Practice question

Maintenance of nonrandom phospholipid distribution across leaflets relies on specialized lipid translocators rather than hydrolytic enzymes. Flippases are P4 ATPases such as ATP11C and ATP11A complexed with CDC50A chaperone that consume ATP to move phosphatidylserine and phosphatidylethanolamine inward against gradient maintaining inner negativity. Floppases are ABC transporters ABCA1 ABCB1 ABCB4 that export phosphatidylcholine and cholesterol outward crucial for HDL biogenesis and bile secretion. Scramblases like TMEM16F ANO6 and Xkr8 family are calcium activated or caspase activated providing bidirectional nonselective pathways randomizing composition during platelet coagulation and apoptosis for phagocytosis. These activities are probed with NBD labeled phosphatidylserine internalization assays N ethylmaleimide inhibition and ionomycin calcium triggering. Lipases including phospholipase A2 cleaving sn two acyl, PLC hydrolyzing PIP2 to DAG IP3, PLD generating PA are catabolic signaling enzymes altering lipid identity but not transporting intact lipid across bilayer. Hence lipase unrelated to asymmetry maintenance distinguishing translocation from hydrolysis preventing confusion interpreting knockout phenotypes and drug targets in cell biology.