BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

6. PHYSIOLOGY OF METABOLISM

6.7. Sulfate Assimilation

Plants absorb sulfur in the form of sulfate (SO₄^2-, oxidation state +VI) and reduce it to sulfide (S^2-, oxidation state -II). These reactions occur predominantly in METABOLISM/14.html">Chloroplasts and require products of Photosynthesis (ATP, ferredoxin); however, in higher plants, this process can also take place in The ROOT System, where its intracellular localization remains insufficiently clear. Unlike nitrogen, which is exclusively incorporated into Organic compounds in its reduced form, sulfur can be integrated into certain organic compounds in an oxidized form as well—for example, in the synthesis of sulfolipids (see 1.5.2, Fig. 1.21), glucosinolates (see 6.16.4), and sulfated Flavonoids. Nevertheless, the predominant portion of sulfur is utilized in the form of sulfide. Sulfur with an oxidation state of -II is present in Amino Acids and Proteins, the reductant Glutathione, certain Coenzymes, and the iron-sulfur centers of redox proteins (such as ferredoxin, see Fig. 6.56). Only Bacteria, Fungi, and green plants are capable of sulfate assimilation, whereas animals must obtain reduced sulfur compounds through their diet.

The reduction of sulfate proceeds in two stages, much like the reduction of nitrate:

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Contrary to previous assumptions, not only in fungi and bacteria, but also in green plants, the intermediate sulfite (SO₃^2-) is formed in a free state during this process and is subsequently reduced to sulfide (S^2-) (Fig. 6.90).

Fig. 6.90. Photosynthetic sulfate assimilation: GSH — reduced glutathione; GSSG — oxidized glutathione (see Fig. 6.62)

The sequence of metabolic reactions begins with The formation of activated sulfate from ATP and sulfate:

The equilibrium of this strongly endergonic reaction lies heavily on the side of the reactants. Through the coupling (see 6.1.5) of two exergonic reactions

PP_i + H₂O = 2P_i ∆G⁰′ = -33.5 kJ • mol^-1

and

APS + ATP = PAPS + ADP G⁰' = -25 kJ • mol^-1

the overall reaction of sulfate activation becomes exergonic:

SO₄^2- + 2ATP = PAPS + 2P_i + ADP G⁰' = -13.5 kJ • mol^-1.

In this equilibrium reaction, along with the predominant PAPS (3'-phosphoadenosine 5'-phosphosulfate, Fig. 6.90), very small amounts of APS are present. The activation of sulfate to APS and PAPS involves the formation of a phosphoanhydride bond (G⁰' = -71 kJ • mol^-1). In this form, the sulfate group can be readily reduced. The enzyme responsible for this primarily reacts with APS, which is why PAPS acts as a reserve pool of active sulfate. APS reductase transfers 2 electrons to the sulfur in APS, releasing sulfite (SO₃^2-). The electrons are supplied by reduced glutathione. Upon the uptake of 6 electrons, sulfite is reduced to sulfide without notable intermediates (see Fig. 6.90), with electrons provided by ferredoxin. This reaction not only bears a formal resemblance to nitrite reduction: sulfite reductase is also structurally similar to nitrite reductase and catalyzes The transfer of 6 electrons using the same Fe₄S₄-siroheme cofactor.

The resulting hydrogen sulfide is used directly for the synthesis of Cysteine. The high substrate affinity of cysteine synthase ensures that hydrogen sulfide does not accumulate within The Cell. The reaction proceeds via the thiolysis of the acceptor SH-group of O-acetylserine, which is why the enzyme is also referred to as O-acetylserine (thiol)-lyase (or O-acetylserine sulfhydrylase); it contains Pyridoxal phosphate as a prosthetic group. Cysteine serves as the Starting Material for The Biosynthesis of Methionine and other low-molecular-weight thiols, such as glutathione or phytochelatins (see 6.2.2.4). According to current data, cysteine is the source of acid-labile sulfur in iron-sulfur centers (see Fig. 6.56).