Sulfate assimilative reduction

Sulfate reduction. All plants, animals, and bacteria metabolize sulfur in order to synthesize amino acids such as cysteine and methionine. The sulfur may be assimilated as sulfate or as organic molecules containing sulfate. The reduction of sulfate in biosynthesis is termed assimilatory sulfate reduction and can take place in aerobic or anaerobic environments (cf. Goldhaber and Kaplan 1974 Rheinheimer 1981 Cullimore 1991). 

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 (S7 ). Enzymes catalyzing the 6-electron reduction of sulfite to sulfide have been observed in bacteria (339, 38S-397), yeast (398-401), fungi (40S-404), and higher plants (3S9, 405). These enzymes may be divided into two classes depending on... 

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 ... 

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. 

Sulfate reduction reduction of the sulfote ion S04 " to the sulfide ion S, in which the hexavalent, positive sulfur of the sulfate is converted to the divalent, negative form. S.r. must be preceded by Sulfate activation (see). The substrate of enzymatic S.r. is therefore either adenosine phosphosulfate (APS) or phosphoadenosinephosphosulfate (PAPS). The enzy-mology of S.r. has been studied in particular in enzyme preparations from baker s yeast (Saccharomy-ces cerevisiae) and the anaerobic bacterium Desulfo-vibrio desulfuricans. The former organism performs assimilatory S.r. (see Sulfate assimilation), Ae latter dissimilatory S.r. (see Sulfate respiration). 

Thus, they concluded that ATP-sulfurylase was subject to feed-forward stimulation by a reaction product of nitrate reduction and that nitrate reductase was subject to an analogous feed forward by a product of sulfate, thus coordinating the first step of each of the two pathways. Some support for this mechanism has come fi om experiments with cell cultures of Ipomea (Zink, 1984) and from measurements of ATP-sulfurylase and nitrate reductase in the leaves of whole tobacco plants (Barney and Bush, 1985), though in the latter study the response of ATP-sulfurylase to nitrogen stress was far less pronounced than the response of nitrate reductase to sulfur stress. However, as discussed below, APS sulfo-transferase rather than ATP-sulfurylase appears to be the principal determining site for the feed-forward control of sulfate assimilation by nitrogen assimilation products in Lemna. 

Cysteine not only is an essential constituent of proteins but also lies on the major route of incorporation of inorganic sulfur into organic compounds.443 Autotrophic organisms carry out the stepwise reduction of sulfate to sulfite and sulfide (H2S). These reduced sulfur compounds are the ones that are incorporated into organic substances. Animals make use of the organic sulfur compounds formed by the autotrophs and have an active oxidative metabolism by which the compounds can be decomposed and the sulfur reoxidized to sulfate. Several aspects of cysteine metabolism are summarized in Fig. 24-25. Some of the chemistry of inorganic sulfur metabolism has been discussed in earlier chapters. Sulfate is reduced to H2S by sulfate-reducing bacteria (Chapter 18). The initial step in assimilative sulfate reduction, used by... 

Sulfur isotopic measurements can shed light on the origin of sulfur in coal. The 34S/32S ratio depends on the source of sulfur and the geologic processes involved during coal formation. For example, isotopic compositions are different for the two principal sources of sulfur in coal 1) the sulfur preserved from the precursor plant material, and 2) the sulfur derived from the bacterial reduction of dissolved sulfate in ambient waters. Plant assimilation of sulfur results in a slight depletion of 34S (4-4.5%c) relative to the 834S in the dissolved sulfate source (102.103). In contrast, the dissimilatory bacterial reduction of sulfate results in a large isotopic fractionation sulfide sulfur can be depleted as much as 60%o in the heavy isotope (89.104-106). 

Sediments deposited in Flodelle Creek spring pool and the Great Lakes have similar and relatively uncomplicated sulfur geochemistry that is controlled by two processes. These processes are the assimilation of sulfur into living biota and its subsequent deposition as organosulfur when the organism dies, and the complete reduction of the pore-water sulfate to H2S that forms sulfide minerals. Low dissolved sulfate concentrations limit the amount of sulfide minerals formed. The 834S value of most of the Smin is essentially the same as the dissolved sulfate. The possible exceptions are minerals formed in sediment from which some 34S-depleted H2S had diffused. 

The most important metabolic reaction is the assimilation of sulfur into organic forms which ultimately require the reduction of oxidized sulfur to the oxidation level of sulfide. This reduction is effected by the majority of microorganisms (bacteria, algae, fungi) and plants and, because of its abundance, sulfate is the dominant precursor of reduced sulfur. Pathways of assimilatory sulfate reduction are discussed briefly in Chapter 6.2 and depicted in Fig. 6.2.1 (p. 317). 

Figure 28. Hypothetical anaerobic nitrogen cycle based on the following thermodynamically permissible reactions (1) ammonium oxidation to dinitrogen by carbon dioxide,. sulfate or ferric iron (no evidence at present, possibly kinetically limited) (2) dinitrogen fixation by various organic and inorganic reductants (known) (3) ammonium oxidation by nitrite or nitrate producing dinitrogen (known) (4) denitrification (known) (5) nitrite or nitrate respiration (known) (6) ferric iron oxidation of ammonium to nitrite or nitrate (no evidence at present) (7) nitrate assimilation (known) (8) ammonium assimilation and di.s,similation (known) (Fenchel etai, 1998).

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