BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 4. MOLECULAR ORGANIZATION AND BIOLOGICAL FUNCTIONS OF MEMBRANES

4.2. Molecular Organization of Membranes

4.2.2. Membrane Proteins

Many Proteins span Cell/29.html">The Lipid Bilayer (Fig. 4.6) and are referred to as transmembrane (integral) proteins. The Structure/106.html">Hydrophobicity of certain proteins is enhanced by covalently attached fatty acid chains, while other proteins associate with the lipid bilayer through covalent bonds involving a specific oligosaccharide linked to phosphatidylinositol. Certain proteins are bound to the membrane without direct lipid contacts, through non-covalent interactions with other intrinsic Membrane Proteins. They are readily released from the membrane upon Changes in the Ionic strength or pH of the solution. Such proteins are called peripheral. Because peptide groups are polar and Water molecules are scarce within the lipid matrix, all peptide groups in the lipid bilayer tend to form Hydrogen Bonds with one another. The number of hydrogen bonds between such peptide groups is maximized when the region of the polypeptide chain traversing the bilayer forms a regular right-handed α-Helix. The requirement to maximize the number of H-bonds in the absence of water implies that the polypeptide chain within the lipid bilayer maintains a straight course without bends, as bends would reduce the number of regular H-bonds. This may explain why proteins only partially immersed in the lipid bilayer have not been identified to date. Cytoplasmic domains of the protein do not form Disulfide Bonds due to the reducing environment of the Cytosol.

Class="center">

Fig. 4.6. Various modes of association of membrane proteins with the lipid matrix of the membrane:

A: 1 - integral protein (succinate dehydrogenase); 2 - electrostatic binding of a peripheral protein to phospholipid polar heads (myelin basic protein); 3 - hydrophobic binding of a peripheral protein (via a terminal helical fragment) outside the hydrophilic lipid HEAD groups (Pyruvate oxidase); 4 - insertion of an integral protein via a short terminal fragment (cytochrome); 5 - integral protein with a single transmembrane α-helix (Glycophorin); 6 - integral protein with multiple transmembrane α-helices (lactose permease, rhodopsin); 7, 8 - peripheral proteins bound to the membrane via a Fatty acid and via phosphatidylinositol (alkaline phosphatase). B: Transmembrane glycoprotein spanning the bimolecular lipid layer once: 9 - extracellular protein domain with S-S bonds and Oligosaccharides; 10 - intramembrane α-helix; 11 - cytoplasmic domain of the protein with SH groups

Transmembranous proteins are distributed asymmetrically within the membrane, reflecting the asymmetric nature of their Biosynthesis and insertion into The Endoplasmic reticulum membrane, as well as the distinct Functions of the cytoplasmic and extracellular domains of these proteins. The oligosaccharide chains of membrane Glycoproteins and Proteoglycans (see Chapter 3) are invariably localized on the extracellular face of the membrane. Evidence for protein Asymmetry includes intra- and interchain disulfide bonds confined exclusively to extracellular domains.

The arrangement of proteins on the cytoplasmic face of The Plasma Membrane is best studied using erythrocyte "ghosts", which are easily obtained by placing erythrocytes in a hypotonic solution. Water enters the Cells, causing them to swell and rupture, thereby releasing their intracellular contents. Studies on such preparations have established that the majority of erythrocyte membrane proteins are peripheral. The most abundant among them is spectrin. This rod-like molecule serves as a crucial component of the erythrocyte Cytoskeleton, maintaining cell integrity and shape. The spectrin molecule is composed of two Polypeptides: α-spectrin (240 kDa) and β-spectrin (220 kDa). These spectrin heterodimers spontaneously aggregate head-to-head to form a tetramer 200 nm in length. The ends of several tetramers are linked together by an Actin filament and protein band 4.1, forming a junctional complex (Fig. 4.7). This creates a lattice-like network on the cytoplasmic side of the membrane, enabling erythrocytes to withstand membrane stress when passing through narrow capillaries. The spectrin cytoskeleton is anchored to the membrane via ankyrin: one end of this protein binds to β-spectrin, while the other binds to the integral band 3 protein (band 3 and 4.1 proteins are named after the electrophoretic bands in which they localize). Additionally, band 4.1 links the junctional complex to the cytoplasmic domain of the integral protein glycophorin. This protein, with up to 10⁵ molecules per erythrocyte, was the first membrane protein whose complete Amino Acid Sequence—131 residues—was determined. The greater part of glycophorin is localized in the outer monolayer of the membrane, containing its hydrophilic N-terminus and 15 oligosaccharide side chains (~100 Monosaccharides, accounting for 60% of the total glycoprotein mass), which are rich in sialic acids that impart a negative charge to The Cell surface. The hydrophilic C-terminus of glycophorin is immersed in the cytosol, whereas the hydrophobic α-helical domain (20 residues)

is localized within the lipid matrix. Glycophorin belongs to the class of membrane glycoproteins that cross the lipid bilayer a single time as an α-helix (Fig. 4.7)

Fig. 4.7. Schematic representation of the localization of erythrocyte membrane Skeleton proteins:

1 - spectrin; 2 - ankyrin; 3 - membrane lipid bilayer; 4 - band 3 protein; 5 - glycophorin; 6 - band 4.1 protein;

7 - actin; 8 - junctional complex

Band 3 protein (Fig. 4.7), much like glycophorin, is a transmembrane protein that spans the lipid bilayer 10 times. Its primary function is to participate in O₂ and CO₂ exchange via the counter-transport of HCO₃^- and Cl^- ions. Similar (though more complex) cytoskeletal networks underlie the Plasma Membranes of nucleated cells.

Many cell types possess The ability to restrict membrane proteins to specific domains. For instance, in intestinal epithelial cells, certain plasma membrane Enzymes and transport proteins are localized exclusively on the apical surface of the cell, whereas others are confined to the basolateral surface. The Lipid Composition of these two domains also differs. Such spatial segregation of proteins and Lipids in these cells is maintained by tight Intercellular junctions. The restriction of lateral mobility of membrane proteins is further reinforced by their attachment to macromolecular structures both outside and inside the cell, including the cytoskeleton and the Extracellular matrix.

Thus, membrane proteins function as receptors and enzymes, facilitate the Transmembrane Transport of substances, and mediate intercellular interactions, among other roles. Some of these proteins traverse the lipid bilayer as a single α-helix or a series of such helices, while others associate with either face of the membrane through covalent linkages to lipids and non-covalent interactions with integral proteins. Most proteins are capable of lateral diffusion, albeit at a significantly slower rate than lipids. Certain proteins are immobilized within specific domains.