Sulfate assimilation

Bick JA, JJ Dennis, GJ Zylstra, J Nowack T Leustek (2000) Identification of a new class of 5 adenylsulfate (APS) reductases from sulfate-assimilating bacteria. J Bacterial 182 135-142. 

Passera, C., Nicolao, L., Ferretti, M., Rascio, N., and Ghisi, R. (1991). Effect of humic substances of enzyme-activities of sulfate assimilation and chloroplast ultrastructure of maize leaves. Photosynthetica 25(1), 39-45. 

Sulfate assimilated py plankton is subsequently removed from the water column by sedimentation. Sediment trap data show that this is an important... 

Fig. 40. A proposed unified scheme of sulfate assimilation in algae. Adenylyl sulfate (APS) transfers the sulfo group via APS-sulfotransferase to form Car-S-SOr (Car = carrier), which is reduced further by thio-sulfonate reductase to Car-S-S which yields the thiol group of cysteine. In addition, if sulfite is released from Car-S-SOa (i.e., by thiol or from mutated sites) or if it enters the cell from outside, it can be reduced via a separate sulhte reductase. From Abrams and Schiff (57i).

As pointed out in the preceding section, sulfate assimilation in yeast has been shown to involve the activation of sulfate by ATP successively to adenosine 5 -phosphosulfate and once again to 3 -phosphoadenosine 5 -phosphosulfate. The latter is then reduced in the presence of NADPH to sulfite and 3, 5 -diphosphoadenosine (37 ). Enzymes catalyzing the... 

Alanine residues, glyceraldehyde-3-phos-phate dehydrogenase, 11,12 Alcohols, catalase smd, 388, 398, 401 Aldehydes, glyceraldehyde-3-phosphate dehydrogenase smd, 39 Algae, sulfate assimilation by, 279, 280 Alkali... 

It is now thought that PAPS is not involved in sulfate assimilation by plants. (Editors note.)... 

Smith, I.K. Regulation of sulfate assimilation in tobacco cells Plant Physiol. 66 (1980) 877-883. 

Metabolism of sulfate in plants, as in all sulfate-assimilating organisms, begins with the activation of sulfate by ATP to form APS. The reaction is catalyzed by the enzyme ATP sulfurylase (E.C. 2.7.7.4)... 

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... 

The various APS and PAPS hydrolases described in Secticm III could be of some significance in the regulation of sulfate assimilation. However, since the synthesis of APS and PAPS are known to occur in chloroplasts, these possibilities must remain speculative until the subcellular localization of the enzymes catalyzing Eqs. (4)-(6) are established especially since Burnell and Anderson (1973a) reported that nonspecific 3 -nucleotidase is not associated with chloroplasts. Recently, Sawhney and Nicholas (1976b) have reported that particulate fractions from Anabaena degrade PAPS to sulfate. ADP sul-furylase, which catalyzes the reaction... 

Most of the inorganic sulfate assimilated and reduced by plants appears ultimately in cysteine and methionine. These amino acids contain about 90% of the total sulfur in most plants (Allaway and Thompson, 1966). Nearly all of the cysteine and methionine is in protein. The typical dominance of protein cysteine and protein methionine in the total organic sulfur is illustrated in Table I by analyses of the sulfur components of a lower plant (Chlorella) and a higher plant (Lemna). Thede novo synthesis of cysteine and methionine is one of the key reactions in biology, comparable in importance to the reduction of carbon in photosynthesis (Allaway, 1970). This is so because all nonruminant animals studied require a dietary source of methionine or its precursor, homocysteine. Animals metabolize methionine via cysteine to inorganic sulfate. Plants complete the cycle of sulfur by reduction of inorganic sulfate back to cysteine and methionine, and are thus the ultimate source of the methionine in most animal diets (Siegel, 1975). 

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 ... 

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. 

As pointed out by Reuveny and Filner (1977), the fact that cysteine repression of sulfate adenylyltransferase in bacteria is complete, whereas in tobacco cells cysteine repression is incomplete may reflect the utilization of the sulfate assimilation pathway by bacteria mainly for sulfate reduction, whereas in plants sulfate assimilation is also required for synthesis of sulfate esters and sulfonolipids (de Meio, 1975). Complete repression by the product of one branch might be deleterious since it would deprive the plant of end-products of the other branch. 

The findings that in tobacco cells sulfur starvation prevents induction of nitrate reductase (P. Filner, unpublished observation cited in Reuveny and Filner, 1977), and that repression of nitrate reductase by a number of nonsulfur amino acids is antagonized by cysteine (Filner, 1966) led Reuveny and Filner (1977) to suggest that a product of the sulfate assimilation pathway may also play a positive role in control of nitrate assimilation. If so, a reciprocal relationship would exist between the nitrogen and sulfur assimilation pathways such that a product of each of these pathways is required as positive effector for the other. 

Adenylyisulfate reductases enzymes of sulfur metabolism which reduce either phosphoadenylylsul-fate (APS reductase) or adenylylsulfate. Adenylylsul-fate reductase (EC 1.8.99.2) is identical with one component of the sulfate reductase in sulfate assimilation, since adenylylsulfate is the donor of the sulfate group. Properties of some of these reductases are shown in the Table. In every case, the reductase is a complex of 3 components, an adenylylsulfate transferase (see Sulfate assimilation, Fig.1), a low-molecular-mass carrier, and the actual adenylylsulfate reductase. Phos-phoadenylylsulfate reductase from Saccharomyces cerevisiae requires NADPH, and has been partly purified. 

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