InterPro domain: IPR001045

The nearly ubiquitous polyamines (putrescine, spermidine and spermine) arepolycationic mediators of cell proliferation and differentiation whosefunctions likely provide both stability and neutralisation for nucleic acids.The following polyamine biosynthetic enzymes are evolutionary related [ 1 ]:

• Spermidine synthase ( 2.5.1.16 ) (putrescine aminopropyltransferase). It catalyzes the last step in the biosynthesis of spermidine from arginine and methionine; the conversion of putrescine to spermidine using decarboxylated S-adenosylmethionine as the cofactor.
• Spermine synthase ( 2.5.1.22 ) (spermidine aminopropyltransferase). It converts spermidine into spermine using decarboxylated S-adenosylmethionine as the cofactor.
• Putrescine N-methyltransferase ( 2.1.1.53 ). It catalyzes a step in the biosynthesis of nicotine in plants; the methylation of putrescine to N- methylputrescine using S-adenosylmethionine as the cofactor.

The Thermotoga maritima spermidine synthase monomer consists of two domains:an N-terminal domain composed of six beta-strands, and a Rossmann-like C-terminal domain. The larger C-terminal catalytic core domainconsists of a seven-stranded beta-sheet flanked by nine alpha helices. Thisdomain resembles a topology observed in a number of nucleotide anddinucleotide-binding enzymes, and in S-adenosyl-L-methionine (AdoMet)-dependent methyltransferase (MTases) [ 2 ].

1. Molecular cloning of plant spermidine synthases. Plant Cell Physiol. 39, 73-9
2. The crystal structure of spermidine synthase with a multisubstrate adduct inhibitor. Nat. Struct. Biol. 9, 27-31