PubMed Health⌕ Search

Biomedical subjects

D B Knaff

Publications and source records attributed to D B Knaff.

At least 109 records · Page 6Linked to original sources
PubMed1977

The role of plastoquinone and beta-carotene in the primary reaction of plant photosystem II.

Extraction of Triton Photosystem II chloroplast fragments with 0.2% methanol in hexane for 3 h results in the removal of 90 to 95% of the plastoquinone in the original preparation. The extracted fragments (chlorophyll:plastoquinone ratio, 900: 1) showed no P-680 photooxidation at 15 K after a single laser flash. The extracted fragments also showed no light-induced C-550 absorbance change at 77 K. Reconstitution of the primary reaction of Photosystem II, as evidenced by restoration of low-temperature photooxidation of P-680, could be obtained by the addition of plastoquinone A but not by the addition of beta-carotene. The addition of beta-carotene plus plastoquinone A restored the C-550 absorbance change. These results indicate that plastoquinone functions as the primary electron acceptor of Photosystem II and that beta-carotene does not play a direct role in the primary photochemistry but is required for the C-550 absorbance change.

Carotenoids↗
PubMed1976

Iron-sulfur proteins of the green photosynthetic bacterium Chlorobium.

The iron-sulfur proteins of the green photosynthetic bacterium Chlorobium have been characterized by oxidation-reduction potentiometry in conjunction with low-temperature electron paramagnetic resonance spectroscopy. Chlorobium ferredoxin was the only iron-sulfur protein detected in the soluble fraction; no high-potential iron-sulfur protein was observed. In addition, high-potential iron-sulfur protein was not detected in the chromatophores. Four chromatophore-bound iron-sulfur proteins were detected. One is the "Rieske" type iron-sulfur protein with a g-value of 1.90 in the reduced state; the protein has a midpoint potential of + 160 mV (pH 7.0), and this potential is pH dependent. Three g=1.94 chromatophore-bound iron-sulfur proteins were observed, with midpoint potentials of -25, -175, and about -550 mV. A possible role for the latter iron-sulfur protein in the primary photochemical reaction in Chlorobium is considered.

Binding Sites↗
PubMed1975

Cytochrome b and photosynthetic sulfur bacteria.

Chromatophores isolated from the purple sulfur bacterium Chromatium and the green sulfur bacterium Chlorobium exhibit absorbance changes in the cytochrome alpha-band region consistent with the presence of a b-type cytochrome. Cytochrome content determined by reduced minus oxidized difference spectra and by heme photochemically active bacteriochlorophyll (reaction-center bacteriochlorophyll). The b-type cytochrome in Chromatium has an alpha-band maximum at 560 nm and a midpoint oxidation-reduction potential of -5 mV at pH 8.0. The b-type cytochrome in Chlorobium has an alpha-band maximum at 564 nm and an apparent midpoint oxidation-reduction potential near -90 mV. Chromatophores isolated from both Chromatium and Chlorobium cells catalyze a photoreduction of cytochrome b that is enhanced in the presence of antimycin A. Antimycin A and 2-n-heptyl-4-hydroxyquinoline-N-oxide inhibit endogenous (but not phenazine methosulfate-mediated) cyclic photophosphorylation in Chromatium chromatophores and non-cyclic electron flow from Na-2S to NADP in Chlorobium chromatophores. These observations suggest that b-type cytochromes may function in electron transport reactions in photosynthetic sulfur bacteria.

Antimycin A↗
PubMed1975

The effect of o-phenanthroline on the midpoint potential of the primary electron acceptor of photosystem II.

The primary electron acceptor of Photosystem II has a midpoint oxidation-reduction potential of +95 mV at pH 7.0 in Photosystem II chloroplast fragments prepared by digitonin treatment. The midpoint potential of the acceptor has a pH dependence of -60 mV/pH unit. At concentrations that inhibit oxygen evolution, o-phenanthroline shifts the midpoint potential of the primary acceptor by +70 mV. The shifted potential retains the same dependence on pH. The effect of o-phenanthroline suggests that it interacts directly with the primary electron acceptor of photosystem II in a manner similar to that reported previously for the primary electron acceptor in purple photosynthetic bacteria.

Chloroplasts↗