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Biomedical subjects

F Jähnig

Publications and source records attributed to F Jähnig.

14 recordsLinked to original sources
PubMed1992

Refolding and oriented insertion of a membrane protein into a lipid bilayer.

We have studied the refolding and membrane insertion of the outer membrane protein OmpA of Escherichia coli. The protein was extracted from its native membrane by sonication in the presence of urea and dissolved in the urea/water mixture in unfolded form. In this form it was purified. Upon addition of preformed lipid vesicles, the protein spontaneously refolded and inserted into the vesicle membranes. The vesicles had to be small and the lipids had to be in the fluid state. The insertion occurred in an oriented manner.

Bacterial Outer Membrane Proteins↗
PubMed1992

Modeling of the structure of bacteriorhodopsin. A molecular dynamics study.

Secondary structure predictions for membrane proteins are relatively reliable and permit the construction of model structures that may serve as initial conformations for molecular dynamics simulations. This might provide a scheme to predict the three-dimensional structures of membrane proteins. The feasibility of such an approach is tested for bacteriorhodopsin. We were not able to fully predict the kidney-shaped structure of bacteriorhodopsin. However, features compatible with this structure developed in a simulation starting from a circular arrangement of the seven predicted helices. When instead we started from the kidney shape, assigning the seven predicted helices in different ways to those on the structure, we could distinguish between the different assignments on the basis of energy and tilt of the helices. In this way we could select the correct assignment from a few others. For the correct assignment, the helices spontaneously adopted a tilt that agrees remarkably well with the experimental model structure derived by others. The root-mean-square deviation between our best molecular dynamics structure and the experimental model structure is 3.8 A, caused mainly by deviations in the internal degrees of freedom of the helices.

Amino Acid Sequence↗
PubMed1992

Conformational transition of an alpha-helix studied by molecular dynamics.

Molecular dynamics simulations were performed on a 20-residue polyalanine helix and a spontaneous transition from a kinked to a straight conformation was observed. The kinetics of the transition was analyzed within the framework of the Kramers model for chemical reactions and within a random walk model. The Kramers model which is based on diffusion along a one-dimensional reaction pathway and the crossing of an energy barrier was found to be inadequate. Instead, a random walk model based on diffusion in the high-dimensional phase space of the system was found to be compatible with the data. The high dimensionality of the phase space permits the system to circumvent high energy barriers and diffuse rapidly at about constant energy, but decelerates the reaction since in the labyrinth of pathways the transition state is reached rarely.

Biophysical Phenomena↗
PubMed1992

A synthetic analogue of melittin aggregates in large oligomers.

An analogue of melittin synthesized in the group of E. T. Kaiser (DeGrado, W. F., F. J. Keźdy, and E. T. Kaiser. 1981. J. Am. Chem. Soc. 103:679-681) was investigated by Raman spectroscopy and fluorescence anisotropy decay. In water, the analogue is completely alpha-helical and aggregates in large oligomers of about 50 monomers. In vesicle membranes, it undergoes orientational fluctuations similar to melittin. The most significant difference from melittin, therefore, is the formation of straight helixes and their aggregation in large oligomers in water. We interpret this as a consequence of the lacking proline residue in the analogue. We, furthermore, hypothesize that the increased tendency for aggregation causes the increased hemolytic activity of the analogue.

Amino Acid Sequence↗
PubMed1991

Biosensors based on membrane transport proteins.

We propose a novel class of biosensors based on membrane bound receptors or transport proteins as the sensing element. The protein is incorporated in a planar lipid bilayer which covers the transducer. The transducer may detect an electric current, a voltage, or a change in fluorescence. A prototype lactose sensor is presented which consists of a quartz slide covered by a lipid membrane containing the protein lactose permease from Escherichia coli. This protein is a lactose/H+ cotransporter, hence lactose in the external medium initiates lactose/H+ cotransport across the lipid membrane. This leads to a rise in proton concentration in the small volume between the lipid membrane and the quartz surface which can be detected by a pH-sensitive fluorescence dye.

Arylsulfonates↗
PubMed1991

Collective vibrations of an alpha-helix. A molecular dynamics study.

The internal dynamics of a 20-residue polyalanine helix was investigated by molecular dynamics simulations. Special attention was paid to the collective vibrations of the helix backbone. The stretch and bend vibrations could be assigned unambiguously to oscillations with periods of 1.4 and 4.3 ps, respectively. The influence of the environment on the dynamics of the collective vibrations was studied by coupling the helix to a heat bath and by adding water molecules. In the presence of water, the stretch vibration becomes more strongly dampened, but still exists as a vibration , while the bend vibration becomes overdamped and degenerates into a relaxation process. The results are compared with available experimental data.

Elasticity↗
PubMed1991

The opacity proteins of Neisseria gonorrhoeae strain MS11 are encoded by a family of 11 complete genes.

Variants of Neisseria gonorrhoeae MS11 show distinct colony morphologies because of the expression of a class of surface components called opacity (Opa, PII) proteins. Southern analyses combined with molecular cloning of genomic DNA from a single variant of MS11 has identified 11 opa genes contained in separate loci. These opa genes code for distinct opacity proteins which are distinguishable at their variable domains. The opa gene analyses were also extended to divergent variants of MS11. These studies have shown that, during in vitro and in vivo culture, 10 of the 11 opa genes did not undergo significant change in their primary sequence. However, in these variants, one gene (opaE) underwent non-reciprocal inter-opa recombinations to generate newer Opa variants. Phylogenic analysis of the opa gene sequences suggests that the opa gene family have evolved by a combination of gene duplication, gene replacement and partial inter-opa recombination events.

Amino Acid Sequence↗
PubMed1990

Structure predictions of membrane proteins are not that bad.

A simple generalization of the well-known hydrophobicity analysis is sufficient to predict membrane-spanning amphiphilic alpha-helices and beta-strands. The use of this method is illustrated for bacteriorhodopsin, OmpA protein and the photosynthetic reaction centre.

Amino Acid Sequence↗
PubMed1979

Electrostatic interactions at charged lipid membranes. Electrostatically induced tilt.

The changes in bilayer structure induced by surface charges in the case of an ionizable lipid were studied by X-ray diffraction, Raman spectroscopy, and film-balance measurements. With increasing surface charge in the ordered phase, the X-ray results show a decrease in bilayer thickness, whereas the hydrocarbon chain packing stays essentially constant, the Raman data signify that the internal chain ordering does not change, and the monolayer studies show a lateral expansion of the bilayer. These results are interpreted in terms of a tilt of the chains caused by the surface charges on the polar heads. The tilt angle between the direction of the chains and the bilayer normal is obtained by a detailed theoretical evaluation. The tilt allows for a better understanding of the electrostatically induced shift of the phase transition temperature and of the shift induced by the binding of water in the case of lecithin in contrast ethanolamine.

Hydrogen-Ion Concentration↗
PubMed1979

Structural order of lipids and proteins in membranes: evaluation of fluorescence anisotropy data.

The limiting long-time value of fluorescence anisotropy in membranes is correlated with the orientational order parameter, which characterizes the structural anisotropy of membranes. Existing experimental results for diphenylhexatriene in lipid bilayers are evaluated for the order parameter of lipid order. Steady-state measurements of fluorescence anisotropy can provide the order parameter in good approximation. Proteins in a fluid lipid phase increase the lipid order parameter so determined. Upon comparison with the order parameter from deuterium magnetic resonance, it is concluded that proteins increase the order of the surrounding lipids in off-normal directions. Order parameters of protein order obtained from the limiting value of protein fluorescence anisotropy are discussed with respect to the influence of lipid order on protein order.

Fluorescence Polarization↗
PubMed1978

Electrostatic interactions at charged lipid membranes. Measurement of surface pH with fluorescent lipoid pH indicators.

The 5-dimethylaminonapthalene-1-sulfonyl (dansyl) chromophore attached to the polar head groups of lipids has been used as a fluorescent lipoid pH indicator to evaluate the interfacial pH in lipid-water lamellar systems prepared from negatively charged lipids. The pH in the vicinity of the charged lipid bilayers is different from the pH of the bulk aqueous phase and the difference is a function of the electrolyte concentration in the aqueous phase and of the lipid packing in the bilayer. At a fixed electrolyte concentration in the aqueous phase, the observed interfacial pH is 0.6 to 0.7 pH units lower above the thermal phase transition of the lipid than it is below this temperature. A quantitative interpretation of the results is given on the basis of the Gouy-Chapman theory. The results indicate that the dansyl chromophore is located in front of the charged surface and its distance from this surface increases with a decrease in lipid packing.

Chemical Phenomena↗
PubMed1976

Electrostatic free energy and shift of the phase transition for charged lipid membranes.

For a charged membrane in an electrolyte solution the electrostatic free energy is derived treating the system as a diffuse double layer. The dependence of the energy on external parameters like surface charge density and temperature is obtained and the physical basis discussed. As an application the charges are shown to exert an electrostatic surface pressure on the lipid chain packing which leads to a shift in the phase transition of membranes. The results confirm the interpretation of experimental data as given by Träuble et al. in the accompanying paper.

Lipids↗