PLASMA MEMBRANE 1. The plasma membrane separates the cell from the external environment. It has highly selective permeability propeies so that the entry and exit of compounds are regulated. The cellular metabolism is in turn influenced and probably regulated by the membrane. The membrane is metabolically very active. 2. The enzyme, nucleotide phosphatase (5&; nucleotidase) and alkaline phosphatase are seen on the outer pa of cell membrane; they are therefore called ectoenzymes. 3. Membranes are mainly made up of lipids, proteins and small amount of carbohydrates. The contents of these compounds vary according to the nature of the membrane. The carbohydrates are present as glycoproteins and glycolipids. Phospholipids are the most common lipids present and they are amphipathic in nature. Cell membranes also contain cholesterol.4. Fluid Mosaic Model The lipid bilayer was originally proposed by Davson and Danielle in 1935. Later, the structure of the biomembranes was described as a fluid mosaic model (Singer and Nicolson, 1972). 4-A. The phospholipids are arranged in bilayers with the polar head groups oriented towards the extracellular side and the cytoplasmic side with a hydrophobic core. The distribution of the phospholipids is such that choline-containing phospholipids are mainly in the external layer and ethanolamine and serine containing phospholipids in the inner layer. 4-B. Each leaflet is 25 A thick, with the head poion 10 A and tail 15 A thick. The total thickness is about 50 to 80 A.4-C. The lipid bilayer shows free lateral movement of its components, hence the membrane is said to be fluid in nature. Fluidity enables the membrane to perform endocytosis and exocytosis. 4-D. However, the components do not freely move from inner to outer layer, or outer to inner layer (flip-flop movement is restricted). During apoptosis (programmed cell death), flip-flop movement occurs. This Flip-flop movement is catalyzed by enzymes. Flippases catalyse the transfer of amino phospholipids across the membrane. Floppases catalyse the outward directed movement which is ATP dependent. This is mainly seen in the role of ABC proteins mediating the efflux of cholesterol and the extrusion of drugs from cells. The MDR (multidrug resistance) associated p-glycoprotein is a floppase. Ernst Ruska designed the first electron microscope in 1939. Gerd Binning and Heinrich Rohrer introduced the scanning electron microscopy in 1981 by which the outer and inner layers of membranes could be visualized separately. All the three workers were awarded Nobel prize in 1986.4-E. The cholesterol content of the membrane alters the fluidity of the membrane. When cholesterol concentration increases, the membrane becomes less fluid on the outer surface, but more fluid in the hydrophobic core. The effect of cholesterol on membrane fluidity is different at different temperatures. At temperature below the Tm cholesterol increases fluidity and thereby permeability of the membrane. At temperatures above the Tm, cholesterol decreases fluidity. In spur cell anemia and alcoholic cirrhosis membrane studies have revealed the role of excess cholesterol. The decrease in membrane fluidity may affect the activities of receptors and ion channels. This has been implicated in conditions like LCAT deficiency, Alzheimer's disease and hypeension. Fluidity of cellular membranes responds to variations in diet and physiological states. Increased release of reactive oxygen species (ROS), increase in cytosolic calcium and lipid peroxidation have been found to adversely affect membrane fluidity. Anesthetics may act by changing membrane fluidity. 4-F. The nature of the fatty acids also affects the fluidity of the membrane, the more unsaturated cis fatty acids increase the fluidity.The fluidity of the membrane is maintained by the length of the hydrocarbon chain, degree of unsaturation and nature of the polar head groups. Trans fatty acids (TFA) decrease the fluidity of membranes due to close packing of hydrocarbon chains. Cis double bonds create a kink in the hydrocarbon chain and have a marked effect on fluidity. Second OH group of glycerol in membrane phospholipids is often esterified to an unsaturated fatty acid, monounsaturated oleic or polyunsaturated linoleic, linolenic or arachidonic. The nature of fatty acids and cholesterol content varies depending on diet. A higher propoion of PUFA which increases the fluidity ors the binding of insulin to its receptor, a transmembrane protein. 5. Membrane Proteins 5-A. The peripheral proteins exist on the surfaces of the bilayer . They are attached by ionic and polar bonds to polar heads of the lipids.5. Membrane Proteins 5-A. The peripheral proteins exist on the surfaces of the bilayer . They are attached by ionic and polar bonds to polar heads of the lipids. 5-B. Anchoring of proteins to lipid bilayers: Several peripheral membrane proteins are tethered to the membranes by covalent linkage with the membrane lipids. Since the lipids are inseed into the hydrophobic core, the proteins are firmly anchored. A typical form of linkage is the one involving phosphatidylinositol which is attached to a glycan. This glycan unit has ethanolamine, phosphate and several carbohydrate residues. This glycan chain includes a glucose covalently attached to the C terminus of a protein by the ethanolamine and to the phosphatidylinositol by the glucosamine. The fatty acyl groups of the phosphatidylinositol diphosphate (PIP2) are firmly inseed into the lipid membrane thus anchoring the protein. This is referred to as glycosylphosphatidylinositol (GPI) anchor.5-C. Microdomains on membranes: GPI anchored proteins are often attached to the external surface of plasma membrane at microdomains called lipid rafts. They are areas on the membrane having predominantly glycosphingolipids and cholesterol. The localization and activity of the protein can be regulated by anchoring and release. Defective GPI anchors are implicated in PNH (paroxysmal nocturnal hemoglobinuria). Lipid rafts have a role in endocytosis, G protein signaling and binding of viral pathogens. The GPI anchors that tether proteins to the membrane are also seen at the lipid rafts. Membrane proteins may be anchored by covalent bonding, palmitoylation, and myristoylation. 5-D. Caveolae are flask-shaped indentations on the areas of lipid rafts that are involved in membrane transpo and signal transduction. Caveolae contain the protein caveolin, along with other receptor proteins. Transpo of macromolecules (IgA) from the luminal side occurs by caveolae mediated transcytosis. The endocytosis of cholesterol containing lipoproteins may be caveolae mediated. Similarly the fusion and budding of viral paicles are also mediated by caveolae. 5-E. The integral membrane proteins are deeply embedded in the bilayer and are attached by hydrophobic bonds or van der Waals forces. 5-F. Some of the integral membrane proteins span the whole bilayer and they are called transmembrane proteins. The hydrophobic side chains of the amino acids are embedded in the hydrophobic central core of the membrane. The transmembrane proteins can serve as receptors (for hormones, growth factors, neurotransmitters), tissue specific antigens, ion channels, membrane-based enzymes, etc.Ref: DM Vasudevan - Textbook of Biochemistry, 8th edition, page no: 11-12