Chloroplasts reductive sulfate assimilation

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 broad outline, the reduction and assimilation of inorganic sulfate and nitrate in plants have several features in common. Both processes entail 8 c reductions to inorganic forms (sulfide and ammonia, respectively) in energy-requiring reactions prior to incorporation into appropriate acceptor molecules. With the exception of the partial reduction of nitrate to nitrite in the cytoplasm, assimilation of sulfate and nitrite occurs in chloroplasts in reactions which are dependent on light for a supply of Fdred and ATP. However, the processes differ in many aspects of detail. For example ATP is required for activation of sulfate prior to reduction but in the nitrate assimilation pathway ATP is required after reduction for the incorporation of ammonia into glutamine. In addition, sulfate activation has no counterpart in nitrate reduction and, whereas sulfate remains bound to a carrier during reduction, the intermediates of nitrate remain free. 

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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