Temperature and solvent effects on reaction centers from Chloroflexus aurantiacus and Chromatium tepidum

Calcium ions are required for the enhanced thermal stability of the light-harvesting-reaction center core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum

Thermochromatium tepidum is a thermophilic purple sulfur photosynthetic bacterium collected from the Mammoth Hot Springs, Yellowstone National Park. A previous study showed that the light-harvesting-reaction center core complex (LH1-RC) purified from this bacterium is highly stable at room temperature (Suzuki, H., Hirano, Y., Kimura, Y., Takaichi, S., Kobayashi, M., Miki, K., and Wang, Z.-Y. (2007) Biochim. Biophys. Acta 1767, 1057-1063). In this work, we demonstrate that thermal stability of the Tch. tepidum LH1-RC is much higher than that of its mesophilic counterparts, and the enhanced thermal stability requires Ca2+ as a cofactor. Removal of the Ca2+ from Tch. tepidum LH1-RC resulted in a complex with the same degree of thermal stability as that of the LH1-RCs purified from mesophilic bacteria. The enhanced thermal stability can be restored by addition of Ca2+ to the Ca2+-depleted LH1-RC, and this process is fully reversible. Interchange of the thermal stability between the two forms is accompanied by a shift of the LH1 Qy transition between 915 nm for the native and 880 nm for the Ca2+-depleted LH1-RC. Differential scanning calorimetry measurements reveal that degradation temperature of the native LH1-RC is 15 degrees C higher and the enthalpy change is about 28% larger than the Ca2+-depleted LH1-RC. Substitution of the Ca2+ with other metal cations caused a decrease in thermal stability of an extent depending on the properties of the cations. These results indicate that Ca2+ ions play a dual role in stabilizing the structure of the pigment-membrane protein complex and in altering its spectroscopic properties, and hence provide insight into the adaptive strategy of this photosynthetic organism to survive in extreme environments using natural resources.

Purification, characterization and crystallization of the core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum

A light-harvesting-reaction center (LH1-RC) core complex has been highly purified from a thermophilic purple sulfur bacterium, Thermochromatium tepidum. The bacteriochlorophyll (BChl) a molecules in the LH1 exhibit a Q(y) transition at 914 nm, more than 25 nm red-shift from those of its mesophilic counterparts. The LH1-RC complex was isolated in a monomeric form as confirmed by sucrose density gradient centrifugation, blue native PAGE and size-exclusion chromatography. Four subunits (L, M, H and a tetraheme cytochrome) in RC and two polypeptides (alpha and beta) in LH1 were identified. Spirilloxanthin was determined to be the predominant carotenoid in the core complex. The purified core complex was highly stable, no significant change in the LH1 Q(y) transition was observed over 10 days of incubation at room temperature in dark. Circular dichroism spectrum of the LH1 complex was characterized by low intensity and nonconservative spectral shape, implying a high symmetry of the large LH1 ring and interaction between the BChl a and carotenoid molecules. A dimeric feature of the BChl a molecules in LH1 was revealed by magnetic circular dichroism spectrum. Crystals of the core complex were obtained which diffracted X-rays to about 10 A.

Kinetic analysis of the thermal stability of the photosynthetic reaction center from Rhodobacter sphaeroides

The temperature-induced denaturation of the photosynthetic reaction center from Rhodobacter sphaeroides has been studied through the changes that occur in the absorption spectrum of the bound chromophores on heating. At elevated temperatures, the characteristic absorbance bands of the bacteriochlorins bound to the polypeptides within the reaction center are lost, and are replaced by features typical of unbound bacteriochlorophyll and bacteriopheophytin. The kinetics of the spectral changes cannot be explained by a direct conversion from the functional to the denatured form of the protein, and require the presence of at least one intermediate. Possible mechanisms for the transformation via an intermediate are examined using a global analysis of the kinetic data, and the most likely mechanism is shown to involve a reversible transformation between the native state and an off-pathway intermediate, coupled to an irreversible transformation to the denatured state. The activation energies for the transformations between the three components are calculated from the effect of temperature on the individual rate constants, and the likely structural changes of the protein during the temperature-induced transformation are discussed.

On the role of basic residues in adapting the reaction centre-LH1 complex for growth at elevated temperatures in purple bacteria

The purple photosynthetic bacterium Thermochromatium tepidum is a moderate thermophile, with a growth optimum of 48-50 degrees C. The X-ray crystal structure of the reaction centre from this organism has been determined, and compared with that from mesophilic bacteria such as Blastochloris viridis and Rhodobacter sphaeroides (Nogi T et al. (2000) Proc Natl Acad Sci USA 97: 13561-13566). Structural features that could contribute to the enhanced thermal stability of the Thermochromatium tepidum reaction centre were discussed, including three arginine residues exposed at the periplasmic side of the membrane that are not present in reaction centres from mesophilic organisms, and potentially could increase the affinity of the complex for the surrounding membrane. In the present report these arginine residues, plus a histidine identified from an extensive sequence alignment, were engineered into structurally homologous positions in the Rhodobacter sphaeroides reaction centre, and the effect on the thermal stability of the Rhodobacter sphaeroides complex was examined. We find that these residues do not enhance the thermal stability of the reaction centre, as assessed by absorbance spectroscopy of the bacteriochlorin cofactors in membrane-bound reaction centres. Possible roles of these residues in the Thermochromatium tepidum reaction centre are discussed, and it is proposed that they facilitate stronger binding of the reaction centre to the encircling LH1 antenna complex, through ionic interactions with acidic residues at the C-terminal end of the LH1 alpha-polypeptide. Such an interaction could enhance the stability of the so-called 'RC-LH1 core' complex that is formed between the reaction centre and the LH1 antenna, and which represents the minimal functional photosynthetic unit in all known purple photosynthetic bacteria. Stronger bonding interactions between the two complexes could also contribute to an increase in the rigidity of the photosynthetic membrane in Thermochromatium tepidum, in accord with the general finding that the cytoplasmic membrane from thermophilic eubacteria is less fluid than its counterpart in mesophilic bacteria.

Thermodynamics of Light-induced and Thermal Degradation of Bacteriochlorins in Reaction Center Protein of Photosynthetic Bacteria

The rate constants of thermal (irreversible) damage of bacteriochlorin pigments (bacteriochlorophyll monomer [B], bacteriochlorophyll dimer [P] and bacteriopheophytine [H]) in reaction center [RC] protein from the photosynthetic bacterium Rhodobacter sphaeroides were studied in the dark and during intense (400 mW x cm(-2)) laser light excitation (wavelengths 488 and 515 nm) under deoxygenated conditions. While the kinetics of degradation of P and B were monoexponential, the decay kinetics of H were overlapped by an initial lag phase at elevated (>40 degrees C) temperature. This is explained by removal of the central metal ion from the bacteriochlorophylls as part of their degradation processes. At all temperatures, the rates of damage were very similar for all bacteriochlorin pigments and were larger in the light than in the dark. The logarithm of the rate constant of pigment degradation and loss of photochemistry as a function of reciprocal (absolute) temperature (Arrhenius/Eyring plot) showed single phase in the light and double phases in the dark. Below 20 degrees C, the rate of pigment degradation in the RC decreased so dramatically in the dark that it became limited by the natural degradation process of bacteriochlorophyll measured in solution. The function of loss of photochemistry in the dark was also biphasic and had a break point at 40 degrees C. The damage in the dark required high enthalpy change (DeltaH(++) = 64 kcal/mol for P and DeltaH(++) = 60 kcal/mol for B) and entropy increase (T x DeltaS(++) = 38 kcal/mol for P and T x DeltaS(++) = 34 kcal/mol for B at T = 300 K), whereas significantly smaller enthalpy change (DeltaH(++) = 21 kcal/mol for P and B and DeltaH(++) = 13 kcal/mol for H) and practically no (T x DeltaS(++) = -1 kcal/mol for P and B at T = 300 K) or small (T x DeltaS(++) = -9 kcal/mol for H at T = 300 K) entropy change was needed in the light. The thermodynamic parameters of activation reveal major steps common in the degradation of all bacteriochlorin pigments: ring opening reactions at C5 or C20 meso-bridges (or both) and breaking/removal of the phytyl chain. Their contribution in the degradation is probably reflected in the observed enthalpy/entropy compensation at an almost constant (DeltaG(++) = 22-26 kcal/mol at T = 300 K) free energy change of activation.

Protein-lipid interactions in the purple bacterial reaction centre

The purple bacterial reaction centre uses the energy of sunlight to power energy-requiring reactions such as the synthesis of ATP. During the last 20 years, a combination of X-ray crystallography, spectroscopy and mutagenesis has provided a detailed insight into the mechanism of light energy transduction in the bacterial reaction centre. In recent years, structural techniques including X-ray crystallography and neutron scattering have also been used to examine the environment of the reaction centre. This mini-review focuses on recent studies of the surface of the reaction centre, and briefly discusses the importance of the specific protein-lipid interactions that have been resolved for integral membrane proteins.

Structure of the H subunit of the photosynthetic reaction center from the thermophilic purple sulfur bacterium, Thermochromatium tepidum Implications for the specific binding of the lipid molecule to the membrane protein complex

The photosynthetic reaction center (RC) is a transmembrane protein complex that catalyzes light-driven electron transport across the photosynthetic membrane. The complete amino-acid sequence of the H subunit of the RC from a thermophilic purple sulfur bacterium, Thermochromatium tepidum, has been determined for the first time among purple sulfur bacteria. The H subunit consists of 259 amino acids and has a molecular mass of 28 187. The deduced amino-acid sequences of this H subunit showed a significant (40%) degree of identity with those from mesophilic purple nonsulfur bacteria. The determined primary structure of the H subunit was compared with the structures of mesophilic B. viridis and R. sphaeroides based on the three-dimensional structure of the H subunit from T. tepidum, which has been recently determined by X-ray crystallography. One lipid molecule was found in the crystal structure of the T. tepidum RC, and the head group of the lipid appears to be stabilized by the electrostatic interactions with the conserved basic residues in the H subunit. The above comparison has suggested the existence of a lipid-binding site on the molecular surface at which a lipid molecule can interact with the RC in a specific manner.

Crystal structures of photosynthetic reaction center and high-potential iron-sulfur protein from Thermochromatium tepidum: thermostability and electron transfer

The reaction center (RC) of photosynthetic bacteria is a membrane protein complex that promotes a light-induced charge separation during the primary process of photosynthesis. In the photosynthetic electron transfer chain, the soluble electron carrier proteins transport electrons to the RC and reduce the photo-oxidized special-pair of bacteriochlorophyll. The high-potential iron-sulfur protein (HiPIP) is known to serve as an electron donor to the RC in some species, where the c-type cytochrome subunit, the peripheral subunit of the RC, directly accepts electrons from the HiPIP. Here we report the crystal structures of the RC and the HiPIP from Thermochromatium (Tch.) tepidum, at 2.2-A and 1.5-A resolution, respectively. Tch. tepidum can grow at the highest temperature of all known purple bacteria, and the Tch. tepidum RC shows some degree of stability to high temperature. Comparison with the RCs of mesophiles, such as Blastochloris viridis, has shown that the Tch. tepidum RC possesses more Arg residues at the membrane surface, which might contribute to the stability of this membrane protein. The RC and the HiPIP both possess hydrophobic patches on their respective surfaces, and the HiPIP is expected to interact with the cytochrome subunit by hydrophobic interactions near the heme-1, the most distal heme to the special-pair.

Hydrogen-bond interactions of the primary donor of the photosynthetic purple sulfur bacterium Chromatium tepidum

We have used near-infrared Fourier transform (pre)resonance Raman spectroscopy to determine the protein interactions with the bacteriochlorophyll (BChl) dimer constituting the primary electron donor, P, in the reaction center (RC) from the thermophilic purple sulfur bacterium Chromatium tepidum. In addition, we report the alignment of partial sequences of the L and M protein subunits of C. tepidum RCs in the vicinity of the primary donor with those of Rhodobacter sphaeroides and Rhodopseudomonas viridis. Taken together, these results enable us to propose the hydrogen-bonding pattern and the H-bond donors to the conjugated carbonyl groups of P. Selective excitation (1064-nm laser radiation) of the FT (pre)-resonance Raman spectra of P in its neutral (P degree) and oxidized (P degree +) states were obtained via their electronic absorption bands at 876 and 1240 nm, respectively. The P degree spectrum exhibits vibrational frequencies at 1608, 1616, 1633, and 1697 cm-1 which bleach upon P oxidation. The P degree + spectrum exhibits new bands at 1600, 1639, and 1719 cm-1. The 1608-cm-1 band, which downshifts to 1600 cm-1 upon oxidation, is assigned to a CaCm methine bridge stretching mode of the P dimer, indicating that each BChl molecule possesses a single axial ligand (His L181 and His M201, from the sequence alignment). The 1616- and 1633-cm-1 bands correspond to two H-bonded pi-conjugated acetyl carbonyl groups of each BChl molecule. with different H-bond strengths: the 1616-cm-1 band is assigned to the PL C2 acetyl group which is H-bonded to a histidine residue (His L176), while the 1633-cm-1 band is assigned to the PM C2 acetyl carbonyl, H-bonded to a tyrosine residue (Tyr M196). Both PL and PM C9 keto carbonyls are free from interactions and vibrate at the same frequency (1697 cm-1). Thus, the H-bond pattern of the primary donor of C. tepidum differs from that of Rb. sphaeroides in the extra H-bond to the PM C2 acetyl carbonyl group; that of PL is H-bonded to a histidine residue in both primary donors (His L168 in Rb. sphaeroides and His L176 in C. tepidum). The P degree/P degree + redox midpoint potentials were measured to be +497 and +526 mV for isolated C. tepidum RCs with and without the associated tetraheme cytochrome c subunit, respectively, and +502 mV for intracytoplasmic membranes. The positive charge localization was estimated to be 69% in favor of PL, indicating a more delocalized situation over the primary donor of C. tepidum than that of Rb. sphaeroides (estimated to be 80% on PL). These differences in physicochemical properties are discussed with respect to the proposed structural model for the microenvironment of the primary donor of C. tepidum.

Preliminary X-ray crystallographic studies of photosynthetic reaction center from a thermophilic sulfur bacterium, Chromatium tepidum

A membrane protein complex, photosynthetic reaction center purified from the thermophilic purple sulfur bacterium, Chromatium tepidum has been crystallized from a phosphate buffer containing a detergent, n-octyl-beta-D-glucopyranoside and a precipitant, polyethylene glycol 4000. The crystals diffracted X-rays beyond 3A resolution with synchrotron radiation and are suitable for high-resolution X-ray crystallographic studies. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) with unit-cell dimensions of a = 136A, b = 197A, and c = 82A. Assuming that they contain one reaction center complex in the asymmetric unit, VM was calculated to be 4.3 A3/Da, which agrees with the values obtained in the membrane protein complexes.