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The Glycerate and Phosphorylated Pathways of Serine Synthesis in Plants: The Branches of Plant Glycolysis Linking Carbon and Nitrogen Metabolism - PubMed

  • ️Mon Jan 01 2018

Review

The Glycerate and Phosphorylated Pathways of Serine Synthesis in Plants: The Branches of Plant Glycolysis Linking Carbon and Nitrogen Metabolism

Abir U Igamberdiev et al. Front Plant Sci. 2018.

Erratum in

Abstract

Serine metabolism in plants has been studied mostly in relation to photorespiration where serine is formed from two molecules of glycine. However, two other pathways of serine formation operate in plants and represent the branches of glycolysis diverging at the level of 3-phosphoglyceric acid. One branch (the glycerate - serine pathway) is initiated in the cytosol and involves glycerate formation from 3-phosphoglycerate, while the other (the phosphorylated serine pathway) operates in plastids and forms phosphohydroxypyruvate as an intermediate. Serine formed in these pathways becomes a precursor of glycine, formate and glycolate accumulating in stress conditions. The pathways can be linked to GABA shunt via transamination reactions and via participation of the same reductase for both glyoxylate and succinic semialdehyde. In this review paper we present a hypothesis of the regulation of redox balance in stressed plant cells via participation of the reactions associated with glycerate and phosphorylated serine pathways. We consider these pathways as important processes linking carbon and nitrogen metabolism and maintaining cellular redox and energy levels in stress conditions.

Keywords: glycerate serine pathway; glycolysis; phosphorylated serine pathway; plastid; γ-aminobutyric acid (GABA).

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Figures

FIGURE 1
FIGURE 1

Non-phosphorylated (glycerate) and phosphorylated (phosphohydroxypyruvate) pathways of serine formation. Enzymes: 1, PGA phosphatase; 2, glycerate dehydrogenase; 3, serine: glyoxylate and serine: pyruvate aminotransferase; 4, PGA dehydrogenase; 5, phosphoserine: 2-oxogutarate aminotransferase; 6, phosphoserine phosphatase. 2-OG – 2-oxoglutarate. In all figures amino acids are depicted in blue, adenylates, phosphate, and pyridine nucleotides – in red.

FIGURE 2
FIGURE 2

Pathways of serine metabolism. In the reaction catalyzed by serine hydroxymethyltransferase (1), serine is converted to glycine and 5,10-methylene-tetrahydrofolate (5,10-CH2-THF). Glycine is transaminated to glyoxylate by serine: glyoxylate or glutamate: glyoxylate aminotransferase (2). Glyoxylate is reduced to glycolate by glyoxylate reductase (3). 5,10-CH2-THF is converted to 10-formyl-tetrahydrofolate (10-CHO-THF) by the bifunctional enzyme methylenetetrahydrofolate dehydrogenase – methenyltetrahydrofolate cyclohydrolase (4), the intermediate is 5,10-methenyl-tetrahydrofolate (5,10-CH = THF). 10-CHO-THF is converted to formate and THF by formate-tetrahydrofolate ligase (5), the reaction yields ATP. Formate can be oxidized to CO2 by formate dehydrogenase (6) or reduced to formaldehyde by formaldehyde dehydrogenase which is also S-nitrosoglutathione reductase (alcohol dehydrogenase type III) (7). Formate can be formed from glyoxylate in a non-enzymatic reaction with hydrogen peroxide (8), glyoxylate in turn can be formed from formate in a putative glyoxylate synthetase reaction (not shown).

FIGURE 3
FIGURE 3

Putative γ-aminobutyrate-isocitrate cycle. Glutamate is decarboxylated by Ca-dependent glutamate decarboxylase (1), the reaction consumes proton and yields γ-aminobutyric acid (GABA). GABA is transaminated to succinic semialdehyde (SSA) by aminotransferases using glyoxylate or pyruvate (2). SSA is oxidized to succinate by SSA dehydrogenase (3). Succinate in the reaction with glyoxylate forms isocitrate, the reaction is catalyzed by the cytosolic form of isocitrate lyase (4). The latter is oxidized to 2-oxogutarate (2-OG) by isocitrate dehydrogenase (5). 2-OG is transaminated to glutamate by aminotransferases using glycine or alanine (6), use of other amino donors such as serine or aspartate is also possible (not shown). SSA can be converted to γ-hydroxybutyrate (γ-HBA) by SSA reductase which is also glyoxylate reductase (7).

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