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flippase
Flippases (rarely spelled flipases) are transmembrane lipid transporter proteins located in the membrane responsible for aiding the movement of phospholipid molecules between the two leaflets that compose a cell's membrane (transverse diffusion, also known as a "flip-flop" transition). The possibility of active maintenance of an asymmetric distribution of molecules in the phospholipid bilayer was predicted in the early 1970s by Mark Bretscher. Although phospholipids diffuse rapidly in the plane of the membrane, their polar head groups cannot pass easily through the hydrophobic center of the bilayer, limiting their diffusion in this dimension. Some flippases - often instead called scramblases - are energy-independent and bidirectional, causing reversible equilibration of phospholipid between the two sides of the membrane, whereas others are energy-dependent and unidirectional, using energy from ATP hydrolysis to pump the phospholipid in a preferred direction.〔Holthuis, Joost CM, and Levine, T.P.,"Lipid traffic: floppy drives and a superhighway." Nature reviews Molecular cell biology 6.3 (2005): 209-220.()〕 Flippases are described as transporters that move lipids from the exoplasmic to the cytosolic face, while floppases transport in the reverse direction.〔
Many cells maintain asymmetric distributions of phospholipids between their cytoplasmic and exoplasmic membrane leaflets.〔Lodish, H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J. (2004). Molecular Cell Biology, 5th, New York: WH Freeman.〕 The loss of asymmetry, in particular the appearance of the anionic phospholipid phosphatidylserine on the exoplasmic face, can serve as an early indicator of apoptosis.〔Castegna A, Lauderback CM, Mohmmad-Abdul H, Butterfield DA. (2004). Modulation of phospholipid asymmetry in synaptosomal membranes by the lipid peroxidation products, 4-hydroxynonenal and acrolein: implications for Alzheimer's disease. ''Brain Res'' 1004(1-2):193-7.〕 This effect has been observed in neurons as a response to amyloid beta peptides, thought to be a primary cause of the neurodegenerative effects of Alzheimer's disease.〔Mohmmad Abdul H, Butterfield DA. (2005). Protection against amyloid beta-peptide (1-42)-induced loss of phospholipid asymmetry in synaptosomal membranes by tricyclodecan-9-xanthogenate (D609) and ferulic acid ethyl ester: implications for Alzheimer's disease. ''Biochim Biophys Acta'' 1741(1-2):140-8.〕
==See also==
*Scramblase
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