Biological Membrane Asymmetry and its Role in Bone Mineralization

The membranes of animal cells consist of a bilayer of lipid molecules (phospholipids and cholesterol) and embedded protein molecules. However, in living cells the phospholipids are not distributed equally between the bilayer's cytoplasmic and extracellular leaflets. Phosphatidylserine (PS) and...

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Bibliographic Details
Published inMacromolecular symposia. Vol. 396; no. 1
Main Author Clarke, Ronald J.
Format Journal Article
LanguageEnglish
Published Weinheim Wiley Subscription Services, Inc 01.04.2021
Subjects
Biomedical materials
Calcium
Calcium ions
Cell surface
Cholesterol
flippase
floppase
Hydroxyapatite
Lecithin
Lipids
Membrane proteins
Membranes
Metal ions
Mineralization
Osteoblasts
Phosphatidylcholine
Phosphatidylethanolamine
Phosphatidylserine
Phospholipids
Proteins
Pumps
scramblase
Skewed distributions
Sphingomyelin
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Summary:The membranes of animal cells consist of a bilayer of lipid molecules (phospholipids and cholesterol) and embedded protein molecules. However, in living cells the phospholipids are not distributed equally between the bilayer's cytoplasmic and extracellular leaflets. Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are concentrated in the membrane's inner leaflet, whereas phosphatidylcholine (PC) and sphingomyelin (SM) are concentrated in the outer leaflet. This non‐equilibrium situation is maintained by lipid pumps in the membrane, i.e., protein molecules which actively use energy in the form of ATP to transport lipids in both directions across the membrane. Pumps which transfer lipids from the external leaflet to the inner leaflet are termed flippases. Those that transfer lipids from the inner to the outer leaflet are termed floppases. The asymmetric distribution of PS is particularly important because of the negative charge of its polar head. This allows it to interact with divalent metal ions in the neighbouring solution. Although normally maintained on the cytoplasmic side of the membrane by a PS‐flippase, activation of scramblase proteins allows its distribution across the membrane to equilibrate, leading to PS exposure on the extracellular surface. In osteoblasts, the cells which produce bone, PS exposure also plays a key role in skeletal development. The negative charges of PS provides a template, attracting Ca2+ ions and promoting the deposition of hydroxyapatite Ca10(PO4)6(OH)2, the major mineral component of bone.
ISSN:1022-1360
1521-3900
DOI:10.1002/masy.202000243