Lysines in the amino-terminal alpha-helix are important to the stability of Rhodobacter capsulatus cytochrome c2

The protein stabilities of wild type and four site-directed mutants of Rhodobacter capsulatus cytochrome c2 have been characterized. The integrity of the cytochrome c2 iron-sulfur environment was ascertained by titration of the 696-nm absorbance band with alkali, and the conformational stability was determined by titration of the 220-nm circular dichroism signal with Gdn-HCl. Analysis of the alkaline transition pK value of K12D (lysine-12 substituted by aspartate) indicated that the K12D iron-sulfur environment was destabilized by 0.6 kcal/mol relative to the wild-type cytochrome c2 at low ionic strength. In contrast, the alkaline transition pK values of K14E (lysine-14 substituted by glutamate), K32E (lysine-32 substituted by glutamate), and K14E/K32E (lysines-14 and -32 substituted by glutamates) were indistinguishable from the wild type, indicating that these substitutions have no effect on the stability of the iron-sulfur environment. Gdn-HCl denaturation of K12D and K14E indicated that both these mutations decreased conformational stability by 1.3 kcal/mol. In contrast, mutant K32E exhibited a small stabilizing effect of 0.2 kcal/mol. Gdn-HCl denaturation of K14E/K32E indicated that this mutation decreased conformational stability by 1.3 kcal/mol, which is consistent with the additive effects of the single charge mutations at positions 14 and 32. The conformational instability of mutants possessing negative charges at position 12 or 14 is best explained by their positioning at the carboxy-terminal region of the amino-terminal alpha-helix of R. capsulatus cytochrome c2. Accordingly, introduction of negatively charged groups into this region appears to destabilize cytochrome c2 through energetically unfavorable interactions with the dipole of the amino-terminal helix.(ABSTRACT TRUNCATED AT 250 WORDS)

Stark effect spectroscopy of carotenoids in photosynthetic antenna and reaction center complexes

The effects of electric fields on the absorption spectra of the carotenoids spheroidene and spheroidenone in photosynthetic antenna and reaction center complexes (wild-type and several mutants) from purple non-sulfur bacteria are compared with those for the isolated pigments in organic glasses. In general, the field effects are substantially larger for the carotenoid in the protein complexes than for the extracted pigments and larger for spheroidenone than spheroidene. Furthermore, the electrochromic effects for carotenoids in all complexes are much larger than those for the Qx transitions of the bacteriochlorophyll and bacteriopheophytin pigments which absorb in the 450-700 nm spectral region. The underlying mechanism responsible for the Stark effect spectra in the complexes is found to be dominated by a change in permanent dipole moment of the carotenoid upon excitation. The magnitude of this dipole moment change is found to be considerably larger in the B800-850 complex compared to the reaction center for spheroidene; it is approximately equivalent in the two complexes for spheroidenone. These results are discussed in terms of the effects of differences in the carotenoid functional groups, isomers and perturbations on the electronic structure from interactions with the organized environment in the proteins. these data provide a quantitative basis for the analysis of carotenoid bandshifts which are used to measure transmembrane potential, and they highlight some of the pitfalls in making such measurements on complex membranes containing multiple populations of carotenoids. The results for spheroidenone should be useful for studies of mutant proteins, since mutant strains are often grown semi-aerobically to minimize reversion.

Fluorescence polarization and low-temperature absorption spectroscopy of a subunit form of light-harvesting complex I from purple photosynthetic bacteria

Measurements of polarized fluorescence and CD were made on light-harvesting complex 1 and a subunit form of this complex from Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodobacter capsulatus. The subunit form of LH1, characterized by a near-infrared absorbance band at approximately 820 nm, was obtained by titration of carotenoid-depleted LH1 complexes with the detergent n-octyl beta-D-glucopyranoside as reported by Miller et al. (1987) [Miller J. F., Hinchigeri, S. B., Parkes-Loach, P. S., Callahan, P. M., Sprinkle, J. R., & Loach, P. A. (1987) Biochemistry 26, 5055-5062]. Fluorescence polarization and CD measurements at 77 K suggest that this subunit form must consist of an interacting bacteriochlorophyll a dimer in all three bacterial species. A small, local decrease in the polarization of the fluorescence is observed upon excitation at the blue side of the absorption band of the B820 subunit. This decrease is ascribed to the presence of a high-energy exciton component, perpendicular to the main low-energy exciton component. From the extent of the depolarization, we estimate the oscillator strength of the high-energy component to be at most 3% of the main absorption band. The optical properties of B820 are best explained by a Bchl a dimer that has a parallel or antiparallel configuration with an angle between the Qy transition dipoles not larger than 33 degrees. The importance of this structure is emphasized by the results showing that core antennas from three different purple bacteria have a similar structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a coproporphyrin-protein complex from Rhodobacter capsulatus

Rhodobacter capsulatus strain AJB530 excretes large amounts of coproporphyrin into the culture supernatant. The coproporphyrin was precipitable with ammonium sulfate, suggesting that it was part of a macromolecular complex. Analysis of an ammonium sulfate fraction indicated that the coproporphyrin was bound to a 66-kDa protein with a pI' of 4.0. The same protein was found in the culture supernatant of the bch+ strain PAS100.

The structure of porin from Rhodobacter capsulatus at 1.8 A resolution

The structure of the porin from Rhodobacter capsulatus was determined at a resolution of 1.8 A. The analysis started from a closely related crystal structure that had been solved at a medium resolution of 3 A using multiple isomorphous replacement and solvent flattening. The new structure contains the complete sequence of 301 amino acid residues. Refinement of the model is under way; the present R-factor is 22% with good geometry. Except for the lengths of several loops, the resulting chain fold corresponds to the medium resolution model. The membrane channel is lined by a large number of ionogenic side chains with characteristic segregation of differently charged groups.

The effects of surface charges on the redox potential of cytochrome c2 from the purple phototrophic bacterium Rhodobacter capsulatus

Four site-directed mutants of Rhodobacter capsulatus cytochrome c2, which substitute lysines at three positions with aspartate or glutamate, have been prepared. Mutations included the single charge substitutions K12D, K14E, and K32E and a double charge substitution K14E/K32E. Characterization of the ionic strength dependence of the wild-type and mutant redox potentials in the "nonbinding" buffer Tris-cacodylate suggests that (i) at zero ionic strength introduction of negatively charged groups stabilizes the oxidized state by 11-14 mV per charge and (ii) at high ionic strengths where the charged groups are masked, the effects of single charge substitutions are overcome; however, the redox potential of the double charge substitution is still affected. These results indicate that at physiological ionic strengths charge distribution only affects redox potential when the heme environment has been perturbed by a structural perturbation and that the determinants of redox potential in c-type cytochromes is primarily due to the local heme environment.

Inactivation of T5 phage by cis-vaccenic acid, an antivirus substance from Rhodopseudomonas capsulata, and by unsaturated fatty acids and related alcohols

The antiviral extract from Rhodopseudomonas capsulata was purified and the predominant active component was defined as cis-vaccenic acid (Cl-8:1 delta 11) by gas-liquid chromatography and gas chromatography-mass spectrometry analyses. Antiviral activities of unsaturated fatty acids and related alcohols against T5 phage were also tested. Among them, linoelaidic acid, oleic acid, and petroselenyl alcohol inactivated 98%, 53%, 67% of T5 phage at the concentration of 50 micrograms/ml, respectively.

A negatively charged N terminus in the alpha polypeptide inhibits formation of light-harvesting complex I in Rhodobacter capsulatus

Light-harvesting complex I (LHI) of Rhodobacter capsulatus contains bacteriochlorophyll and carotenoids which are noncovalently bound to two different apoproteins (alpha and beta polypeptides) carrying oppositely charged N-terminal ends. The contribution of these charged segments to the assembly of LHI was studied with mutants having oppositely charged amino acids in the alpha or beta polypeptide. The influence of these mutations on the insertion and assembly process of the LHI complex was investigated by means of spectroscopic analysis of isolated intracytoplasmic membranes and pulse-chase experiments. Exchange of four positively charged amino acids to negatively charged amino acids on the N-terminal domain of the alpha subunit inhibited completely the assembly of the LHI complex. Although this mutant has no antenna, the reaction center is active and the cells were able to grow anaerobically in the light. Conversely, mutation of the four negatively charged amino acids of the N-terminal segment of the beta polypeptide did not prevent the assembly of the LHI complex, although the stability of the complex and the size of the photosynthetic unit were affected. The presence of the mutated beta polypeptide was confirmed by protein sequencing.

Cytokine induction by lipopolysaccharide (LPS) corresponds to lethal toxicity and is inhibited by nontoxic Rhodobacter capsulatus LPS

Many pathological effects of gram-negative bacteria are produced by their cell wall-derived lipopolysaccharides (LPSs). Differing pathogenicity of gram-negative LPSs, however, may depend on their capacities to induce cytokines. Thus, we studied the lethal toxicity of four nonenterobacterial LPSs and compared it with their capacity to induce mononuclear cell (MNC)-derived interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF). Unstimulated MNC did not release these cytokines. LPS from the phototrophic strain Rhodobacter capsulatus 37b4 elaborated little toxicity in galactosamine-treated mice (10 micrograms of LPS per mouse was the 100% lethal dose [LD100]) and induced IL-1 and IL-6 release only at high concentrations (10 to 50 micrograms of LPS per ml). R. capsulatus LPS failed to induce TNF activity even at the highest concentration tested (100 micrograms of LPS per ml). In contrast, LPS derived from Pseudomonas diminuta NCTC 8545 or the nodulating species Bradyrhizobium lupini DSM 30140 and Rhizobium meliloti 10406 expressed lethal toxicity (LD100, 1,000, 100, and 10 ng per mouse, respectively) and induced IL-1 or IL-6 (10 to 100, 10, and 1 ng of LPS per ml, respectively) at concentrations 1,000- to 10,000-fold lower than effective levels of R. capsulatus LPS. LPSs from P. diminuta, B. lupini, and R. meliloti also stimulated TNF production and release. MNC accumulated cell-associated IL-1 activities under circumstances in which released activity was readily detected. The cells contained only scant IL-6 activity, indicating release of this mediator rather than intracellular accumulation. Antisera to the respective cytokines inactivated biological activities of the samples selectively. The R. capsulatus LPS inhibited cytokine induction by LPS from P. diminuta, B. lupini, and R. meliloti in coincubation experiments. These results show that the in vivo lethality of the LPSs tested correlates with the induction of monocyte-derived cytokines in vitro. The results of this study suggest that the different lethality of various LPSs from gram-negative bacteria may be due to the differential ability of these LPSs to induce cytokine production.

Resonance Raman studies of genetically modified reaction centers from Rhodobacter capsulatus

Resonance Raman (RR) spectra are reported for the photosynthetic reaction center (RC) proteins from Rhodobacter capsulatus wild type and the genetically modified systems GluL104----Leu and HisM200----Leu. The spectra were obtained with a variety of excitation wavelengths, spanning the UV, violet, and yellow-green regions of the absorption spectrum, and at temperatures of 30 and 200 K. The RR data indicate that the structures of the bacteriochlorin pigments in RCs from Rb. capsulatus wild type are similar to those in RCs from Rhodobacter sphaeroides wild type. The data also show that the amino acid modifications near the primary electron acceptor (GluL104----Leu) and special pair (HisM200----Leu) perturb only those bacteriochlorin pigments near the site of the mutation and do not influence the structures of the other pigments in the RC. In the case of the GluL104----Leu mutant, elimination of the hydrogen bond to the C9 keto group of BPhL results in frequency shifts of RR bands of certain skeletal modes of the macrocycle. This allows the assignment of bands to the individual BPhL and BPhM pigments. In the case of the HisM200----Leu mutant, in which the special pair is comprised of a bacteriochlorophyll (BChl)-bacteriopheophytin (BPh) heterodimer rather than the BChl2 unit bound in the wild type, certain skeletal vibrations due to the additional BPh unit are identified. The frequencies of these modes are similar to those of the analogous vibrations BPhL and BPhM, which indicates that the structure of the BPh in the heterodimer is not unusual in any discernible way.(ABSTRACT TRUNCATED AT 250 WORDS)