Sulfate assimilation bound pathway

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. 

Fig. 2. Summary of the free and bound pathways of sulfate assimilation in plants. Some related reactions and points of entry of several forms of inorganic sulfur are also shown. The reaction sequence catalyzed by (1) ATP sulfurylase, (2) APS sulfotransferase, (3) thiosulfonate reductase, and (4) cysteine synthase constitutes the bound sulfate assimilation pathway. The synthesis of OAS is catalyzed by (5) serine transacetylase. The reaction sequence (I), (6)-(9)or (1), (2), (10), (8), (9) constitutes the free pathway reactims (7) and (10) are nonenzymatic, (6) is catalyzed by APS sulfotransferase, (8) by sulfite reductase, and (9) by cysteine synthase. APS and PAPS are interrelated via (11) APS kinase and (12) NDP phophohydrolase. APS can be hydrolyzed via (13) APS sulfohydrolase or (14) APS cyclase.

Sulfur Fluxes for Some Reactions of the Free and Bound Sulfate Assimilation Pathways in Intact Chloroplasts... 

Done, 1978) although it is not known whether this is sufficient to satisfy the nitrogen assimilation requirements of the cell (however see Miffin and Lea, this volume. Chapter 4). Notwithstanding this uncertainty, then, since an atomic ratio for N S of about 30 is considered adequate in plant material (Epstein, 1972), a theoretical sulfur flux for sulfate to cysteine of approximately 0.4 /Ag atoms mg/Chl h might be expected. The data shown in Table I suggest that the free pathway, if operative in chloroplasts, could account for this rate. The sulfur flux for sulfate to cysteine in intact spinach chloroplasts is three orders of magnitude less than the assumed theoretical rate (Table I). Some rates for the incorporation of sulfate into intermediates of the bound pathway have been reported for osmotically shocked spinach chloroplasts (Schwenne/ al., 1976) but this data does not indicate the flux capacity of the component reactions. 

Fig. 3. Regulation of the bound pathway for the assimilation of sulfate into cysteine and associated processes. Carrier refers to an endogenous thiol of uncertain identity in higher plants. Enzymes associated with the sulfate assimilation pathway and the synthesis of O-acetylseiine are (1) high-ailinity sulfate uptake mechanism, (2) ATP-sulfurylase, (3) adenosine S -phosphosulfate (APS) sulfotransferase, (4) organic thiosulfate reductase, (5) cysteine synthase, and (6) serine transacetylase. Cysteine sulfhydrase (7), an enzyme of cysteine catabolism, and nitrate reductase (8), the first enzyme of the nitrate assimilation pathway, are also shown. Inhibitory control of the pathways is shown by discontinuous lines (----) and enhancement by continuous lines (------).

Fig. 4. Intercellular location of the enzymes of the bound and free pathways for the assimilation of inorganic sulfur in maize, a C4 plant. Sulfate is delivered to the leaf by the vascular bundle (V.B.) and assimilated via the bound pathway in the bundle sheath cells. Assimilation of free sulfite (including SO2) and free sulfide (including H2S) occurs via the free pathway in mesophyll cells. C, Carrier of the bound pathway. (Adapted from Schmutz and Brunold, 1985.)...

The sulfur moiety of cysteine is derived ultimately by the reductive assimilation of inorganic sulfate. Sulfate can be reduced in plants by two pathways. One pathway involves free sulfite as an intermediate which is reduced by sulfite reductase to form free sulfide. The other involves carrier-bound sulfite (carrier-S-SOj) which is reduced by thiosulfonate reductase to yield carrier-bound sulfide (carrier-S-S ). Although the relative physiological importance of the two pathways has not been firmly established, the indispensability of thiosulfonate reductase (even in the presence of sulfite reductase) for sulfate reduction in Chlorella mutants indicates the physiological importance of bound sulfite for this organism (Schmidt et al., 1974). Further details of the reduction of sulfate are presented in Chapter 5. 

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