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The phytotherapeutic bromelain is a heterogeneous protein mixture, extracted from pineapple stem, with high proteolytic activity based on cysteine proteases. Its global protein chemical composition was analyzed qualitatively and quantitatively by SDS-PAGE and RP-HPLC. A SDS-PAGE method with elaborate sample pretreatment was developed, to cope with the bromelain's self-digestion properties and the hypothetical disulfide scrambling during electrophoresis. Both can produce misleading results, if not considered. RP-HPLC was applied for its high separation power for bromelain proteinaceous compounds. A peak identification and assignment to different protein classes in bromelain was done by enzyme kinetics and MS. The method was successfully applied for the quantitative determination of the molar ratio between inhibitor and enzyme and resulted to be approximately 3:2. Bromelain contains, from a molar point of view, inhibitor molecules as major component, which thus might be considered as a natural pharmaceutical excipient in Bromelain, because it protects the enzymes against autolysis. We described two methods to separate the inhibitor fraction from the enzyme fraction, RP-HPLC and size exclusion chromatography. A pineapple derived Jacalin-like-lectin, herein called 'Anlec', was identified and quantified by RP-HPLC-MS in bromelain and its content was determined to be 5%, related to all proteins in bromelain. Anlec binds specifically to mannose-containing glycans and is discussed in literature to possess anti-HIV medical potential. Bromelain could therefore be a possible and economic source for the production of Anlec. An isolation strategy of Anlec from bromelain, in high purity, is shown in this work. The presented RP-HPLC results are comprehensive in chemical information, and the method is expedient to provide appropriate bromelain protein isolations but also to accomplish quality control, covering all relevant protein components. It is furthermore shown, that proteins in bromelain may react with reducing sugars in a Maillard reaction to form glycated proteins. Maillard reaction products in bromelain are detected and characterized and could be responsible for the limited stability and storage times at room temperature of bromelain. Even the active center thiol group could be potentially glycated., (Copyright © 2020 Elsevier B.V. All rights reserved.)

In eukaryotic and prokaryotic cells, F-ATP synthases provide energy through the synthesis of ATP. The chloroplast F-ATP synthase (CF 1 F O -ATP synthase) of plants is integrated into the thylakoid membrane via its F O -domain subunits a, b, b' and c Subunit c with a stoichiometry of 14 and subunit a form the gate for H + -pumping, enabling the coupling of electrochemical energy with ATP synthesis in the F 1 sector.Here we report the crystallization and structure determination of the c14-ring of subunit c of the CF 1 F O -ATP synthase from spinach chloroplasts. The crystals belonged to space group C2, with unit-cell parameters a=144.420, b=99.295, c=123.51 Å, and β=104.34° and diffracted to 4.5 Å resolution. Each c-ring contains 14 monomers in the asymmetric unit. The length of the c-ring is 60.32 Å, with an outer ring diameter 52.30 Å and an inner ring width of 40 Å., (© 2014 The Author(s).)

The release of reactive oxygen species (ROS) as side products of aerobic metabolism in the mitochondria is an unavoidable consequence. As the capacity of organisms to deal with this exposure declines with age, accumulation of molecular damage caused by ROS has been defined as one of the central events during the ageing process in biological systems as well as in numerous diseases such as Alzheimer's and Parkinson's Dementia. In the filamentous fungus Podospora anserina, an ageing model with a clear defined mitochondrial etiology of ageing, in addition to the mitochondrial aconitase the ATP synthase alpha subunit was defined recently as a hot spot for oxidative modifications induced by ROS. In this report we show, that this reactivity is not randomly distributed over the ATP Synthase, but is channeled to a single tryptophan residue 503. This residue serves as an intra-molecular quencher for oxidative species and might also be involved in the metabolic perception of oxidative stress or regulation of enzyme activity. A putative metal binding site in the proximity of this tryptophan residue appears to be crucial for the molecular mechanism for the selective targeting of oxidative damage., (Copyright © 2011 Elsevier B.V. All rights reserved.)

ATP synthases are part of the sophisticated cellular metabolic network and therefore multiple interactions have to be considered. As discussed in this review, ATP synthases form various supramolecular structures. These include dimers and homooligomeric species. But also interactions with other proteins, particularly those involved in energy conversion exist. The supramolecular assembly of the ATP synthase affects metabolism, organellar structure, diseases, ageing and vice versa. The most common approaches to isolate supercomplexes from native membranes by use of native electrophoresis or density gradients are introduced. On the one hand, isolated ATP synthase dimers and oligomers are employed for structural studies and elucidation of specific protein-protein interactions. On the other hand, native electrophoresis and other techniques serve as tool to trace changes of the supramolecular organisation depending on metabolic alterations. Upon analysing the structure, dimer-specific subunits can be identified as well as interactions with other proteins, for example, the adenine nucleotide translocator. In the organellar context, ATP synthase dimers and oligomers are involved in the formation of mitochondrial cristae. As a consequence, changes in the amount of such supercomplexes affect mitochondrial structure and function. Alterations in the cellular power plant have a strong impact on energy metabolism and ultimately play a significant role in pathophysiology. In plant systems, dimers of the ATP synthase have been also identified in chloroplasts. Similar to mammals, a correlation between metabolic changes and the amount of the chloroplast ATP synthase dimers exists. Therefore, this review focusses on the interplay between metabolism and supramolecular organisation of ATP synthase in different organisms., (2011 Elsevier B.V. All rights reserved.)

Native electrophoresis is a powerful tool for the separation of intact protein complexes. By incubating such gels in a suitable reaction solution, specific enzyme activities can be screened comprehensively. The recent standard procedure for determination of ATP hydrolysis activity in blue or clear native gels is based on formation of a lead phosphate precipitate. The resulting white bands are challenging for detection and documentation of low activities. For the analysis of photosynthetic ATP synthases, the method has to be adapted to deregulate the inhibition of latent ATPase functions. Therefore, we introduced an incubation of gels in detergent solution, whereby taurodeoxycholate turned out to be the most efficient activator. In order to detect low ATPase activities, a short additional incubation step subsequent to the formation of lead phosphate is recommended. By adding ammonium sulfide, the white bands are converted into brownish-black bands of lead sulfide. Our new procedure sustains the linear quantitation range of the original lead phosphate protocol and moreover expands the detection limit.

By the elucidation of high-resolution structures the view of the bioenergetic processes has become more precise. But in the face of these fundamental advances, many problems are still unresolved. We have examined a variety of aspects of energy-transducing membranes from large protein complexes down to the level of protons and functional relevant picosecond protein dynamics. Based on the central role of the ATP synthase for supplying the biological fuel ATP, one main emphasis was put on this protein complex from both chloroplast and mitochondria. In particular the stoichiometry of protons required for the synthesis of one ATP molecule and the supramolecular organisation of ATP synthases were examined. Since formation of supercomplexes also concerns other complexes of the respiratory chain, our work was directed to unravel this kind of organisation, e.g. of the OXPHOS supercomplex I(1)III(2)IV(1), in terms of structure and function. Not only the large protein complexes or supercomplexes work as key players for biological energy conversion, but also small components as quinones which facilitate the transfer of electrons and protons. Therefore, their location in the membrane profile was determined by neutron diffraction. Physico-chemical features of the path of protons from the generators of the electrochemical gradient to the ATP synthase, as well as of their interaction with the membrane surface, could be elucidated by time-resolved absorption spectroscopy in combination with optical pH indicators. Diseases such as Alzheimer's dementia (AD) are triggered by perturbation of membranes and bioenergetics as demonstrated by our neutron scattering studies.

Native gels enable the analysis of protein complexes on a proteome-wide scale in a single experiment. The protocols described in this unit are based on separation of protein complexes by blue native polyacrylamide electrophoresis (BN-PAGE), the most versatile native gel system, and the closely related milder colorless native PAGE (CN-PAGE). Both BN-PAGE and CN-PAGE are described on analytical to preparative scales. In addition, methods for subsequent analysis of protein complexes are given, including electroelution from native gels as well as denaturing and native two-dimensional PAGE. Finally, the removal of Coomassie dye from electroeluted proteins is detailed along with a discussion of fundamental considerations for the solubilization of membrane protein complexes from various biological samples, which are exemplified for mitochondria, chloroplasts (thylakoids), and cyanobacteria.

Due to its unmatched resolution, gel electrophoresis is an indispensable tool for the analysis of diverse biomolecules. By adaptation of the electrophoretic conditions, even fragile protein complexes as parts of intracellular networks migrate through the gel matrix under sustainment of their integrity. If the thickness of such native gels is significantly increased compared to the analytical version, also high sample loads can be processed. However, the cage-like network obstructs an in-depth analysis for deciphering structure and function of protein complexes and other species. Consequently, the biomolecules have to be removed from the gel matrix into solution. Several approaches summarized in this review tackle this problem. While passive elution relies on diffusion processes, electroelution employs an electric field to force biomolecules out of the gel. An alternative procedure requires a special electrophoresis setup, the continuous elution device. In this apparatus, molecules migrate in the electric field until they leave the gel and were collected in a buffer stream. Successful isolation of diverse protein complexes like photosystems, ATP-dependent enzymes or active respiratory supercomplexes and some other bioparticles demonstrates the versatility of preparative electrophoresis. After liberating particles out of the gel cage, numerous applications are feasible. They include elucidation of the individual components up to high resolution structures of protein complexes. Therefore, preparative electrophoresis can complement standard purification methods and is in some cases superior to them.

For functional characterization, we isolated the F1FO-ATP synthase of the thermophilic cyanobacterium Thermosynechococcus elongatus. Because of the high content of phycobilisomes, a combination of dye-ligand chromatography and anion exchange chromatography was necessary to yield highly pure ATP synthase. All nine single F1FO subunits were identified by mass spectrometry. Western blotting revealed the SDS stable oligomer of subunits c in T. elongatus. In contrast to the mass archived in the database (10,141 Da), MALDI-TOF-MS revealed a mass of the subunit c monomer of only 8238 Da. A notable feature of the ATP synthase was its ability to synthesize ATP in a wide temperature range and its stability against chaotropic reagents. After reconstitution of F1FO into liposomes, ATP synthesis energized by an applied electrochemical proton gradient demonstrated functional integrity. The highest ATP synthesis rate was determined at the natural growth temperature of 55 degrees C, but even at 95 degrees C ATP production occurred. In contrast to other prokaryotic and eukaryotic ATP synthases which can be disassembled with Coomassie dye into the membrane integral and the hydrophilic part, the F1FO-ATP synthase possessed a particular stability. Also with the chaotropic reagents sodium bromide and guanidine thiocyanate, significantly harsher conditions were required for disassembly of the thermophilic ATP synthase.

Native gels enable the analysis of protein complexes on a proteome-wide scale in a single experiment. The protocols described in this unit are based on separation of protein complexes by blue native polyacrylamide electrophoresis (BN-PAGE), the most versatile native gel system, and the closely related milder colorless native PAGE (CN-PAGE). Both BN-PAGE and CN-PAGE are described on analytical to preparative scales. In addition, methods for subsequent analysis of protein complexes are given, including electroelution from native gels as well as denaturing and native two-dimensional PAGE. Finally, the removal of Coomassie dye from electroeluted proteins is detailed along with a discussion of fundamental considerations for the solubilization of membrane protein complexes from various biological samples, which are exemplified for mitochondria, chloroplasts (thylakoids), and cyanobacteria., ((c) 2008 by John Wiley & Sons, Inc.)

ATP synthases - rotary nano machines - consist of two major parts, F(O) and F(1), connected by two stalks: the central and the peripheral stalk. In spinach chloroplasts, the central stalk (subunits gamma, epsilon) forms with the cylinder of subunits III the rotor and transmits proton motive force from F(O) to F(1), inducing conformational changes of the catalytic centers in F(1). The epsilon subunit is an important regulator affecting adjacent subunits as well as the activity of the whole protein complex. Using a combination of chemical cross-linking and mass spectrometry, we monitored interactions of subunit epsilon in spinach chloroplast ATP synthase with III and gamma. Onto identification of interacting residues in subunits epsilon and III, one cross-link defined the distance between epsilon-Cys6 and III-Lys48 to be 9.4 A at minimum. epsilon-Cys6 was competitively cross-linked with subunit gamma. Altered cross-linking yields revealed the impact of nucleotides and Mg(2+) on cross-linking of subunit epsilon. The presence of nucleotides apparently induced a displacement of the N-terminus of subunit epsilon, which separated epsilon-Cys6 from both, III-Lys48 and subunit gamma, and thus decreasing the yield of the cross-linked subunits epsilon and gamma as well as epsilon and III. However, increasing concentrations of the cofactor Mg(2+) favoured cross-linking of epsilon-Cys6 with subunit gamma instead of III-Lys48 indicating an approximation of subunits gamma and epsilon and a separation from III-Lys48.

Dimers and oligomers of F-type ATP synthases have been observed previously in mitochondria of various organisms and for the CF(o)F(1) ATP synthase of chloroplasts of Chlamydomonas reinhardtii. In contrast to mitochondria, however, dimers of chloroplast ATP synthases dissociate at elevated phosphate concentration. This suggests a regulation by cell physiological processes. Stable isotope labeling of living cells and blue-native PAGE have been employed to quantitate changes in the ratio of monomeric to dimeric CF(o)F(1) ATP synthase. Chlamydomonas reinhardtii cells were cultivated photoautotrophically in the presence of (15)N and photomixotrophically at natural (14)N abundance, respectively. As compared to photoautotrophic growth, an increased assembly of ATP synthase dimers on the expense of preexisting monomers during photomixotrophic growth was observed, demonstrating a metabolic control of the dimerization process.

ATP synthases convert an electrochemical proton gradient into rotational movement to produce the ubiquitous energy currency adenosine triphosphate. Tension generated by the rotational torque is compensated by the stator. For this task, a peripheral stalk flexibly fixes the hydrophilic catalytic part F1 to the membrane integral proton conducting part F(O) of the ATP synthase. While in eubacteria a homodimer of b subunits forms the peripheral stalk, plant chloroplasts and cyanobacteria possess a heterodimer of subunits I and II. To better understand the functional and structural consequences of this unique feature of photosynthetic ATP synthases, a procedure was developed to purify subunit I from spinach chloroplasts. The secondary structure of subunit I, which is not homologous to bacterial b subunits, was compared to heterologously expressed subunit II using CD and FTIR spectroscopy. The content of alpha-helix was determined by CD spectroscopy to 67% for subunit I and 41% for subunit II. In addition, bioinformatics was applied to predict the secondary structure of the two subunits and the location of the putative coiled-coil dimerization regions. Three helical domains were predicted for subunit I and only two uninterrupted domains for the shorter subunit II. The predicted length of coiled-coil regions varied between different species and between subunits I and II.

In the inner mitochondrial membrane, the respiratory chain complexes generate an electrochemical proton gradient, which is utilized to synthesize most of the cellular ATP. According to an increasing number of biochemical studies, these complexes are assembled into supercomplexes. However, little is known about the architecture of the proposed multicomplex assemblies. Here, we report the electron microscopic characterization of the two respiratory chain supercomplexes I1III2 and I1III2IV1 in bovine heart mitochondria, which are also two major supercomplexes in human mitochondria. After purification and demonstration of enzymatic activity, their structures in projection were determined by single particle image analysis. A difference map between the supercomplexes I1III2 and I1III2IV1 closely fits the x-ray structure of monocomplex IV and shows its location in the assembly. By comparing different views of supercomplex I1III2IV1, the location and mutual arrangement of complex I and the complex III dimer are discussed. Detailed knowledge of the architecture of the active supercomplexes is a prerequisite for a deeper understanding of energy conversion by mitochondria in mammals.

Higher plant mitochondria have many unique features compared with their animal and fungal counterparts. This is to a large extent related to the close functional interdependence of mitochondria and chloroplasts, in which the two ATP-generating processes of oxidative phosphorylation and photosynthesis, respectively, take place. We show that digitonin treatment of mitochondria contaminated with chloroplasts from spinach (Spinacia oleracea) green leaves at two different buffer conditions, performed to solubilize oxidative phosphorylation supercomplexes, selectively extracts the mitochondrial membrane protein complexes and only low amounts of stroma thylakoid membrane proteins. By analysis of digitonin extracts from partially purified mitochondria of green leaves from spinach using blue and colorless native electrophoresis, we demonstrate for the first time that in green plant tissue a substantial proportion of the respiratory complex IV is assembled with complexes I and III into "respirasome"-like supercomplexes, previously observed in mammalian, fungal, and non-green plant mitochondria only. Thus, fundamental features of the supramolecular organization of the standard respiratory complexes I, III, and IV as a respirasome are conserved in all higher eukaryotes. Because the plant respiratory chain is highly branched possessing additional alternative enzymes, the functional implications of the occurrence of respiratory supercomplexes in plant mitochondria are discussed.

The chloroplast H(+)-ATP synthase is a key component for the energy supply of higher plants and green algae. An oligomer of identical protein subunits III is responsible for the conversion of an electrochemical proton gradient into rotational motion. It is highly controversial if the oligomer III stoichiometry is affected by the metabolic state of any organism. Here, the intact oligomer III of the ATP synthase from Chlamydomonas reinhardtii has been isolated for the first time. Due to the importance of the subunit III stoichiometry for energy conversion, a gradient gel system was established to distinguish oligomers with different stoichiometries. With this methodology, a possible alterability of the stoichiometry in respect to the metabolic state of the cells was examined. Several growth parameters, i.e., light intensity, pH value, carbon source, and CO(2) concentration, were varied to determine their effects on the stoichiometry. Contrary to previous suggestions for E. coli, the oligomer III of the chloroplast H(+)-ATP synthase always consists of a constant number of monomers over a wide range of metabolic states. Furthermore, mass spectrometry indicates that subunit III from C. reinhardtii is not modified posttranslationally. Data suggest a subunit III stoichiometry of the algae ATP synthase divergent from higher plants.

H+-ATP synthase is the dominant ATP production site in mitochondria and chloroplasts. So far, dimerization of ATP synthase has been observed only in mitochondria by biochemical and electron microscopic investigations. Although the physiological relevance remains still enigmatic, dimerization was proposed to be a unique feature of the mitochondrion [Biochim. Biophys. Acta 1555 (2002) 154]. It is hard to imagine, however, that closely related protein complexes of mitochondria and chloroplast should show such severe differences in structural organization. We present the first evidences for dimerization of chloroplast ATP synthases within the thylakoid membrane. By investigation of the thylakoid membrane of Chlamydomonas reinhardtii by blue-native polyacrylamide gel electrophoresis, dimerization of the chloroplast ATP synthase was detected. Chloroplast ATP synthase dimer dissociates into monomers upon incubation with vanadate or phosphate but not by incubation with molybdate, while the mitochondrial dimer is not affected by the incubation. This suggests a distinct dimerization mechanism for mitochondrial and chloroplast ATP synthase. Since vanadate and phosphate bind to the active sites, contact sites located on the hydrophilic CF1 part are suggested for the chloroplast ATP synthase dimer. As the degree of dimerization varies with phosphate concentration, dimerization might be a response to low phosphate concentrations.

Proton ATP synthases carry out energy conversion in mitochondria, chloroplasts, and bacteria. A key element of the membrane integral motor CFO in chloroplasts is the oligomer of subunit III: it converts the energy of a transmembrane electrochemical proton gradient into rotational movement. To enlighten prominent features of the structure-function relationship of subunit III from spinach chloroplasts, new isolation methods were established to obtain highly pure monomeric and oligomeric subunit III in milligram quantities. By Fourier-transform infrared (FTIR) and CD spectroscopy, the predominantly alpha-helical secondary structure of subunit III was demonstrated. For monomeric subunit III, a conformational change was observed when diluting the SDS-solubilized protein. Under the same conditions the conformation of the oligomer III did not change. A mass of 8003 Da for the monomeric subunit III was determined by MALDI mass spectrometry (MALDI-MS), showing that no posttranslational modifications occurred. By ionisation during MALDI-MS, the noncovalent homooligomer III14 disaggregated into its III monomers.

Three fundamentally different chloroplast ATP synthase samples of increasing complexity were visualized by atomic force microscopy. The samples are distinguishable in respect to the isolation technique, the detergent employed, and the final subunit composition. The homo-oligomer III was isolated following SDS treatment of ATP synthase, the proton-turbine III+IV was obtained by blue-native electrophoresis, and complete CFO was isolated by anion exchange chromatography of NaSCN splitted ATP synthase. In all three ATP synthase subcomplexes 14 and only 14 circularly arranged subunits III composed the intact transmembrane rotor. Therefore, 14 protomers built the membrane-resident proton turbine. The observed stoichiometry of 14 is not a biochemical artifact or affected by natural growth variations of the spinach, as previously suggested. A correlation between the presence of subunit IV in the imaged sample and the appearance of a central protrusion in the narrower orifice of the oligomeric cylinder III14 has been observed. In contrast to current predictions, in chloroplast FO the subunit IV can be found inside the cylinder III14 and not at its periphery, at least in the reconstituted 2D arrays imaged.

Analysis of the membrane integral proteome is mainly dependent on the ability of protein separation. Blue-native polyacrylamide gel electrophoresis (BN-PAGE) is a technique capable of efficient membrane protein separation, so far mainly applied to the mitochondrial oxidative phosphorylation machinery. Applying BN-PAGE to the thylakoid membranes after mild solubilization with digitonin we succeeded in displaying the response of the green algae Chlamydomonas reinhardtii to altered culture conditions. In addition, by peptide mass fingerprinting and matrix assisted laser desorption/ionization-mass spectrometry (MALDI-MS) extremely hydrophobic subunits of the photosystem complexes with 5-11 transmembrane helices were identified, which could not be accessed by in-gel digestion in previous studies.

Recent structural data suggest that the number of identical subunits (c or III) assembled into the cation-powered rotor of F1F0 ATP synthase depends on the biological origin. Atomic force microscopy allowed individual subunits of the cylindrical transmembrane rotors from spinach chloroplast and from Ilyobacter tartaricus ATP synthase to be directly visualized in their native-like environment. Occasionally, individual rotors exhibit structural gaps of the size of one or more subunits. Complete rotors and arch-shaped fragments of incomplete rotors revealed the same diameter within one ATP synthase species. These results suggest the rotor diameter and stoichiometry to be determined by the shape of the subunits and their nearest neighbor interactions.

The proton-ATP synthase of thylakoid membranes from spinach chloroplasts (CF(O)F(1)) and its subcomplexes CF(O) and CF(1) were isolated by blue native electrophoresis (BN-PAGE) [Neff, D. and Dencher, N.A. (1999) Biochem. Biophys. Res. Commun. 259, 569-575] and subsequently electroeluted from the gel. A method was developed to remove most of the dye Coomassie G-250 (CBG) using gel filtration, a prerequisite for many biophysical investigations. The dye was removed from the electroeluted CF(O)F(1), CF(O) or CF(1) and exchanged with the detergent CHAPS. ATP hydrolysis activity of CF(1) and ATP synthesis activity of reconstituted CF(O)F(1) were determined before and after dye removal. The secondary structure of CF(O) was studied by CD spectroscopy in the presence and the absence of the dye. CBG neither abolishes the catalytic activity of the isolated CF(O)F(1) and CF(1) nor affects the subunit composition and the high alpha-helical content of CF(O). In crystallization attempts, 2D arrays of CF(O)F(1) and of CF(O) before and after dye removal were obtained. In the aggregates of CF(O), circular structures with a mean diameter of 6.7 nm were observed. Our results indicate that the combination of BN-PAGE and dye removal by gel filtration is a suitable approach to obtain catalytically active protein complexes for further functional and structural characterization.

n-Dodecyl-beta-D-maltoside was used as a detergent to solubilize the ammonium sulphate precipitate of chloroplast F(O)F(1)-ATP synthase, which was purified further by dye-ligand chromatography. Upon reconstitution of the purified protein complex into phosphatidylcholine/phosphatidic acid liposomes, ATP synthesis, driven by an artificial DeltapH/Deltapsi, was observed. The highest activity was achieved with ATP synthase solubilized in n-dodecyl-beta-D-maltoside followed by chromatography with Red 120 dye. The optimal dye for purification with CHAPS was Green 5. All known subunits were present in the monodisperse proton-translocating ATP synthase preparation obtained from chloroplasts.

To gain a pure enzyme preparation for functional and crystallization studies, an additional purification step in the isolation of the chloroplast ATP synthase (CF(0)F(1)) has been introduced. By applying gel filtration or anion exchange perfusion chromatography in presence of the detergents CHAPS and n-dodecyl-beta-d-maltoside, respectively, Rubisco and other contaminants were separated from CF(0)F(1). The purity and activity depended on the chromatographic method and the detergent employed. The highest purity and activity were achieved by anion exchange chromatography for the detergent dodecyl-maltoside and by gel filtration for the detergent CHAPS. The detergent Triton X-100, which is frequently used to solubilize CF(0)F(1), was found to be inadequate to stabilize the ATP synthase during chromatography., (Copyright 1999 Academic Press.)

By neutron scattering experiments and time-resolved absorption spectroscopy we have investigated picosecond equilibrium fluctuations and the kinetics of the photocycle of bacteriorhodopsin (BR) in the purple membrane (PM). Natural PM samples composed of 75% BR (w/w) and 25% lipid (w/w) as well as delipidated PM having only 5% lipid (w/w) were measured at different levels of hydration. We observed a reduced 'flexibility', due to a diminished weight of stochastic large-amplitude motions occurring in the delipidated PM as compared to the natural PM. This effect is more pronounced for wet samples, indicating the importance of lipid hydration for protein dynamics. The reduced flexibility is accompanied by significantly larger time constants describing the decay of the M-intermediate. Therefore, a correlation between the dynamical behavior of the protein-lipid complex and BR function emerges.