Catalyzes the formation of phosphatidylethanolamine (PtdEtn) from phosphatidylserine (PtdSer). Plays a central role in phospholipid metabolism and in the interorganelle trafficking of phosphatidylserine. a 1,2-diacyl-sn-glycero-3-phospho-L-serine + H(+) = a 1,2-diacyl-sn-glycero-3-phosphoethanolamine + CO2 Binds 1 pyruvoyl group covalently per subunit. Phospholipid metabolism; phosphatidylethanolamine biosynthesis. Heterodimer of a large membrane-associated beta subunit and a small pyruvoyl-containing alpha subunit. Anchored to the mitochondrial inner membrane through its interaction with the integral membrane beta chain. 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 autoendoproteolytic cleavage occurs by a canonical serine protease mechanism, 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 prosthetic group on the alpha chain. During this reaction, the Ser that is part of the protease active site of the proenzyme becomes the pyruvoyl prosthetic group, which constitutes an essential element of the active site of the mature decarboxylase. This protein may be expected to contain an N-terminal transit peptide but none has been predicted. Belongs to the phosphatidylserine decarboxylase family. PSD-B subfamily. Eukaryotic type I sub-subfamily.