Catalyzes the decarboxylation of S-adenosylmethionine to S-adenosylmethioninamine (dcAdoMet), the propylamine donor required for the synthesis of the polyamines spermine and spermidine from the diamine putrescine. H(+) + S-adenosyl-L-methionine = CO2 + S-adenosyl 3-(methylsulfanyl)propylamine Binds 1 pyruvoyl group covalently per subunit. Allosterically activated by metal cations, which are absolutely required for activity. The presumed physiological activator is Mg(2+), but can also be activated in vitro by other divalent cations such as Mn(2+), Fe(2+) and Ca(2+), by the monovalent cation Li(+), and by trivalent cations such as Eu(3+), Tb(3+) and Gd(3+). Competitively inhibited by methylglyoxal bis-guanylhydrazone. Also inhibited by Zn(2+), inhibition may be due to interaction with the active site cysteine. Inactivated by treatment with the imine reductant NaCNBH(3) only in the presence of substrate. Optimum pH is 7.4. Active from pH 6.7 to 8.5. Amine and polyamine biosynthesis; S-adenosylmethioninamine biosynthesis; S-adenosylmethioninamine from S-adenosyl-L-methionine: step 1/1. Heterooctamer of four alpha and four beta chains arranged as a tetramer of alpha/beta heterodimers. Is synthesized initially as an inactive proenzyme. Formation of the active enzyme involves a self-maturation process in which the active site pyruvoyl group is generated from an internal serine residue via an autocatalytic post-translational modification. Two non-identical subunits are generated from the proenzyme in this reaction, and the pyruvate is formed at the N-terminus of the alpha chain, which is derived from the carboxyl end of the proenzyme. The post-translation cleavage follows an unusual pathway, termed non-hydrolytic serinolysis, in which the side chain hydroxyl group of the serine supplies its oxygen atom to form the C-terminus of the beta chain, while the remainder of the serine residue undergoes an oxidative deamination to produce ammonia and the pyruvoyl group blocking the N-terminus of the alpha chain. Spermidine-deficient mutants show a small decrease in growth rate and increased sensibility to superoxide toxicity. Belongs to the prokaryotic AdoMetDC family. Type 2 subfamily.