Synonym(s): Spermidine aminopropyltransferase<p>A group of polyamine biosynthetic enzymes involved in the fifth (last) step in thebiosynthesis of spermidine from arginine and methionine which includes; spermidine synthase (<db_xref db="EC" dbkey="2.5.1.16"/>), spermine synthase (<db_xref db="EC" dbkey="2.5.1.22"/>) and putrescine N-methyltransferase (<db_xref db="EC" dbkey="2.1.1.53"/>) [<cite idref="PUB00004525"/>].</p><p>The <taxon tax_id="2336">Thermotoga maritima</taxon> spermidine synthase monomer consists of two domains:an N-terminal domain composed of six beta-strands, and a Rossmann-like C-terminal domain [<cite idref="PUB00015422"/>]. 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) [<cite idref="PUB00015422"/>].</p>