High-light illumination of photosynthetic organisms stimulates the production of singlet oxygen by photosystem II (PSII) and causes photo-oxidative stress. In the PSII reaction centre, singlet oxygen is generated by the interaction of molecular oxygen with the excited triplet state of chlorophyll (Chl). The triplet Chl is formed via charge recombination of the light-induced charge pair. Changes in the midpoint potential of the primary electron donor P(680) of the primary acceptor pheophytin or of the quinone acceptor Q(A), modulate the pathway of charge recombination in PSII and influence the yield of singlet oxygen formation. The involvement of singlet oxygen in the process of photoinhibition is discussed. Singlet oxygen is efficiently quenched by beta-carotene, tocopherol or plastoquinone. If not quenched, it can trigger the up-regulation of genes, which are involved in the molecular defence response of photosynthetic organisms against photo-oxidative stress.
Plastoquinone, Herbicide Binding Protein. Membrane Proteins, Photosystem II, Thylakoid MembraneThe 32 kDa herbicide and O b binding peptide (D-l protein) and its homologous 34 kDa peptide (D-2 protein) are integral membrane subunits of photosystem II. A model for their folding through the thylakoid membrane in five transmembrane a-helices is proposed from the compari son of amino acid sequence and hydropathy index plot homologies with subunits of the bacterial system. Following recent data on the X-ray structure of a bacterial photosystem the binding niche for O b is interpreted on the basis of the amino acid changes found in the 32 kDa peptide in herbicide tolerant higher plants and algae.Photosystem II consists of five integral peptide subunits of 47, 44, 34, 32 and 10 kDa molecular weight. They bind the functional components of photosystem II: the reaction center chlorophyll, core antenna chlorophylls, pheophytin. Fe, cytochrome £>559, two plastoquinones Q A und Q B and a primary electron donor (for review see [1 -3]). The 32 kDa peptide has been given particular attention, as it was the first of the photosystem II peptides of which the specific function was recognized. It was shown to bind herbicides [4] and to be identical with the rapid ly turning over peptide [5] encoded by the D-l gene. From the known mode of action of these herbicides on the acceptor side of photosystem II it is indicated that the 32 kDa peptide is involved in 0 B binding. It was also the first integral membrane peptide of the electron transport system, whose complete amino acid sequence was determined [6] (now all of the genes for photosystem II peptides have been se quenced as well as most of the cytochrome b/f-com plex and of photosystem I, for review see [7,8]). From predictions of its secondary structure [9] the suggestion developed that integral peptides of the photosynthetic membrane may fold through the membrane several times in hydrophobic helices [8]. Although this general statement seems to be ac cepted, the exact folding of the 32 kDa peptide re mained controversial.Rao et al. [9] suggested that the 32 kDa peptide folds seven times through the membrane. However.Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen 0341 -0382/86/0100 -0240 $01.30/0 other predictions of the folding of the 32 kDa pep tide have been proposed [10, 1 1 ] because it appeared that certain sequence stretches in the Rao model were not hydrophobic enough or too short to span the membrane. On the other hand hydrophobic se quences. not considered by Rao et al. [9], might cross the membrane [10,11]. A main argument against any of these folding predictions is that these models do not fit well mutant data that indicate which amino acids participate in herbicide and from there O b binding. The 32 kDa peptide of herbicide tolerant mutants of higher plants and algae is shown to have specific amino acid changes [12][13][14][15][16][17]. These changes: val219, phe25s, ser264, leu275 are not readily accomo dated, according to the Rao et al. [9] prediction, in...
Sequence homology and structural similarity between cytochrome b of mitochondrial complex III ABSTRACTThe amino acid sequences of cytochrome b of complex III from five different mitochondrial sources (human, bovine, mouse, yeast, and Aspergilus nidulans) and the chloroplast cytochrome b6 from spinach show a high degree of homology. Calculation of the distribution of hydrophobic residues with a "hydropathy" function that is conserved in this family of proteins implies that the membrane-folding pattern of the 42-kilodalton (kDa) mitochondrial cytochromes involves 8-9 membrane-spanning domains. The smaller 23-kDa chloroplast cytochrome appears to fold in five spanning domains that are similar to the first five of the mitochondria. Four highly conserved histidines are considered to be the likely ligands for the two hemes. The positions of the histidines along the spanning segments and in a cross section of the membrane-spanning a helices implies that two ligand pairs, His-82-His-
K K-Tocopherol concentrations were determined at low and high light intensities and compared with the rate of photosynthesis, photosystem II (PS II) and its reaction center D1 protein. Blocking of tocopherol biosynthesis at the 4-hydroxyphenylpyruvate dioxygenase by the herbicide pyrazolynate led to a quick disappearance of K K-tocopherol in high light, as well as of PS II activity and the D1 protein. Homogentisic acid rescued all activities. It is concluded that K K-tocopherol has a continuous turnover as a scavenger of the singlet oxygen that arises from the quenching by oxygen of the triplet of the PS II reaction center and triggers the degradation of the D1 protein. Thus tocopherols are essential to keep photosynthesis active. We suggest that this is why plants make and need tocopherols. Chemical quenchers of singlet oxygen, notably diphenylamines, completely protect PS II, prevent D1 protein degradation and keep tocopherol levels even at very high light intensities. This supports the notion that 1 O 2 is the intermediate in light triggered D1 protein turnover. ß
H erbicide Target, H erbicide R esistance, Q uinone Binding Protein, P hotosystem II, Thylakoid M embrane Protein The folding through the m embrane o f the plastoquinone and herbicide binding protein subunits of photosystem II and the topology o f the binding niche for plastoquinone and herbicides is described. The m odel is based on the hom ology in amino acid sequence and folding prediction from the hydropathy analysis o f the D -l and D-2 subunits o f photosystem II to the reaction center polypeptides L and M o f the bacterial reaction center. It incorporates the am ino acid changes in the D -l polypeptide in herbicide tolerant plants and those indicated by chem ical tagging to be involved in Q B binding. It proposes hom ologous amino acids in the D -l/D -2 polypeptides to those indicated by the X-ray structure o f the bacterial reaction center to be involved in Fe-, quinoneand reaction center chlorophyll-binding. The different chemical com pounds known to interfere with Q b function are grouped into two families depending on their orientation in the Q B binding niche.
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