Enzymes chloroplast

As described earlier, translation of the EPSPS mRNA of plants results in the formation of a protein which has an AJ-terminal extension. The AJ-terminal extension, referred to as the chloroplast transit peptide, is necessary and sufficient for the import of the preprotein by the chloroplast. Once imported by the chloroplast, the transit peptide is cleaved releasing the mature enzyme. As expected, introduction of the EPSPS transit peptide to other protein sequences results in the importation of the fusion protein by the chloroplast. 

Bioprocess The process that uses complete living cells or their components (e.g., enzymes, organelles, and chloroplasts) to effect desired chemical and/or physical changes. 

FIGURE 22.27 Light-induced pH changes in chloroplast compartments. Illumination of chloroplasts leads to proton pumping and pH changes in the chloroplast, such that the pH within the thylakoid space falls and the pH of the stroma rises. These pH changes modulate the activity of key Calvin cycle enzymes. 

An extract from the soluble stromal proteins of purified and intact spinach-leaf chloroplasts was prepared by lysis of the cells in buffer, centrifugation of the suspension of broken cells, and concentration of the supernatant with removal of insoluble material. This extract contained all of the enzymes involved in the condensation of the cyclic moieties of thiamine, thiazole, and pyramine. Thus, the synthesis of thiamine in this extract following the addition of pyramine and putative precursors was a proof that the system had the possibility of building the thiazole. It was found that L-tyrosine was the donor of the C-2 carbon atom of thiazole, as in E. coli. Also, as in E. coli cells, addition of 1 -deoxy-D-f/irco-pen-tulose permitted synthesis of the thiamine structure. The relevant enzymes were localized by gel filtration in a fraction covering the 50- to 350-kDa molecular-mass range. This fraction was able to catalyze the formation of the thiazole moiety of thiamine from 0.1 -mM 1-deoxy-D-t/ireo-pentulose at the rate of 220 pmol per mg of protein per hour, in the presence of ATP and Mg2+. 

In be complexes bci complexes of mitochondria and bacteria and b f complexes of chloroplasts), the catalytic domain of the Rieske protein corresponding to the isolated water-soluble fragments that have been crystallized is anchored to the rest of the complex (in particular, cytochrome b) by a long (37 residues in bovine heart bci complex) transmembrane helix acting as a membrane anchor (41, 42). The great length of the transmembrane helix is due to the fact that the helix stretches across the bci complex dimer and that the catalytic domain of the Rieske protein is swapped between the monomers, that is, the transmembrane helix interacts with one monomer and the catalytic domain with the other monomer. The connection between the membrane anchor and the catalytic domain is formed by a 12-residue flexible linker that allows for movement of the catalytic domain during the turnover of the enzyme (Fig. 8a see Section VII). Three different positional states of the catalytic domain of the Rieske protein have been observed in different crystal forms (Fig. 8b) (41, 42) ... 

Smirnoff, J. Colombe, S.V. (1988). Drought influences the activity of enzymes of the chloroplast hydrogen peroxide scavenging system. Journal of Experimental Botany, 39, 1097-1109. 

Mayoral, M.L., Atsmon, D., Gromet-Elhanan, Z. Shimshi, D. (1981). Effect of water stress on enzyme activities in wheat and related wild species Carboxylase activity, electron transport and photophosphorylation in isolated chloroplasts. Australian Jourrml of Plant Physiology, 8, 385-94. 

Younis, H.M., Weber, G. Boyer, J.S. (1983). Activity and conformational changes in chloroplast coupling factor induced by ion binding Formation of a magnesium-enzyme-phosphate complex. Biochemistry, 22, 2505-12. 

Chlorophyll b occurs as an accessory pigment of the light-harvesting systems in land plants and green algae, and comprises one-third (or less) of total chlorophyll. The biosynthesis of chlorophyll b has been an area of active research particularly regarding its compartmentalization in chloroplast membranes, identification of the gene for chlorophyllide a oxidase, and characterization of the enzymes involved. ... 

Although chlorophyll and chlorophyllin colorants seem to be easily obtained, in practice their production as natural food colorants is rather difficult. The sensitivity of chlorophylls to certain enzymes, heat, and low pH, and their low tinctorial strength greatly limit their manufacture and application as food additives, principally when the pigments are isolated from the protective environment of the chloroplasts. The well-known instability of chlorophylls prompted extensive research for developing... 

Bonk, M. et al., Chloroplast import of four carotenoid biosynthetic enzymes in vitro reveals differential fates prior to membrane binding and oligomeric assembly, Eur. J. Biochem. 247, 942, 1997. 

Amon, D.E., Copper enzymes in isolated chloroplast. Polyphenoloxidase in Beta vulgaris. Plant Physiol., 24, 1, 1949. 

Pyraflufen-ethyl is also used as the defoliant for cotton and as a desiccant for potatoes. Pyraflufen-ethyl is a novel inhibitor of protoporphyrinogen IX oxidase. Inhibition of this enzyme in chloroplasts causes accumulation of protoporphyrinogen IX, which results in peroxidation of foliar cell membrane lipids under the light and finally death of cells. 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

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