The function of PTDSS1 (phosphatidylserine synthase 1, Ensembl gene identifier ENSG00000156471) is as follows. 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). .