FIGURE 1.

Overview of serine biosynthesis pathways in plants. In plants, serine can be synthesized via three pathways: the phosphorylated serine biosynthesis pathway, the photorespiratory pathway and the glycerate pathway. The first reaction within the phosphorylated serine biosynthesis pathway is catalyzed by the phosphoglycerate dehydrogenase (PGDH) enzyme. The PGDH enzyme oxidizes 3-phosphoglycerate (3-PGA) to 3-phosphohydroxypyruvate (3-PHP) by reducing NAD to NADH(H). In the next step the amino group of glutamate is transferred to 3-PHP by the phosphoserine aminotransferase (PSAT) enzyme, yielding phosphoserine (Pser) and 2-oxoglutarate (2-OG). Finally, Pser is dephosphorylated to serine by the phosphoserine phosphatase (PSP). The photorespiratory pathway starts with the conversion of 2-phosphoglycolate (2-PG) to glyoxylate. The glyoxylate glutamate aminotransferase (GGAT) transfers the amino group from glutamate to glyoxylate yielding glycine and 2-OG. Glycine is further decarboxylated by the glycine decarboxylase complex (GDC) releasing CO₂ and NH₃ and yielding one molecule of methylated tetrahydrofolate (5,10-CH₂-THF). The methyl group of 5,10-CH₂-THF is transferred to an additional glycine to form serine catalyzed by the serine hydroxymethyltransferase (SHMT). The third serine biosynthesis pathway catalyzes serine from 3-PGA. 3-PGA is dephosphorylated to glycerate by the 3-PGA phosphatase (PGAP). Glycerate is further oxidized to hydroxypyruvate (HP) and finally converted to serine by transferring the amino group from alanine. The last reaction is catalyzed by the alanine – HP aminotransferase (AH-AT).