Purple Bacterial Reaction Centers Research Articles

ENDOR and special triple resonance studies of chlorophyll cation radicals in photosystem 2.

Electron nuclear double resonance (ENDOR) and special triple (ST) resonance spectroscopies have been used to study the cation radicals of the primary donor, P680, and two secondary donor chlorophylls (Chl) in photosystem 2 (PS2). Two different preparations were employed, Tris-washed PS2 membranes and PS2 reaction centers (D1-D2-I-Cytb559 complex). One secondary donor Chl a cation radical, Chl1.+, was generated in the Tris-washed preparation, while the P680.+ radical cation and a further Chl a cation radical, Chl2.+, were produced in the reaction center preparation. The ENDOR spectrum of the primary donor radical cation of photosystem 1 (P700.+) is also presented for comparison. Hyperfine coupling constants for methyl groups have been measured for all three PS2 radical species and assigned by comparison with previously published spectra of Chl a radicals in vitro. Electron spin densities were calculated from these hyperfine couplings. Comparison of ENDOR spectral features with those of Chla.+ in vitro indicates similar values for Chl1.+ and Chl2.+ radicals but an apparent reduction in unpaired electron spin density for P680.+. It has been proposed from the more detailed studies of purple bacterial reaction centers that such a reduction in spin density can be interpreted as a delocalization over two Chl a molecules. Our calculations therefore suggest that P680.+ is a weakly coupled chlorophyll pair with 82% of the unpaired electron spin located on one chlorophyll of the pair at 15 K. Environmental or geometrical changes to the chlorin ring structure to give a novel monomeric primary donor are also possible.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and binding site of the primary electron acceptor in the reaction center of Chlorobium.

In isolated, chlorosome-free reaction centers from Chlorobium limicola f thiosulphatophilum, a chlorin pigment exhibits a Qy absorption band at 672 nm (Feiler, U., Nitschke, W., & Michel, H. (1992) Biochemistry 31, 2608-2614). To characterize the chemical nature of this chlorin pigment and its interactions within the reaction-center protein, selective enhancement of its Raman scattering was achieved by resonant excitation within its Soret band. This is the first time that structural studies of this pigment were performed on the native reaction-center protein. The obtained resonance Raman spectra were consistent with a single population of a chlorophyll a(-like) pigment, possessing a vinyl group on ring I, but not with bacteriochlorophyll c or bacteriophaeophytin c. The stretching frequencies of the C9-keto carbonyl of this pigment indicates that it is H-bonded to the reaction-center protein. The strength of this H-bond is very close to those of the keto carbonyls of the primary electron acceptors in purple bacterial reaction centers and D1/D2 particles. Since in membranes of Chlorobiaceae a transient bleaching at 670 nm is due to the primary acceptor in the reaction center (Nuijs, A. M., Vasmel, H., Joppe, H. L. P., Duysens, L. N. M., & Amesz, J. (1985a) Biochim. Biophys. Acta 907, 24-34), we thus conclude that the primary acceptor in Chlorobium reaction centers is the characterized chlorophyll a(-like) pigment.(ABSTRACT TRUNCATED AT 250 WORDS)

Evolution of photosynthetic reaction centers and light harvesting chlorophyll proteins

It is proposed that there is a single evolutionary origin for photosynthetic reaction centers and also for most light-harvesting chlorophyll proteins. It is generally accepted that the purple bacterial reaction center (quinone-reducing photosystem) and the plant and cyanobacterial PSII (oxygen-evolving photosystem) are homologous. It is also apparent that the green sulfur bacterial reaction center is homologous to cyanobacterial PSI (the pyridine nucleotide reducing photosystem). However, it is less obvious that PSI is related to the purple bacterial reaction center. It is herein proposed that PSI represents a gene fusion of the precursors of small light harvesting bacteriochlorophyll proteins from purple bacteria and purple bacterial reaction centers. Furthermore, it is proposed that reaction centers evolved from the membrane-spanning cytochrome b of the cytochrome bc 1 complex and that most membrane-spanning cytochromes may have a common origin.

Excitation transfer and charge separation kinetics in purple bacteria. (1) Picosecond fluorescence of chromatophores from Rhodobacter capsulatus wild type

The fluorescence kinetics of chromatophores from Rhodobacter capsulatus wild type have been measured with picosecond time resolution over the wavelength range from 850 to 940 nm. The data have been analyzed both by global lifetime analysis yielding lifetimes and decay-associated spectra (DAS) and by global target analysis techniques yielding rate constants and species-associated spectra (SAS). In the global lifetime analysis five lifetime components were necessary for a good fit. The lifetimes are: τ 1 = 9 ps showing a DAS with positive and negative amplitudes; τ 2 = 40 ps with a DAS maximum at ≈ 890 nm; τ 3 = 95 ps with a DAS maximum at ≈ 895 nm; τ 4 = 260 ps with a DAS similar to that of lifetime component τ 3; a fifth lifetime component of 940 ps has nearly negligible amplitude. In the global target analysis several kinetic models were tested. A homogeneous model with sequential energy transfer LHC II ↔ LHC I harr; RC does formally fit the data only if the detailed charge separation and charge recombination processes at the reaction center (RC) are taken into account explicitly. However, despite the good fit, such a model must be excluded as a valid description both on the basis of simple thermodynamic considerations as well as due to the fact that it predicts unreasonable RC rate constants. Instead a heterogeneous model, assuming a mixture of chromatophores with open and closed RCs describes the situation both formally and physically quite well. For such a model the rate constants of the RC electron transfer processes and their free energy values as well as the energy transfer rate constants and SAS of the antenna pools were obtained. The extrapolated RC kinetics for open RCs agrees well with that known from isolated purple bacterial RCs. The rate constants for energy transfer processes among antenna pools and from antenna to RCs indicate that at room temperature the exciton kinetics of the entire antenna system is limited and determined by the RC charge separation, i.e., the exciton decay is trap limited. Our data are compared with the corresponding data for Rb. sphaeroides.

Photosynthetic reaction centres: variations on a common structural theme?

From their hybrid properties, the reaction centres of green sulphur bacteria and heliobacteria seem to be the missing links between the two branches of the reaction centre family, typified by higher plant photosystem I and the purple bacterial reaction centre. This suggests that all of the diverse types of photosynthetic reaction centres have closer structura l resemblances than was previously thought.

Evolutionary relationships between “Q-type” photosynthetic reaction centres: Hypothesis-testing using parsimony

Hypotheses concerning the evolutionary relationships between "Q-type" photosynthetic reaction centres are tested using amino acid parsimony analysis of subunit sequences and an alignment based on dot matrix comparisons. Strong evidence is found for independent gene duplications having produced the L and M subunits of the photosynthetic purple bacterial reaction centre and D1 and D2 of Photosystem-II. Much support is also found for the L and M subunits of the green filamentous bacterium Chloroflexus aurantiacus arising from the same gene duplication as the purple bacterial subunits, suggesting there was an ancestral bacterial heterodimeric reaction centre. These conclusions caution against over-extrapolation from the purple bacterial reaction centre to Photosystem-II, and suggest that the latter is more ancient than previously supposed.

A structural basis of light energy and electron transfer in biology

Aspects of intramolecular light energy and electron transfer will be discussed for three protein cofactor complexes, whose three‐dimensional structures have been elucidated by X‐ray crystallography: components of light‐harvesting cyanobacterial phycobilisomes, the purple bacterial reaction centre and the blue multi‐copper oxidases. A wealth of functional data is available for these systems which allow specific correlations between structure and function, and general conclusions about light energy and electron transfer in biological materials to be made.

Nobel lecture. A structural basis of light energy and electron transfer in biology.

Aspects of intramolecular light energy and electron transfer will be discussed for three protein cofactor complexes, whose three-dimensional structures have been elucidated by x-ray crystallography: Components of light harvesting cyanobacterial phycobilisomes, the purple bacterial reaction centre, and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allow specific correlations between structure and function and general conclusions about light energy and electron transfer in biological materials to be made.

Nobel lecture. A structural basis of light energy and electron transfer in biology.

Aspects of intramolecular light energy and electron transfer will be discussed for three protein--cofactor complexes, whose three-dimensional structures have been elucidated by X-ray crystallography: components of light-harvesting cyanobacterial phycobilisomes; the purple bacterial reaction centre; and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allows specific correlations between structure and function and general conclusions about light energy and electron transfer in biological materials to be made.

Stoichiometry of cytochrome b-559 in Photosystem II

A careful determination of the stoichiometry of cytochrome b -559 per Photosystem II reaction centre has been made for different Photosystem II preparations. Using an extinction coefficient determined from the α-band of reduced pyridine haemochrome and chemical analyses of pigment levels it is concluded that there is one cytochrome b -559 haem per Photosystem II reaction centre. This conclusion assumes that by analogy with the purple bacterial reaction centre there are two pheophytin molecules per Photosystem II reaction centre.

Reality of P‐680 chlorophyll protein – Identification of the site of primary photochemistry in oxygenic photosynthesis

Until recently nearly all available experimental evidence seemed to indicate that the largest subunit of about 50 kDa in the photosystem II core complex (psbB gene product) is the site of primary photochemistry, and thus the name “P‐680 apoprotcin” has been given to this subunit. The notion, however, has also been challenged on the basis of deduced amino acid sequence homology between the proteins in the photosystem II and those of the purple bacterial reaction center. The actual preparation of a functionally active photosystem II reaction center completely devoid of the psbB gene product, but consisting of D‐1 and D‐2 proteins and cytochrome b‐559, has now been achieved.

Partial purification, subunit structure and thermal stability of the photochemical reaction center of the thermophilic green bacterium Chloroflexus aurantiacus

Spectrally pure reaction center preparations from Chloroflexus aurantiacus have been obtained in a stable form; however, the product contained several contaminating polypeptides. The reaction center pigment molecules (probably three bacteriochlorophyll a and three bacteriopheophytin a molecules) are associated with two polypeptides ( M r = 30000 and 28000) in a reaction center complex of M r = 52000. No carotenoid is present in the complex. These data together with previous spectral data suggest that the Chloroflexus reaction center represents a more primitive evolutionary form of the purple bacterial reaction center, and that it has little if any relationship to the green bacterial component. A reaction center preparation from Rhodopseudomonas sphaeroides R26 was fully denatured at 50°C while the Chloroflexus reaction center required higher temperatures (70–75°C) for complete denaturation. Thus, an intrinsic membrane protein of a photosynthetic thermophile has been demonstrated to have greater thermal stability than the equivalent component of a mesophile.