H(+) + L-2,4-diaminobutanoate = CO2 + propane-1,3-diamine Amine and polyamine biosynthesis; 1,3-diaminopropane biosynthesis; 1,3-diaminopropane from L-aspartate 4-semialdehyde: step 2/2. Belongs to the group II decarboxylase family.