Reactions, chloroplast-mediated

Of the various biochemical pathways identified as being affected by herbicides, the chloroplast-mediated reactions have received the greatest attention. Approximately 70 percent of the current commercial herbicides, while they may also affect other systems, interfere with chloroplast reactions. Hence, the objectives of this paper are to review some of the work conducted with isolated chloroplasts, evaluate the status of these studies, and relate the observed interferences to the expression of phytotoxicity. 

Calvin cycle (aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast. 

Allelochemicals Evaluated for Effects on Reactions Mediated by Isolated Chloroplasts and Mitochondria... 

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

CO2 are fixed into sugar (glucose) and mediated by the enzyme rubisco (ribulose-l-5-biphosphate carboxylase). It occurs in the stroma of chloroplasts. The Calvin cycle is also known as the dark reaction, as opposed to the first-stage light reactions. 

Animals and bacteria are heterotrophs they obtain carbon in various forms as food and metabolize many forms of it to provide energy and body structure. Plants are autotrophs all their carbon comes from C02 powered by photosynthesis. Photosynthesis occurs within the thylakoid membranes of chloroplasts in plant leaves, and it is mediated by chlorophyll. The light reaction splits water into 02, electrons, and protons (H+). NADPH is produced by electron transport and ATP synthesis by associated proton transport. 

Oxidant-induced reduction of cytochrome b The oxidant-induced reduction of Cyt b means the reduction ofCyt-Z>6 linked to the oxidation ofquinol is mediated by the Rieske iron-sulfur protein. This reaction has long been documented for the mitochondrial and photosynthetic-bacterial Cyt-icj complex. The same reaction is expected to occur in the Cyt-f)6/complex of higher plants and cyanobacteria. Oxidant-induced reduction of Cyt b was illustrated in the early reaction steps in Fig. 11 (C) above. We describe here, with the help of Fig. 12, the work of Hurt and Hauska detailing the spectrophotometric evidence for the reaction steps in the Cyt-b(f complex isolated from spinach chloroplasts. 

Before proceeding to the next topic, we look at another version of artificial phosphorylation by chloroplasts in the dark, i.e., not driven hyphotoinducedelectron transfer. This new type oftwo stage phosphorylation, called dark oxidation-reduction coupled phosphorylation, was reported by Selman and Psczolla and may be considered as a variant of the postillumination or the acid-base transition types discussed above. The authors found that ATP formation in chloroplasts in the dark can be achieved by an artificial, transmembrane redox reaction using ascorbate as the reductant for ferricyanide trapped inside the chloroplasts, provided it is mediated by a redox carrier such as DAD, DCIP or PMS that liberates protons during its oxidation, as illustrated in the scheme in Figure 13. 

In practice, a concentrated chloroplast sample (3 mg Chl/ml) is loaded or charged with a high concentration (100 mM) of ferricyanide by abrief sonication. Ferricyanide must be present during sonica-tion in order for the chloroplasts to be able to synthesize ATP in the dark. The sample is then diluted 15 fold with a buffer that contains ADP and Pj plus 10 mM ascorbate as the reductant and 0.4 mM DAD as the redox mediator. After incubation for two minutes at 20 °C in the dark, the reaction was quenched with HCIO4 and ATP analyzed. This dark redox-coupled phosphorylation has a yield of 70 nmoles ATP/mg Chi, amounting to about one-half to one-fourth of the amount usually obtained by acid-base transition. Ascorbate alone was not sufficient to catalyze ATP synthesis. As expected, the dark phosphorylation was also inhibited by uncouplers. 

Relatively little is known about plant sterols. (Most of the research effort in steroid metabolism has been expended in the investigation of steroid-related human diseases.) It appears, however, that the initial phase of plant sterol synthesis is very similar to that of cholesterol synthesis with the following exception. In plants and algae the cyclization of squalene-2,3-epoxide leads to the synthesis of cycloartenol (Figure 12.30) instead of lanosterol. Many subsequent reactions in plant sterol pathways involve SAM-mediated methylation reactions. There appear to be two separate isoprenoid biosynthetic pathways in plant cells the ER/cyto-plasm pathway and a separate chloroplast pathway. The roles of these pathways in plant isoprenoid metabolism are still unclear. 

Effect of temperature on protein Synthesis by isolated Chloroplasts Intact chloroplasts of V. Sinensis mediated protein synthesis in a light-dependent reaction without the requirement for ATP. No protein synthesis was seen in the dark. Fig.l shows the fluorographic profile of the proteins synthesized at various temperatures. The general profile of the polypeptides synthesized was not altered very much by the change in incubation temperature. However, at 35 C and above, there was a distinct increase in the synthesis and/or accumulation of four polypeptides in the molecular sizes of 85,70, 60 and 23 kDa. All four polypeptides could be observed, at least as faint bands, even at 25 C. The synthesis of HSPs at 40 C in the isolated chloroplasts were completely inihibited by chloramphenicol but not by cycloheximide. The transcriptional inihibitors like actinomycin-D and rifamycin were also inihibitory to the HSP synthesis (Fig 2). 

But as IE is also in close reverse dependence on partial pressure of CO2 in the air (rises with the decrease in partial pressure) /7, 12/ which mesns that isoprene synthesis in which acetyl CoA of chloroplast origin is used is obviously mediated by the connection with the reaction of photosynthetic carboxylation. 

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