U. Smith

Structure of Rhodopseudomonas sphaeroides R-26 reaction center

The molecular replacement method has been succesfully used to provide a structure for the photosynthetic reaction center of Rhodopseudomonas sphaeroides at 3.7 Å resolution. Atomic coordinates derived from the R. viridis reaction center were used in the search structure. The crystallographic R‐factor is 0.39 for reflections between 8 and 3.7 Å. Validity of the resulting model is further suggested by the visualization of amino acid side chains not included in the R. viridis search structure, and by the arrangements of the reaction centers in the unit cell. In the initial calculations quinones or pigments were not included; nevertheless, in the resulting electron density map, electron density for both quinones qa and qb appears along with the bacteriochlorophylls and bacteriopheophytins. Kinetic analysis of the charge recombination shows that the secondary quinone is fully functional in the R. sphaeroides crystal.

Characterization of bacterial photosynthetic reaction center crystals from Rhodopseudomonas sphaeroides R-26 by X-ray diffraction

An orthorhombic crystal form (P2(1)2(1)2(1)) of the reaction center from the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 has been characterized. The crystals were grown from polyethylene glycol; the unit cell dimensions are a = 142.2 A, b = 139.6 A, and c = 78.7 A; and they contain one reaction center in each crystallographic asymmetric unit. The crystals diffract to at least 3.0 A resolution, and are suitable for detailed structural studies.

The chromophore-protein bonds in phycocyanin

Abstract Peptides formed by the proteolysis of C -phycocyanin were purified by TLC and analyzed for amino acid content. The data suggest that phycocyanobilin is linked to apophycocyanin through two bonds, one an ester bond involving the carboxyl group of an aspartic acid side-chain and the hydroxyl group of the enol form of ring A of the bilin, and the other a thio-ether type of bond from a cysteine side-chain to the bilin side-chain at position 2, a linkage analogous to that in cytochrome c. Liberation of phycocyanobilin from apophycocyanin by alcohols would involve transesterification at the ester bond followed by the formation of the keto form of ring A and elimination of the cysteine side-chain.

Extraction and purification of 1H, 2H, and isotope hybrid algal cytochrome, ferredoxin and flavoprotein

Abstract Cytochrome c, ferredoxin, and phytoflavin, three low molecular weight, thermally stable proteins, have been isolated from the thermophilic alga Synechococcus lividus grown in both 1H2O and 2H2O. The isolation, purification, and physical properties of these three proteins are given in detail and their utility for magnetic resonance studies is described.

Heliobacterium chlorum: cell organization and structure.

The basic cellular organization of Heliobacterium chlorum is described using the freeze-etching technique. Internal cell membranes have not been observed in most cells, leading to the conclusion that the photosynthetic apparatus of these organisms must be localized in the cell membrane of the bacterium. The two fracture faces of the cell membrane are markedly different. The cytoplasmic (PF) face is covered with densely packed particles averaging 8 nm in diameter, while the exoplasmic (EF) face contains far fewer particles, averaging approximately 10 nm in diameter. Although a few differentiated regions were noted within these fracture faces, the overall appearance of the cell membrane was remarkably uniform. The Heliobacterium chlorum cell wall is a strikingly regular structure, composed of repeating subunits arranged in a rectangular pattern at a spacing of 11 nm in either direction. We have isolated cell wall fragments by brief sonication in distilled water, and visualized the cell wall structure by negative staining as well as deep-etching.

Studies on the identity of ordinary and deuterio-phycocyanins: End-groups, amino acid compositions, minimum molecular weights, peptide maps

Abstract New data on the amino acid composition of phycocyanins extracted from Phormidium luridum and Synechococcus lividus grown in H2O and in 99.7% 2H2O have been acquired. The presence of tryptophan in these phycocyanins has been established. The minimum molecular weight for phycocyanin from P. luridum is calculated to be 21 000 g/mole, and for phycocyanin extracted from S. lividus, 25 000 g/mole, values considerably smaller than those previously reported. N-terminal amino acid determinations of Plectonema calothricoides, P. luridum, and S. lividus phycocyanins by the 1-fluoro-2,4-dinitrobenzene method showed methionine to be terminal in all the phycocyanins examined. Peptide maps of tryptic digests of both ordinary and deuterio-phycocyanins from P. calothricoides and P. luridum showed a close resemblance, and these serve to confirm the identity between hydrogen and deuterio-phycocyanins originating in a particular blue-green alga.

ESR study of the primary electron donor in highly 13C-enriched Chlorobium limicola f. thiosulfatophilum

The photo‐induced electron spin resonance signal of the primary donor P840+ in unit membrane fractions of the title photosynthetic bacterium was measured. Bacteria containing natural isotopic abundances and 82% 13C‐enrichment were examined. Second moment analysis of the lineshape of the 13C‐enriched P840+ signal shows that P840+ consists of two bacteriochlorophyll a‐type macrocycles. The data thus obtained are independent of difficulties in interpreting either peak‐to‐peak linewidths or ENDOR data with respect to intermolecular spin delocalization.

Comparison of electron spin polarization of P 870 + Qa a − observed in Zn2+-containing and Fe2+-depleted bacterial reaction centers

Recently, a general model has been developed to explain electron spin polarized (ESP) electron paramagnetic resonance (EPR) signals found in systems where radical pairs are formed sequentially. The photosynthetic bacterial reaction center (RC) is such a system in which we can experimentally vary parameters (lifetime, structure, and magnetic interactions in the sequentially formed radical pairs) that affect ESP development in order to test this model. In Fe2+-depleted transfer step from intermediate radical pair, P870+Qa− which is produced in an electron transfer step from intermediate radical pair, P870+I−. (P870+ is the oxidized primary donor, a special pair of bacteriochlorophyll molecules, I− is the reduced bacteriopheophytin acceptor, and Qa− is the reduced primary quinone acceptor.) The lifetime of P870+I− can be shortened relative to the lifetime of P870+I− in Fe2+-depleted RCs by substitution of Zn2+. We report the first observation of X-band and Q-band ESP EPR signals due to P870+Q− from bacterial reaction centers that contain Zn2+. Comparison of these signals to those observed from Fe2+-depleted bacterial reaction centers shows intensity differences and g-factor shifts. The results are discussed in terms of the general sequential radical pair model.

Factors Affecting Electron Spin Polarization in Photosynthetic Systems

The first steps of the plant photosystem I (PSI) primary reactions are believed to be: \({P_{700}}{A_0}{A_1}{^{1*}}{P_{700}}{A_0}{A_1} \to P_{700}^ + A_0^ - {A_1} \to P_{700}^ + {A_0}A_1^ - \) where P700 and A0 are chlorophyll species and A1 is a quinone-like molecule. (1) The initial suggestions that A0 and A1 act as electron acceptors in sequence prior to the acceptor X were based on EPR experiments which showed that two electron acceptors are photoaccumulated at low redox potential. (2,3) Support for this suggestion came from following the triplet state found in PSI as a function of the photoaccumulation of the proposed A 0 − and A 1 − , and interpretations of the transient electron spin polarized (esp) signal believed to be due to P 700 + A 1 − which has been observed by time resolved EPR experiments.