Biosynthesis of Polyamines
PAs are predominantly derived from the amino acids ornithine (Orn) and methionine, while arginine and lysine serve as alternative, secondary sources of these metabolites (Fig. 2). The canonical biosynthesis pathway begins with Orn decarboxylation, by ornithine decarboxylase (ODC), to form Put. Spd and Spm are formed from Put via addition of aminopropyl groups. These are donated by the methionine derivative: decarboxylated S-adenosylmethionine (dcAdoMet) produced by S-adenosylmethionine decarboxylase (AdoMetDC). The addition of aminopropyl groups is mediated by spermidine and spermine synthases (SpdS and SpmS), (reviewed in Refs. [4], [6] and [7]). Put can alternatively be synthesized from arginine via arginine decarboxylase (ADC) and agmatinase (Fig. 2) (reviewed in Refs. [4], [6] and [7]). There are exceptions in which the canonical pathway is not entirely present; here, organisms use alternative pathways to synthesize PAs. Such is the case for Arabidopsis thaliana and Trypanosoma cruzi that lack an ODC gene [8] and [9]. A. thaliana synthesizes Put solely via the ADC route [9], whereas T. cruzi (the insect form) uptakes Put and Cad present in the excreta of its host insect [10] and [11].