Chloroplasts serine synthesis

In the light in the leaves of C3 plants, the major role of serine metabolism is in photorespiration. However, serine can also act as a precursor of cysteine. There is therefore a requirement for serine synthesis through a route not related to photorespiration. Shingles etal. (1984) were able to show that chloroplasts had the capacity to convert formate and glycine to serine, provided that they were supplied at high concentrations. Kleczkowski and Givan (1988) have discussed in detail other potential routes of serine synthesis. 

Phosphorylation of serine residue(s) of the j8-subunit of the carboxyltransferase unit occurs in pea chloroplasts incubated in the light [20]. Alkaline phosphatase treatment reduces ACC activity in parallel to removal of phosphate groups from ACC. This activation by phosphorylation is opposite to the inhibition of animal ACC by phosphorylation but is consistent with the increase in ATP concentration and rates of fatty acid synthesis in chloroplasts in the light and the activation of other plastid enzymes by phosphorylation. These results suggest that the CT subunit reaction is rate determining for overall ACC activity, at least for the multisubunits enzyme of dicots. 

Two separate pathways converge in the reaction catalyzed by cysteine synthase the reductive assimilation of sulfate to sulfide, and the synthesis of OAS. All the reactions of reductive sulfate assimilation are present in chloroplasts, but the quantitative significance of these organelles in providing the sulfur precursor for cysteine synthesis is not clear (see Anderson, this volume. Chapter 5). A recent review (Givan and Harwood, 1976) indicates that serine is formed in chloroplasts fairly directly from intermediates of the carbon reduction pathway, but that this synthesis also requires extrachloro-plastic factors yet to be defined. Serine acetyltransferase has been reported in a fraction consisting mainly of mitochondria (Smith and Thompson, 1%9 ... 

In summary, synthesis of serine from CO requires the cooperation of chloroplasts and extrachloroplastic components. It is not known whether serine acetyltransferase is chloroplastic, but at least some of this enzyme activity is present in particles that are probably mitochondria. Acetyl-CoA may be generated in both chloroplasts and mitochondria. Reductive assimilation of sulfate to sulfide, and the sulfhydration of OAS occur in chloroplasts, but the quantitative significance of these reactions in chloroplasts remains to be assessed. 

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