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. 2022 May 9;23(9):5266. doi: 10.3390/ijms23095266

Figure 1.

Figure 1

The formation of bilayer phospholipids by de novo pathways, methylation, and base-exchange reactions. Phosphorylation of choline by CK is the first step in the CDP:choline branchof the Kennedy pathway. Phosphocholine is then combined with CTP by CCT/PCYT1 to produce CDP:choline. This product is then coupled with DAG by CPT or CEPT to generate PC. PE is formed by the CDP:ethanolamine branch through analogous reactions that are catalyzed, sequentially, by ETNK, ECT/PCYT2, and EPT/SELENO1. PCYT1 and PCYT2 are the main regulatory enzymes of the Kennedy pathway. Additional PC can be produced by PE methylation using PEMT. PS is exclusively made from PC and PE by PTDSS1 and PTDSS2 base-exchange reactions. PS is decarboxylated to PE by PISD. Abbreviations: CK, choline kinase; CCT/PCYT1, phosphocholine cytidylyltransferase; CEPT, choline/ethanolamine phosphotransferase; CPT, choline phosphotransferase; DAG, diacylglycerol; ETNK, ethanolamine kinase; ECT/PCYT2, phosphoethanolamine cytidylyltransferase; EPT, ethanolamine phosphotransferase; PS, phosphatidylserine; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PISD, phosphatidylserine decarboxylase; PS, phosphatidylserine; PEMT, PE N-methyltransferase; PTDSS1, phosphatidylserine synthase 1; PTDSS2, phosphatidylserine synthase 2; SELENO1 (selenoenzyme 1) ethanolamine phosphotransferase 1-EPT1.