In enzymology, a homospermidine synthase (spermidine-specific) () is an enzyme that catalyzes the chemical reaction spermidine + putrescine sym-homospermidine + propane-1,3-diamine Thus, the two substrates of this enzyme are spermidine and putrescine, whereas its two products are sym-homospermidine and propane-1,3-diamine. This enzyme belongs to the family of transferases, specifically those transferring aryl or alkyl groups other than methyl groups. The systematic name of this enzyme class is spermidine:putrescine 4-aminobutyltransferase (propane-1,3-diamine-forming). References EC 2.5.1 Enzymes of unknown structure