Catalyzes a base-exchange reaction in which the polar head group of phosphatidylethanolamine (PE) or phosphatidylcholine (PC) is replaced by L-serine (PubMed:19014349, PubMed:24241535). Catalyzes mainly the conversion of phosphatidylcholine (PubMed:19014349, PubMed:24241535). Also converts, in vitro and to a lesser extent, phosphatidylethanolamine (PubMed:19014349, PubMed:24241535). a 1,2-diacyl-sn-glycero-3-phosphoethanolamine + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + ethanolamine a 1,2-diacyl-sn-glycero-3-phosphocholine + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + choline Requires calcium ions (PubMed:19014349). Inhibited by exogenous phosphatidylserine (PubMed:24241535). Optimum pH for both PC and PE is between 7.0 and 7.5. Phospholipid metabolism; phosphatidylserine biosynthesis. Highly enriched in the mitochondria-associated membrane (MAM). The disease is caused by variants affecting the gene represented in this entry. Belongs to the phosphatidyl serine synthase family.