The cytoplasmic membrane of erythrocytes consists of the bilayer of phospholipids, while proteins either “float” on the surface of membranes, or permeate lipids, providing strength and viscosity of the membranes.
The outer layer is formed by glycoproteins with branched oligosaccharide complexes, which are terminal sections of the group antigens of the blood. Phosphatidylcholine, sphingomyelin and unesterified cholesterol make up the lipid component of the outer layer. Lipids of the outer layer of the erythrocyte membrane play an important role in ensuring the homeostasis of the membrane, the selectivity of its permeability to various substrates and ions. Together with phospholipids, cholesterol regulates the activity of membrane-bound enzymes by changing the viscosity of the membrane, and also participates in the modification of the secondary structure of enzymes. Increased concentration of sphingomyelin results in decreased fluidity of the erythrocyte membrane and changes in membrane properties.
The middle bilayer of the erythrocyte membrane is represented by hydrophobic “tails” of polar lipids. Integral proteins that include enzymes, receptors, and transport proteins are active only if they are present in the hydrophobic part of the bilayer, where they can acquire the spatial configuration required for protein activity. Therefore, any changes occurring in the lipid composition of the erythrocyte membrane always lead to a change in its fluidity and functional impairment of integral proteins.
The inner layer of the erythrocyte membrane faces the cytoplasm and consists of proteins spectrin and actin. Flexible elongated molecules of spectrin that binds to microfilaments of actin and lipids of the inner surface of the membrane, form the skeleton of the erythrocyte. A small percentage of lipids found in the inner layer of the erythrocyte membrane is represented by phosphatidylethanolamine and phosphatidylserine. The presence of the spectrin determines the mobility of proteins that retain double bile lipids. Long, flexible heterodimers formed by spectrin through the lateral association create a network to the cell membrane at two sites, one mediated by ankyrin coupling with spectrin to Band 3, and the second one is mediated by protein 4.1 coupling junctional complex to Glycophorin C. Glycophorin is one of the most important glycoproteins that is located on both the outer and the inner surface of erythrocyte membranes. Glikoforin contains large amounts of sialic acid and is negatively charged.
The structure of the erythrocyte membrane and low viscosity of normal hemoglobin provide unique plasticity to erythrocytes, which allows them to pass easily through the capillaries, twice smaller than the erythrocytes themselves, and acquire various shapes.