A group of polyamine biosynthetic enzymes involved in the fifth (last) step in the biosynthesis of spermidine from arginine and methionine which includes; spermidine synthase (EC: 2.5.1.16), spermine synthase (EC: 2.5.1.22) and putrescine N-methyltransferase (EC: 2.1.1.53) [PMID:9517003].

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 [PMID:11731804]. The larger C-terminal catalytic core domain consists of a seven-stranded beta-sheet flanked by nine alpha helices. This domain resembles a topology observed in a number of nucleotide and dinucleotide-binding enzymes, and in S-adenosyl-L-methionine (AdoMet)- dependent methyltransferase (MTases) [PMID:11731804].