Pyridoxal phosphate (PLP)-dependent decarboxylase involved in the biosynthesis of norcobamides, cofactors in the tetrachloroethene reductive dehalogenase PceA of S.multivorans. Catalyzes the decarboxylation of L-serine O-phosphate to ethanolamine O-phosphate, the precursor for the linkage between the nucleotide loop and the corrin ring in norcobamide. Less active with L-threonine phosphate. No activity with L-serine or L-threonine. Has no aminotransferase activity as no production of L-glutamate with L-histidinol phosphate and 2-oxoglutarate as substrates (PubMed:31325159). Complements growth defects in the S.enterica cobD deletion mutant, but of the cobamides, the norpseudo-vitamin B12 (norpseudo-B12) rather than the pseudo-B12 is produced in the mutant. However, addition of L-threonine phosphate to the culture minimal medium of the mutant results in formation of also the pseudo-B12, indicating the dual substrate specificity of this enzyme (PubMed:27274028). H(+) + O-phospho-L-serine = CO2 + phosphoethanolamine Cofactor biosynthesis. Homodimer. Belongs to the class-II pyridoxal-phosphate-dependent aminotransferase family.