Compartmentalization chloroplasts

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. ... 

It is generally accepted that chloroplasts possess an intact pathway of aromatic amino acid biosynthesis that is tightly regulated. In addition, the subcellular location of some aromatic-pathway isozymes has been shown to be in the cytosol, but whether an intact pathway exists in the cytosol has not yet been proven. The evidence bearing on aromatic amino acid compartmentation and regulation is reviewed, with particular emphasis given to the relationship between primary biosynthesis and secondary metabolism in the cytosol. 

Toxicity of protein Chloroplast compartmentalization minimizes adverse effects of protein May have serious effects in cytoplasm... 

Bagge P, Larsson C. 1986 Biosynthesis of aromatic aminoacids by highly purified spinach chloroplasts—compartmentation and regulation of the reactions. Physiol Plantarum 68 641-647. 

Compartmentation and Regulation of Starch Synthesis and Degradation in Chloroplasts... 

Unique subcellular compartmentation is also present in quinolizidine alkaloid biosynthesis, which occurs in the mesophyll chloroplasts of some legumes.158 One of the enzymes catalyzing the last two acylations of the pathway in Lupinus albus occurs in the cytoplasm, whereas the other resides in the mitochondria/59 Although the quinolizidine nucleus appears to be synthesized in the chloroplast, subsequent modifications can occur only after alkaloid intermediates are transported to the cytosol and mitochondia. Quinolizidine alkaloids appear to accumulate in vacuoles of epidermal cells where their defensive properties are most effective. 

The rate of photosynthesis does not depend on the amount of a single component (e.g., the activity of a particular enzyme). There is a wide range of possible regulatory factors, proven to exist in vitro, but the importance of which in vivo has still to be determined. In particular, there is a multitude of factors affecting the activity of the enzymes involved, with pH, ions, coenzymes, and metabolite effectors modulating the activity of every enzyme studied thus far. Compartmentation is the other key factor. The role of metabolite transport in the cell, particularly between chloroplast and cytosol, but also to and from mitochondria, vacuole, and other organelles, is now considered to be fundamental to the regulation of photosynthesis. In this chapter, we look at the factors considered to be of major importance... 

Plate 3 Compartmentation of biosynthesis and sequestration. Abbreviations SM, secondary metabolites GS-SM, conjugate of SM with glutathione NPAAs, non-protein amino acids ATP, adenosine triphosphate ADP, adenosine diphosphate mt, mitochondrium cp, chloroplast nc, nucleus 1, passive transport 2, free diffusion 3, H+/SM antiporter 4, ABC transporter for SM conjugated with glutathione 5, ABC transporter for free SM 6, H+-ATPase. (Fig. 1.4, p. 9)... 

During the light period, when COi is being fixed in the chloroplasts by the RPP pathway, it is likely that the mechanisms discussed above for C3 photosynthesis are also functional in CAM plants [19]. Additional control mechanisms are expected to provide for the efficient functioning of the diurnal cycle of CO2 fixation. The functioning of CAM cannot, however, be interpreted solely in terms of enzymolo-gy, but rather will involve cellular compartmentation of enzymes and metabolites together with intracellular transport processes [17]. 

Some of the most intriguing differences between the met-allomes of different cell types occur in subcellular organelles or vesicles. Eukaryotic cells in particular have carefully compartmentalized essential transition metals for specific biological purposes. The mitochondria and the chloroplast both contain high levels of metalloproteins relative to the cytoplasm and may have distinct metal quotas. Mammahan cerebrocortical neurons possess zinc-filled vesicles with labile zinc pools that approach... 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

Compartmentalization. A cell is not a stirred reaction vessel. In eukaryotic algae, for example, CO2 is fixed and initial carbohydrates are produced in the chloroplast (also called plastid). Fatty acids are produced in the chloroplast and then exported for use... 

Studies of the kinetics of [ S]0/ incorporation by C. sorokiniana (Giovanelli et al., 1978) and L. paucicostata (P. M. Macnicol, A. H. Datko, J. Giovanelli, and S. H. Mudd, unpublished) show that the soluble cysteine that is an intermediate on the pathway of sulfate assimilation is compartmentalized in a rapidly turning over pool which constitutes less than 2% of the total soluble cysteine in the cell. Recently it has been demonstrated that substantial portions of the cysteine synthases of leaves of spinach, peas and clover reside in the stroma of chloroplasts (Fankhauser ef al., 1976 Ng and Anderson, 1978). Chloroplastic and extrachloroplastic forms of cysteine synthase have been briefly reported (Fankhauser and Brunold, 1978). Earlier reports (Smith, 1972 Masada et al., 1975 Ascano and Nicholas, 1977) of cysteine synthase in a number of plant tissues claimed that the enzyme was predominantly soluble. The degree of intactness of the chloroplasts used in these studies was not determined, and a plausible explanation for recovery of cysteine synthase in the soluble fraction is that the chloroplasts were disrupted during isolation, releasing the enzyme. 

In view of these findings we decided to investigate the PPase enzyme with respect to chloroplast compartmentation. 

CO2, as well as O2 assimilation and evolution should should be simultaneously enhanced, which actually takes place in the investigation of Emerson s effect has been shown in the Table. On the other hand, the enhancement effect of photosynthesis by CO2 and O2 quantitatively differs from one another. The reactions of water photolysis are compartmentalized in the membrane of tilakoids in close proximity from ETC, while reactional centres of carboxyliration are in the strom of the chloroplasts and, thus, should be at greater distance from ETC. 

C- plants (9). One of the features of C. plants, is the presence of "Kranz" anatomy. There is compartmentalization of enzymes in "Kranz" anatomy viz PEPC in the cytosol of mesophyll cells and RuBISCO in the chloroplasts of the bundle sheath cells(ll). Accordingly in the present study, predominent activity of PEPC in the partially green maize callus cultures and lowest portion of young leaf lacking well developed chloroplasts can be explained. 

The subcellular compartmentation of terpenoid metabolism is a matter of controversy. Chloroplasts seem to contain the entire terpenoid biosynthetic pathway and appear to be autonomous in regard to terpene biosynthesis. Carotenoids and the side chains of chlorophyll are synthesized in the chloroplasts. Sterols are synthesized in the cytoplasm (Gershenzon and Croteau, 1990). 

There is no doubt that subcellular compartmentation of hormones exists. Evidence for subcellular compartmentation of plant hormones is now being reported (e.g., de la Fuente and Leopold 1972, Loveys 1977, Ohlrogge et al. 1980). Water stress induces a large rise in extrachloroplastic ABA as opposed to that inside the chloroplasts (Loveys 1977, Walton 1980). Undoubtedly, more evidence for compartmentation will emerge as hormone target sites are analyzed. Compartmentation of hormones should be viewed as a possible complication in the analysis of hormones extractable or diffusible from tissues. 

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