Superpathway polyamine biosynthesis (Saccharomyces cerevisiae)

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In S. cerevisiae, the biosynthesis of putrescine, spermidine, and spermine begins with two intermediates from amino acid metabolism: ornithine, an intermediate of arginine biosynthesis and degradation, and S-adenosylmethionine (AdoMet), a metabolite of methionine and a ubiquitous methyl group donor for many biological reactions. At the start of the pathway, ornithine is decarboxylated to putrescine by ornithine decarboxylase (Spe1p) while S-adenosylmethionine decarboxylase (Spe2p) decarboxylates AdoMet. Spermidine synthase (Spe3p) then catalyzes the transfer of an aminopropyl group from decarboxylated AdoMet to putrescine, converting the two substrates to 5-methylthioadenosine (MTA) and spermidine, respectively. In an analogous reaction, spermidine can go on to react with decarboxylated AdoMet via spermine synthase (Spe4p) to yield spermine and another molecule of MTA. Spermidine but not spermine is essential for growth in S. cerevisiae.

MTA, a major byproduct of the polyamine biosynthesis pathway, is also the principal substrate for the methionine salvage pathway. The methionine salvage pathway recycles MTA produced during polyamine biosynthesis or other cellular processes back into methionine. In S. cerevisiae, the polyamine biosynthesis pathway and the methionine salvage pathway together are estimated to account for 11-19% of the methionine used by a methionine auxotroph grown on limiting methionine.

Description adapted from https://pathway.yeastgenome.org/.