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M Schiffer

Publications and source records attributed to M Schiffer.

At least 73 records · Page 4Linked to original sources
PubMed1992

In bacterial reaction centers protons can diffuse to the secondary quinone by alternative pathways.

The mechanisms of proton conduction to the reduced secondary quinone in bacterial reaction centers were studied in wild-type and genetically modified reaction centers from Rhodobacter capsulatus. In the L212-213AA double mutant (L212Glu----Ala, L213Asp----Ala), reaction center function is severely altered. However, a photocompetent revertant of this strain which carries a third 'compensating' mutation, M231Arg----Leu, at about 15 A from the secondary quinone, displays the normal proton binding function of the reaction center. Furthermore, the apparent pK values of group(s) involved in the stabilization of the semiquinone anion are restored by that mutation. We conclude that L212Glu and L213Asp are not obligatory residues for proton donation to QB in Rb. capsulatus. We suggest that protons can be delivered to the QB site from the cytoplasm via a network of proton channels activated by compensatory mutations, possibly involving water molecules bound in the interior of the reaction center.

Binding Sites↗
PubMed1992

Femtosecond spontaneous-emission studies of reaction centers from photosynthetic bacteria.

Spontaneous emission from reaction centers of photosynthetic bacteria has been recorded with a time resolution of 50 fs. Excitation was made directly into both the special-pair band (850 nm) and the Qx band of bacteriochlorophylls (608 nm). Rhodobacter sphaeroides R26, Rhodobacter capsulatus wild type, and four mutants of Rb. capsulatus were studied. In all cases the fluorescence decay was not single exponential and was well fit as a sum of two exponential decay components. The short components are in excellent agreement with the single component detected by measurements of stimulated emission. The origin of the nonexponential decay is discussed in terms of heterogeneity, the kinetic scheme, and the possibility of slow vibrational relaxation.

Energy Transfer↗
PubMed1992

Subgroups of Tcr alpha chains and correlation with T-cell function.

T-cell receptor (Tcr) alpha chains are classified into four subgroups (I, II, III, and miscellaneous) based on the amino acid residues at positions 61 and 62. Subgroup I has Gly Phe at these positions, subgroup II has Arg Phe, subgroup III has Arg Leu, and subgroup miscellaneous has several other combinations. Variability plots for subgroups I, II, and III sequences show higher values around positions 93-103, 105, 108, 111, 113, and 115, suggesting that these positions may interact with the processed antigen molecules. Smaller peaks are present at various other regions which may bind the major histocompatibility complex class I or II molecules. The patterns of variability within one subgroup are similar for all species, for human alone, and for mouse alone. These subgroup patterns appear much less complicated than patterns for sequences in all subgroups taken together, implying that subgroups may be related to Tcr functions. Among 83 mouse chains, 15 are from cytotoxic cells and 40 from helper cells. Of the 15 from cytotoxic cells, 11, 2, 0, and 2 are in subgroups I, II, III, and miscellaneous; and of the 40 from helper cells, 9, 16, 12, and 3 are in subgroups I, II, III, and miscellaneous, respectively. Thus, a correlation between sequence and function of Tcr alpha chains seems possible.

Amino Acid Sequence↗
PubMed1992

The functions of tryptophan residues in membrane proteins.

Membrane proteins have a significantly higher Trp content than do soluble proteins. This is especially true for the M and L subunits of the photosynthetic reaction center from purple bacteria. The Trp residues are not uniformly distributed through the membrane but are concentrated at the periplasmic side of the complex. In addition, Trp residues are not randomly aligned. Within the protein subunits, many form hydrogen bonds with carbonyl oxygens of the main chain, thereby stabilizing the protein. On the surface of the molecule, they are correctly positioned to form hydrogen bonds with the lipid head groups while their hydrophobic rings are immersed in the lipid part of the bilayer. These observations suggest that Trp residues are involved in the translocation of protein through the membrane and that following translocation, Trp residues serve as anchors on the periplasmic side of the membrane.

Hydrogen Bonding↗
PubMed1992

Legal aspects of telepathology.

The legal aspects of telecommunication are discussed briefly. Telepathology induces no new legal conditions; however, due to its somewhat higher diagnostic error rate specific problems may occur. Both the pathologist and hospital should clarify the internal liability conditions. The pathologist should be aware that he is still liable for errors induced by the technician, even without having the possibility of responsibility or any supervision.

Communication↗
PubMed1991

Structure of the membrane-bound protein photosynthetic reaction center from Rhodobacter sphaeroides.

The structure of the photosynthetic reaction center (RC) from Rhodobacter sphaeroides was determined at 3.1-A resolution by the molecular replacement method, using the Rhodopseudomonas viridis RC as the search structure. Atomic coordinates were refined with the difference Fourier method and restrained least-squares refinement techniques to a current R factor of 22%. The tertiary structure of the RC complex is stabilized by hydrophobic interactions between the L and M chains, by interactions of the pigments with each other and with the L and M chains, by residues from the L and M chains that coordinate to the Fe2+, by salt bridges that are formed between the L and M chains and the H chain, and possibly by electrostatic forces between the ends of helices. The conserved residues at the N-termini of the L and M chains were identified as recognition sites for the H chain.

Amino Acids↗
PubMed1991

Comparison of reaction centers from Rhodobacter sphaeroides and Rhodopseudomonas viridis: overall architecture and protein-pigment interactions.

Photosynthetic reaction centers (RCs) from the photosynthetic bacteria Rhodobacter sphaeroides and Rhodopseudomonas viridis are protein complexes closely related in both structure and function. The structure of the Rps. viridis RC was used to determine the structure of the RC from Rb. sphaeroides. Small but meaningful differences between the positions of the helices and the cofactors in the two complexes were identified. The distances between helices AL and AM, between BL and BM, and between bacteriopheophytins BPL and BPM are significantly shorter in Rps. viridis than they are in Rb. sphaeroides RCs. There are a number of differences in the amino acid residues that surround the cofactors; some of these residues form hydrogen bonds with the cofactors. Differences in chemical properties and location of these residues account in some manner for the different spectral properties of the two RCs. In several instances, the hydrogen bonds, as well as the apparent distances between the histidine ligands and the Mg atoms of the bacteriochlorophylls, were found to significantly differ from the Rb. sphaeroides RC structure previously described by Yeates et al. [(1988) Proc. Natl. Acad. Sci. U.S.A. 85, 7993-7997] and Allen et al. [(1988) Proc. Natl. Acad. Sci. U.S.A. 85, 8487-8491].

Amino Acids↗
PubMed1990

Testing the procedure of simulated annealing by refining homologous immunoglobulin light-chain dimers.

The refinement of antigen-binding fragment structures by the method of simulated annealing was tested. Using the program X-PLOR, we refined the structure of one immunoglobulin light-chain dimer against 2.8 A diffraction data collected for a homologous light-chain dimer. The refinement proceeded smoothly; alpha-carbons of the conserved segments of the domain moved to the positions in the reference structure solved independently. An average movement of approximately 1.5 A for atoms in the variable domains (half of the molecule) was observed. Though the final R-factors and energy terms of the reference and test structures were very similar, some of the chain segments of the hypervariable loops (HVR3s) and the ends of some side chains did not converge to the positions in the reference structure. Therefore, although globally the refinement worked very well, positions of the loops and the side chains that are critical for immunoglobulin function have to be carefully examined by difference Fourier techniques.

Computer Simulation↗
PubMed1989

Structure of spheroidene in the photosynthetic reaction center from Y Rhodobacter sphaeroides.

The structure of the reaction center of Y Rhodobacter sphaeroides has been solved at 3 A resolution, using the atomic coordinates of the reaction center from the carotenoidless mutant R26 Rhodobacter sphaeroides. The structure has been refined by a stimulated annealing with the computer program X-PLOR, leading to a crystallographic R factor of 0.22 using reflections between 8 and 3 A. The spheroidene molecule which is bound to the Y reaction center has been fitted in the electron density map as a 15-cis isomer with a highly asymmetric structure. The cis-bond is located at proximity from ring 1 of the accessory bacteriochlorophyll on the inactive M side. The nature of the cis-bond was confirmed by resonance Raman spectra obtained from Y reaction center crystals. The structure of spheroidene in Y reaction center is compared to that proposed for 1,2-dihydroneurosporene in Rhodopseudomonas viridis reaction center crystals.

Bacterial Proteins↗
PubMed1989

Structure of a second crystal form of Bence-Jones protein Loc: strikingly different domain associations in two crystal forms of a single protein.

We have determined the structure of the immunoglobulin light-chain dimer Loc in a second crystal form that was grown from distilled water. The crystal structure was determined to 2.8-A resolution; the R factor is 0.22. The two variable domains are related by local 2-fold axes and form an antigen binding "pocket". The variable domain-variable domain interaction observed in this crystal form differs from the one exhibited by the protein when crystallized from ammonium sulfate in which the two variable domains formed a protrusion (Chang et al., 1985). The structure attained in the distilled water crystals is similar to, but not identical with, the one observed for the Mcg light-chain dimer in crystals grown from ammonium sulfate. Thus, two strikingly different structures were attained by this multisubunit protein in crystals grown under two different, commonly used, crystallization techniques. The quaternary interactions exhibited by the protein in the two crystal forms are sufficiently different to suggest fundamentally different interpretations of the structural basis for the function of this protein. This observation may have general implications regarding the use of single crystallographic determinations for detailed identification of structural and functional relationships. On the other hand, proteins whose structures can be altered by manipulation of crystallization conditions may provide useful systems for study of fundamental structural chemistry.

Bence Jones Protein↗
PubMed1989

Correlation of paramagnetic states and molecular structure in bacterial photosynthetic reaction centers: the symmetry of the primary electron donor in Rhodopseudomonas viridis and Rhodobacter sphaeroides R-26.

The orientation of the principal axes of the primary electron donor triplet state measured in single crystals of photosynthetic reaction centers is compared to the x-ray structures of the bacteria Rhodobacter (Rb.) sphaeroides R-26 and Rhodopseudomonas (Rps.) viridis. The primary donor of Rps. viridis is significantly different from that of Rb. sphaeroides. The measured directions of the axes indicate that triplet excitation is almost completely localized on the L-subunit half of the dimer in Rps. viridis but is more symmetrically distributed (approximately 63% on the L half of the special pair and approximately 37% on the M half) on the dimeric donor in Rb. sphaeroides R-26. The large reduction of the zero field splitting parameters relative to monomeric bacteriochlorophyll triplet in vitro suggests significant participation of asymmetrical charge transfer electronic configurations in the special pair triplet state of both organisms (approximately 23% in Rps. viridis and approximately 13% in Rb. sphaeroides).

Bacterial Proteins↗
PubMed1988

Analysis of immunoglobulin domain interactions. Evidence for a dominant role of salt bridges.

Available crystallographic data for homologous immunoglobulin constant domains were correlated with measured association constants for these domains. High correlation was found between the association constant and both the buried surface area (number of interdomain contacts) and the number of salt bridges formed in the interaction, whereas no correlation with the number of hydrogen bonds between domains was evident. The total free energy of binding, as determined from the association constant, was related to the number of contacts, hydrogen bonds and salt bridges found in the domain:domain interface by the crystallographic studies. These calculations yielded reasonable average energy terms for each interaction category.

Binding Sites↗