Superpathway of serine and glycine biosynthesis (Saccharomyces cerevisiae)

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There are several pathways for serine and glycine biosynthesis in S. cerevisiae. When glucose is used as the carbon source, glycine can be synthesized from threonine via threonine aldolase (Gly1p) in the threonine pathway, while serine can be synthesized from the glycolytic intermediate 3-phosphoglycerate in the phosphoglycerate pathway. Expression of the genes involved in the latter pathway decreases upon depletion of glucose. When ethanol or other non-fermentable substrates are used as the carbon source, S. cerevisiae can alternatively synthesize glycine from glyoxylate via alanine glyoxylate aminotransferase (Agx1p). Glycine and serine produced in these pathways can be interconverted to one another via the tetrahydrofolate-dependent serine hydroxymethyltransferases Shm1p and Shm2p. Between these, the cytoplasmic isoform Shm2p serves as the major source of glycine (via serine breakdown) in serine-rich conditions. The mitochondrial isoform Shm1p may also contribute to glycine synthesis, however it preferentially catalyzes serine synthesis from glycine under conditions of serine starvation. Description from https://pathway.yeastgenome.org/.