Spermine synthase (, spermidine aminopropyltransferase, spermine synthetase) is an enzyme that converts spermidine into spermine. This enzyme catalyses the following chemical reaction
Spermine synthase is an enzyme involved in polyamine biosynthesis. In humans, it is coded by SMS (gene), but it is present in all eukaryotes and plays a role in a variety of biological functions in plants. Its structure consists of two identical monomers of 41 kDa with three domains each, creating a homodimer formed via dimerization. The interactions between one of the three domains, the N-terminals of the monomers, is responsible for dimerization as that is where the active site is located; the central terminal consisting of four ò- strands structurally forming a lid for the third domain, the C-terminal domain.