摘要:

AbstractRetinochrome is a photo‐isomerase of all‐trans‐retinal to the 11‐cis‐isomer for rhodopsin synthesis. This protein is probably folded into seven α‐helices spanning the membranes. Upon irradiation, it is converted to metaretinochrome, via pre‐lumi‐ and lumiretinochrome, which are stable under 50 and 250 K, respectively. A series of analogue studies revealed that the cyclohexene ring and 6‐s‐cis‐twisted conformation are important for binding. Modification of the 14‐position suggests close location of th

ISSN:0021-2148    

DOI:10.1002/ijch.199500036

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe photoisomerization of all‐trans‐retinal of bacteriorhodopsin to 13‐cisgives rise to a series of unstable states that thermally interconvert on the picosecond to millisecond timescale, and ultimately decay back to the initial state. Since this “photocycle” drives the translocation of a proton from the cytoplasmic to the extracellular side of the membrane, its detailed description is essential to understand the mechanism of the ion transport. The interconversions of the intermediates of the cycle, K610, L550, M410, N560, and O640, proceed with complex multiphasic kinetics, and there is much disagreement over the interpretation of this. However, many groups have described the photocycle as a single reaction sequence with several reversible reactions, some pH‐dependent. Through the use of time‐resolved spectroscopy and site‐specific mutations, such a scheme provides a reasonable model for how the retinal Schiff base first transfers its proton to Asp‐85, followed by release of a proton to the extracellular side; then the Schiff base is reprotonated from Asp‐96, and proton uptake from the cytoplasmic surface restores the initial protonation state of Asp‐96, while the retinal reis

ISSN:0021-2148    

DOI:10.1002/ijch.199500037

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractDifference FTIR spectroscopy of the photointermediates of bacteriorhodopsin is informative for changes in H‐bonding, the protonation states, and the bond orientation of functional residues such as C=O, N‐H, and O‐H of the chromophore, protein residues, peptide bonds, and internal water molecules. The vibrational bands of the chromophore are found at frequencies similar to those observed by resonance Raman spectra. Moreover, FTIR gives clear results on the N— in‐plane bending vibration and C14–C15stretching vibrational modes, both of which are useful for the analysis of the structure of the chromophore. The protonation states and H‐bonding changes of intramembrane protonated aspartic acid residues can be revealed only by FTIR spectroscopy, in combination with proper isotope labeling and site‐directed mutagenesis. The results with Asp‐96 and Asp‐85 in the L, M, and N photointermediates were especially useful for understanding the proton transfer mechanism. Besides amino acid groups in the protein, peptide C=O vibrations were assigned to a specific bond by the use of site‐directed isotope labeling. Water molecules which undergo structural changes upon photoreaction were attributed specifically to those interacting with particular protein residues by mutational studies. On the basis of these FTIR studies, the role of the L intermediate is emphasized as the key intermediate that creates the conditions for the proton transfer reaction from the Schiff base to Asp‐85 and subsequent proton uptake reactions in

ISSN:0021-2148    

DOI:10.1002/ijch.199500038

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractRecent advances in understanding the intramolecular charge transfer and proton release by the light‐driven proton pump bacteriorhodopsin (bR) are critically reviewed. The focus is on the time‐resolved electrical methods, i.e., photocurrent and photovoltage measurements, and on transient absorption experiments with pH‐sensitive dyes. Particular attention is paid to the following topics: charge translocation in the low‐pH forms of bR (acid‐blue and acid‐purple); electrogenicity of the 13‐ciscycle; results with bR mutants; surface‐bound dyes to detect the proton release at specific sites on either side of the protein; the question of kinetic coupling between the deprotonation of the Schiff base and proton release; and rapid long‐range migration of protons along the surface of th

ISSN:0021-2148    

DOI:10.1002/ijch.199500039

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe photochemical reactions of the intermediates of the photochemical cycle of bacteriorhodopsin (bR) are reviewed. These reactions constitute photochemical control of the cycle and provide an independent approach for the investigation of the mechanism of light energy transduction in the purple membrane. The absorption of a light quantum by the K, L, or M intermediates converts them back to bR. These transformations interrupt the photocycle so that no proton transfer occurs after absorption of the second quantum. The action of blue light on the M intermediate causes structural changes of the chromophore, as a result of which the Schiff base is reprotonated from Asp‐85, not from Asp‐96 as in the usual thermal transition of M. The photoreactions of the L, M, N, and O intermediates lead to the formation of new photoproducts. Studies of the photoconversion of the intermediates can serve as an additional source of information on the nature of photoprocesses in bR: they reveal several conformers of K and bR at 90 K, different M states, two N intermediates, and provide direct evidence for the existence of a thermal back reaction from N to M. The study of the photoreactions of the J, K, L, M, N, and O intermediates is a promising method for elucidating the structures and roles of these states. Reversible photoconversions of bR and its photointermediates provide a basis for potential applications of bR in optical registration of informat

ISSN:0021-2148    

DOI:10.1002/ijch.199500040

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe light‐induced proton pump in bacteriorhodopsin is reviewed with emphasis on acid‐base equilibria of protein residues and of the retinal Schiff base moiety. Pump mechanisms in bR and in some of its mutants are classified in terms of light‐induced pKachanges (class I) or light‐induced exposure changes, in which the proton accessibility of the protein changes from the outside to the inside of the membrane (class II). A discussion of the theoretical basis of the factors which determine the pKaof ionizable protein groups is followed by a review of the experimental phenomena associated with the titration of residues in both unphotolyzed bR and during its photocycle. The time‐resolved titrations of the Schiff base and of the Asp‐85 residue are discussed in terms of the accessibility of the two groups to external protons. Finally, the molecular aspects of the pH‐dependent proton pump in native bR and in various mutants are analyzed, focusing on the mechanism of the initial proton release reaction and on the subsequent molecular switch which allows reprotonation from the inside of the cell. Special attention is devoted to the question of coupling between the photocycle intermediates (primary M formation and decay) and the transmembrane proton translocation. Recent work with bR mutants raise the question as to whether proton transfer from the Schiff base to Asp‐85 at the M stage is directly responsible for proton translocation, as well as for the repro

ISSN:0021-2148    

DOI:10.1002/ijch.199500041

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe availability of the structure of bacteriorhodopsin from electron microscopy studies has opened up the possibility of exploring the proton pump mechanism of this protein by means of molecular dynamics simulations. In this review we summarize earlier theoretical investigations of the photocycle of bacteriorhodopsin including relevant quantum chemistry studies of retinal, structure refinement, molecular dynamics simulations, and evaluation of pKavalues. We then review a series of recent modeling efforts which refined the structure of bacteriorhodopsin adding internal water, and which studied the nature of the J intermediate and the likely geometry of the K590and L550intermediates (strongly distorted 13‐cis) as well as the sequence of retinal geometry and protein conformational transitions which are conventionally summarized as the M412intermediate. We also review simulations of the photocycle of light‐adapted bacteriorhodopsin at T=77 K and of the photocycle of dark‐adapted bacteriorhodopsin, both cycles differing from the conventional photocycle through a nonfunctional (pure 13‐cis) retinal geometry of the corresponding K590and L550states. The simulations demonstrate a potentially critical role of water and of minute reorientations of retinal's Schiff base nitrogen in controlling proton pumping in bR568; the simulations also indicate the existence of heterogeneous photocycles. The results exemplify the important role of molecular dynamics simulations in extending investigations on bacteriorhodopsin to a level of detail which is presently beyond experimental resolution, but which needs to be known to resolve the pump mechanism of bacteriorhodopsin. Finally, we outline the major existing challenges in the field of bacteriorhodopsin m

ISSN:0021-2148    

DOI:10.1002/ijch.199500042

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe first section of this paper is a detailed summary of studies made by us and others on metal cations binding to deionized bacteriorhodopsin (dIbR) and its variants. Our studies include the luminescence experiments of Eu3+binding to dIbR and potentiometric studies of Ca2+binding to dIbR, to deionized bR mutants, to bacterioopsin, and to dIbR with its C‐terminus removed. The results suggest the presence of two classes of binding sites, one class has two high‐affinity constants, and one has one low‐affinity constant. For Ca2+binding, there is one metal cation in each of the two high‐affinity sites which are coupled to the charged aspartates 85 and 212 (known to be in the retinal cavity) but not coupled to each other. The low‐affinity class can accommodate 0–6 Ca2+ions and most of them are bound to the surface. Mg2+has a slightly smaller value for its binding constant to the highest‐affinity site. Thus, one expects more Ca2+than Mg2+bound to the two high‐affinity sites. In the second section, we summarize our recent study on the effect of metal cation charge density (Ca2+, Mg2+, Eu3+, Tb3+, Ho3+, Dy3+) on the kinetics of both Schiff base deprotonation and proton transport to the extracellular surface. For all metal cations, the apparent rate constant of the slow components of the deprotonation process is the same as that for the transport process at 22 °C. The temperature studies, however, show this apparent equality to be fortuitous and to result from cancellation of the contribution of the energy and entropy of activation. Thus, while the entropy of activation is positive for the deprotonation process, it is negative for the proton transport process. These kinetic parameters depend weakly on the charge density, but in an opposite sense for the two processes. These results suggest that the deprotonation is not the rate‐limiting step for the proton transport process. A possible mechanism is proposed in which a hydrated metal cation is used to induce the deprotonation of the protonated Schiff base and to dissociate one of its H2O molecules to donate the proton in

ISSN:0021-2148    

DOI:10.1002/ijch.199500043

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA comprehensive review of the physiology, structure, and function of halorhodopsin (HR), the only known light‐driven anion pump, is presented. Beside the well‐studied transport function of HR in intact cells the article focuses on recent results about the molecular properties of HR. Overexpression and in vivo 2‐D crystallization allowed structural investigations at a level of 7 Å resolution. The results demonstrate a very close structural relationship with the proton pump bacteriorhododopsin (BR). The retinal binding site as well as a chloride binding site near the Schiff base in HR can be modeled with the side chains placed into the corresponding positions in the BR structural model. Mechanistically the vectorial catalytic cycle of HR is similar to that of BR, as suggested before (Oesterhelt et al.,J. Bioenerg. Biomembr.1992,24: 181), and consists of photoisomerization of the retinal moiety which triggers chloride movement within the transport site towards the Schiff base. This is followed by an accessibility change from the extracellular (EC) channel to the cytoplasmic (CP) channel allowing chloride to be released into the cytoplasm. After reisomerization and reversion of the accessibility change, a chloride is rebound into the transport site from the extracellular surface through EC. In the absence of transported ions or the additional presence of azide, photoisomerization to 13‐cisis followed directly by an accessibility change and release of a proton from the Schiff base through CP. Blue light causes photoisomerization back totransand the accessibility change is reversed. Uptake of a proton through EC completes proton transport from the outside to the inside. Depending on relative concentrations of chloride and azide, both modes of ion translocation operate in parallel and as alternatives during individual cycles of a molecule. Future experimentation will have to fill in the many details of this molecular model of ion t

ISSN:0021-2148    

DOI:10.1002/ijch.199500044

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe review covers recent progress on microbial sensory rhodopsins, visual pigment‐like retinylidene photoreceptors that function in phototaxis by archaeons, such asHalobacterium salinarium, and by unicellular eukaryotic algae, such asChlamydomonas reinhardtii.Six demonstrably different sensory rhodopsins are known in halophilic archaea. The best characterized is sensory rhodopsin I (SR‐I), a color‐sensitive receptor that relays attractant and repellent photosignals to a tightly bound transducer protein HtrI (halobacterial transducer for sensory rhodopsin I). New advances in the mechanism of signal transduction by the SR‐I/HtrI complex from molecular‐biological and biophysical approaches are summarized. Effects of HtrI on light‐induced proton transfers in the receptor are discussed for their possible role in signaling. Current knowledge concerning the growing family of related archaeal sensory rhodopsins is presented.The evidence for a sensory rhodopsin in phototaxis byC. reinhardtiiand other unicellular eukaryotic algae is reviewed. The molecular information is more limited than for the archaeal organisms, but the physiological information is rich and complex. Compelling data exist for a single retinal‐containing receptor mediating both phototaxis and photophobic responses inC. reinhardtii.From retinal analog studies, the isomeric configuration and ring/chain conformation of the retinal in the receptor appear to be identical to those of the archaeal sensory rhodopsins. Also, photoisomerization from all‐trans‐ to 13‐cis‐retinal appears to be the trigger for signaling, as in the archaeal pigments. Conflicting early studies suggesting an 11‐cis‐retinal chromophore and signaling without photoisomerization are analyzed and possible explanations for those reports are suggested.As a general conclusion, the microbial sensory rhodopsins provide an opportunity to explore photochemistry and protein/protein interaction in photosensory transduction in geneti

ISSN:0021-2148    

DOI:10.1002/ijch.199500045

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY