(-8) - 10(-9) cm2/s), while the mobility of the rest is strongly reduced (D less than 10(-10) cm2/s). At a higher concentration (cp approximately 20 mmol), all probe is immobilized (D less than 10(-10) cm2/s). (b) Incorporation of glycophorin up to cp = 0.4 mmol/mol of total lipid leads to a gradual increase of the fraction of mobile lipid probe due to the lateral-phase separation into a pure P beta 1 phase and a fraction of lipid that is fluidized by strong hydrophilic lipid-protein interaction. (c) The diffusion of the glycophorin molecules is characterized by a slow and a fast fraction. The latter increases with increasing protein content, which is again due to the lateral-phase separation caused by the hydrophilic lipid-protein interaction. The results are interpreted in terms of a fast transport along linear defects in the P beta 1 phase, which form quasi-fluid paths for a nearly one dimensional and thus very effective transport. Evidence for this interpretation of the diffusion measurements is provided by freeze-fracture electron microscopy.

Effect of cerulenin on macromolecule synthesis in chemoheterotrophically and photoheterotrophically grown Rhodopseudomonas sphaeroides

The antibiotic cerulenin causes the immediate cessation of phospholipid biosynthesis in both chemoheterotrophic and photoheterotrophic cultures of Rhodopseudomonas sphaeroides. Macromolecule biosynthesis in photoheterotrophic cells was unaffected by cerulenin for the first 2 h after antibiotic addition and then continued at a reduced rate for an additional 8 h. In contrast, macromolecule biosynthesis in chemoheterotrophic cells was severely affected by cerulenin within the first 2 h of treatment. Pulse-labeling of protein after cerulenin addition revealed that all subcellular fractions were equally affected by the action of cerulenin with chemoheterotrophic cell fractions more profoundly affected than those derived from photoheterotrophic cells. Protein insertion into the intracytoplasmic membrane of photoheterotrophic cells continued for up to 6 h after the onset of cerulenin treatment. Residual macromolecule synthesis was correlated with the presence of the photosynthetic membrane system under all conditions of growth.

The influence of cholesterol incorporation and removal on lipid-bilayer viscosity and electron transfer in rat-liver microsomes

The incorporation and removal of cholesterol from rat liver microsomes was used as a methodical approach to investigate the molecular organization of microsomal redox-chains. It was shown that the incorporation of cholesterol in microsomes increases and removal of cholesterol decreases lipid bilayer viscosity as indicated from the rate of fluorescent probe-pyrene eximerisation in cholesterol-enriched and cholesterol-depleted microsomes. The increase of membrane viscosity slows down the initial rates and decreases the rate constants of cytochrome b5 reduction by NAD(P)H, whereas the decrease of membrane viscosity enhances the initial rates and increases the rate constants of these reactions. The rates of cytochrome P450 reduction by reduced pyridine nucleotides do not depend on the viscosity of lipid bilayer. The incorporation and removal of cholesterol from microsomes was not followed by any essential changes in the rates of dimethylaniline N-demethylation, aniline p-hydroxylation, p-nitroanisole O-demethylation, oxygen consumption, oxidation of NADH and NADPH. Thus the reduction of cytochrome b5 by NADH and NADPH is the diffusion-dependent reaction in the redox-chains of microsomes only.

Thermodynamics and membrane processes

This review represents a personal view of membrane thermodynamics. I do not intend to deal at all with the irreversible thermodynamics of membrane mass transfer processes. This aspect has been covered far more competently and completely by other people (Bittar, 1970; Paterson, 1970; Rottenberg, Caplan & Essig, 1970; Mitchell, 1970; Rothschildet al.1980; Oster, Perelson & Katchalsky, 1973; Kedem & Katchalsky, 1958; Schwartz, 1971). The recent review on osmosis by Hill (1979) is a particularly succinct appraisal of a facet of irreversible membrane thermodynamics. Arata & Nishimura (1980) have considered the coupling of electron transfer to vectorial processes in biological membranes.

Relationship between the density distribution of intramembrane particles and electron transfer in the mitochondrial inner membrane as revealed by cholesterol incorporation

A low pH method of liposome-membrane fusion (Schneider et al., 1980, Proc. Natl. Acad. Sci. U. S. A. 77:442) was used to enrich the mitochondrial inner membrane lipid bilayer 30-700% with exogenous phospholipid and cholesterol. By varying the phospholipid-to-cholesterol ratio of the liposomes it was possible to incorporate specific amounts of cholesterol (up to 44 mol %) into the inner membrane bilayer in a controlled fashion. The membrane surface area increased proportionally to the increase in total membrane bilayer lipid. Inner membrane enriched with phospholipid only, or with phospholipid plus cholesterol up to 20 mol %, showed randomly distributed intramembrane particles (integral proteins) in the membrane plane, and the average distance between intramembrane particles increased proportionally to the amount of newly incorporated lipid. Membranes containing between 20 and 27 mol % cholesterol exhibited small clusters of intramembrane particles while cholesterol contents above 27 mol % resulted in larger aggregations of intramembrane particles. In phospholipid-enriched membranes with randomly dispersed intramembrane particles, electron transfer activities from NADH- and succinate-dehydrogenase to cytochrome c decreased proportionally to the increase in distance between the particles. In contrast, these electron-transfer activities increased with decreasing distances between intramembrane particles brought about by cholesterol incorporation. These results indicate that (a) catalytically interacting redox components in the mitochondrial inner membrane such as the dehydrogenase complexes, ubiquinone, and heme proteins are independent, laterally diffusible components; (b) the average distance between these redox components is effected by the available surface area of the membrane lipid bilayer; and (c) the distance over which redox components diffuse before collision and electron transfer mediates the rate of such transfer.

Rate of lateral diffusion of intramembrane particles: measurement by electrophoretic displacement and rerandomization

A method combining electrophoresis and freeze-fracture electron microscopy is described; the method was used to determine the lateral diffusion coefficient of intramembrane particles (integral proteins) in the mitochondrial inner membrane. An electric current was passed through microsuspensions of purified, spherical inner membranes at pH 7.4, which caused an electrophoretic migration of intramembrane particles in the membrane plane into a single, crowded patch facing the positive electrode. The membrane microsuspensions were quick-frozen at specified times after the packed particles were released from the electrophoretic force and while the particles were diffusing back to a random distribution. Observed concentration gradients of intramembrane particles during this time were quantitatively compared with and found to follow a mathematical model for Fickian diffusion of particles on a spherical membrane. The results determine the kinetics of free diffusion of integral proteins at the resolution of individual proteins. The diffusion coefficient of the integral proteins in the mitochondrial inner membrane was determined to be 8.3 X 10(-10) cm2/sec at 20 degrees C, from which a root-mean-square displacement of 57 nm in 10 msec is predicted.

Lateral diffusion of rhodopsin in photoreceptor cells measured by fluorescence photobleaching and recovery

Frog rod outer segments were labeled with the sulfhydryl-reactive label iodoacetamido tetramethylrhodamine. The bulk of the label reacted with the major disk membrane protein, rhodopsin. Fluorescence photobleaching and recovery (FPR) experiments on labeled rods showed that the labeled proteins diffused rapidly in the disk membranes. In these FPR experiments we observed both the recovery of fluorescence in the bleached spot and the loss of fluorescence from nearby, unbleached regions of the photoreceptor. These and previous experiments show that the redistribution of the fluorescent labeled proteins after bleaching was due to diffusion. The diffusion constant, D, was (3.0 +/- 10(-9) cm2 s-1 if estimated from the rate of recovery of fluorescence in the bleached spot, and (5.3 +/- 2.4) x 10(-9) cm2 s-1 if estimated from the rate of depletion of fluorescence from nearby regions. The temperature coefficient, Q10, for diffusion was 1.7 +/- 0.5 over the range 10 degrees--29 degrees C. These values obtained by FPR are in good agreement with those previously obtained by photobleaching rhodopsin in fresh, unlabeled rods. This agreement indicates that the labeling and bleaching procedures required by the FPR method did not significantly alter the diffusion rate of rhodopsin. Moreover, the magnitude of the diffusion constant for rhodopsin is that to be expected for an object of its diameter diffusing in a bilayer with the viscosity of the disk membrane. In contrast to the case of rhodopsin, FPR methods applied to other membrane proteins have yielded much smaller diffusion constants. The present results help indicate that these smaller diffusion constants are not artifacts of the method but may instead be due to interactions the diffusing proteins have with other components of the membrane in addition to the viscous drag imposed by the lipid bilayer.

Phospholipids containing photoactivable groups in studies of biological membranes

Photoactivable carbene precursors, aryl diazirines and trifluorodiazopropionates, were incorporated synthetically into the omega-positions of fatty acids, which were used to synthesize phospholipids. Extensive intermolecular C--H insertion reactions were demonstrated by photolysis of liposomes prepared from the above phospholipids. Structural analysis of the cross-linked products showed that the predominant sites of cross-linking were in the expected positions within the bilayer. Studies on the topography of a number of membrane proteins using the above phospholipids were initiated. Cross-linking of the photoactivable phospholipids to membrane-embedded proteins, glycophorin A, cytochrome b5, and gramicidin A, was demonstrated.

Relationships between bilayer lipid, motional freedom of oxidoreductase components, and electron transfer in the mitochondrial inner membrane

The relationships between bilayer lipid, diffusional and conformational activities of oxidoreduction components, and electron transfer activity in the mitochondrial inner membrane are considered. Using a new, low pH method to fuse liposome phospholipid (asolectin) with the isolated mitochondrial inner membrane, the membrane bilayer is enriched up to 700% with exogenous phospholipid. During such enrichment, ultrastructural analysis reveals that integral proteins diffuse freely and randomly into the expanding bilayer. Kinetic analysis reveals that a diffusion limited step occurs between succinate- and NADH dehydrogenase and cytochromes bc1, and that the dehydrogenases, ubiquinone, and cytochromes bc1 are free to diffuse independently of one another in the membrane plane. Whether cytochromes bc1 and cytochrome c oxidase codiffuse in the membrane plane, or diffuse independently of one another remains unclear. The specific activities of succinate- and NADH-dehydrogenase as well as cytochrome c oxidase are affected by bilayer enrichment. This most likely occurs through the direct modulation by the newly incorporated phospholipid on conformational activity required in the oxidoreductases for electron transfer.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

Rapid conformational changes of cytochrome P-450: effect of dimyristoyl lecithin

Binding of benzphetamine to purified microsomal cytochrome P-450 from rat liver causes a shift in the heme spin state of the protein to favor the high-spin form. This shift is strongly temperature dependent. A rapid temperature jump of a cytochrome P-450/epsilon benzphetamine mixture, monitored by changes in the Soret absorptions of the heme, reveals two relaxation processes: one in a 50-msec time range (tau(f)) and the other in a 0.3-sec time range (tau(s)). Both relaxations reflect conformational changes of the protein after the substrate binding. No bimolecular reaction of benzphetamine and the enzyme has been resolved. This indicates that there is no absorption change of the heme associated with the initial binding. In the presence of dimyristoyl lecithin, at 25 degrees C tau(f) decreases by nearly one order of magnitude whereas tau(s) decreases to one-third. The enhancement of rates by added phospholipid is both temperature- and concentration-dependent: rates are accelerated only above the gel-liquid crystalline transition temperature, and this effect saturates near the enzyme/lipid ratio of 1:20. In contrast, the lipid does not have significant effect on the equilibrium binding curve of the substrate. These results suggest that the lipid may form an envelope around the enzyme and, depending on its crystalline state, regulates the rate of the substrate-induced conformational changes of cytochrome P-450.