This is the N-terminal domain found in human spermine synthase (EC 2.5.1.22). The N-terminal domain, which forms the major part of the dimerization interface, shows a considerable structural similarity to the AdoMetDC-like fold (S-adenosylmethionine decarboxylase, the enzyme that forms the aminopropyl donor substrate), Pfam:PF02675 . Deletion of the N-terminal domain led to a complete loss of spermine synthase activity, suggesting that dimerization may be required for activity. The N-terminal domain (amino acids 1-117) includes seven beta-strands and two alpha-helices [1].

1: Crystal structure of human spermine synthase: implications of substrate binding and catalytic mechanism. Wu H, Min J, Zeng H, McCloskey DE, Ikeguchi Y, Loppnau P, Michael AJ, Pegg AE, Plotnikov AN; J Biol Chem. 2008;283:16135-16146. PMID:18367445