Chloroplastic binding site

In contrast, experiments with heterotropically grown C. pyrenoidosa (with cysteine as the sole sulfur source) showed that the organism, as expected, lacked sulfolipid but showed normal rates of O2 evolution (Sinensky, 1977). Chlorophyll synthesis was relatively unaffected, but CO2 fixation was very poor, apparently because of a reduction in phosphoribulose kinase activity. Sinensky (1977) suggested that sulfolipid might be involved in the enzyme s action, and since the purified enzyme showed no lipid requirement (Racker, 1957), possibly by acting as a chloroplast binding site for the kinase. 

Surprisingly, too, there are claims of higher oxidation states of Mn in some systems, e.g. Mnlv in photosynthetic(II) chloroplast systems and Mn111 in acid phosphatase. In the latter enzyme Tyr and Cys residues appear to form part of the metal-binding site. The metal is also involved in the phosphate binding. While superoxide dismutase (SOD) is more generally found with Cu and Zn as the active metals, an Mn-SOD form is found in certain bacteria. The Mn oscillates between different oxidation states in its catalytic activity.149... 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

A chloroplast location of the gene for Cyt / was suggested by two plastome mutants of Oenothera which were deficient in spectrally detected Cyt/[102], and by the maternal mode of inheritance of Cyt /in interspecific Fj hybrids of Nicotiana [103]. The structural gene (petA) for the Cyt / polypeptide was initially localized and characterized from pea chloroplast DNA [104,105], and subsequently from wheat [106], spinach [107] and Oenothera [108]. Open reading frames of 320 codons have been detected in chloroplast DNA from each of these plants. Comparison with the determined N-terminal sequence of Cyt / indicates the presence of a 35 amino acid residue pre-sequence and a mature polypeptide of 285 amino acid residues. A putative haem-binding site, Cys-Ala-Asn-Cys-His, is located near the... 

Fig. 5. Model for the protein binding site for protoheme in cytochrome b559 (A) and modei for the orientation of the two histidine imidazole rings in cytochrome bS59 (B). See text for discussion. Figures adapted from Babcock, Widger, Cramer, Oertling and Metz (1985) Axial ligands of chloroplast cytochrome b559 identification and requirement for a heme-cross-linked polypeptide structure. Biochemistry 24 3643.

The Q-cycle hypothesis and other alternative versions of it were attempts to explain the two important reactions occurring during electron transport and proton translocation in the mitochondrial cytochrome be complex and also in the chloroplast cytochrome complex by the so-called oxidant-induced reduction of Cyt b and the interheme electron transport in the Cyts b. Abundant experimental work to obtain evidence for these two reactions as well as other aspects relating to the structure and function of the be complexes has been performed. In addition to what has been mentioned above, we will present several selected examples to elucidate the oxidant-induced reduction of Cvt b and the need for two quinone-binding sites, using the chloroplast CyX-b(f complex or the Cyi-bcx complex from photosynthetic bacteria as examples, all monitored by absorbance changes of cytochromes induced by either steady or flash illumination. 

Plastocyanin cycles between the Cu and Cu oxidation states, and transfers electrons from cytochrome / to the P700 component of photosystem I in the chloroplasts of higher plants and algae.The low molecular weight (10.5 kDa) and availability of detailed structural information have made this protein an attractive candidate for mechanistic studies, which, when taken together, point to two distinct surface binding sites (i.e., regions on the... 

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