BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 4. MOLECULAR ORGANIZATION AND BIOLOGICAL FUNCTIONS OF MEMBRANES

4.2. Molecular organization of membranes

4.2.3. Carbohydrates, water, and inorganic ions

CARBOHYDRATES account for up to 10% of the total mass of Cell/30.html">The Plasma Membrane. However, Membrane Carbohydrates do not exist in a free state; rather, they are covalently linked to Lipids (Glycolipids) and Proteins (Glycoproteins and Proteoglycans). Most Membrane Proteins are glycoproteins. At the same time, only a tenth of the lipid molecules in the outer monolayer of the membrane are bound to carbohydrates. Yet, because lipid molecules outnumber protein molecules in the membrane by 50-fold, glycolipids are more abundant than glycoproteins and proteoglycans.

Like lipids and proteins, carbohydrates exhibit an asymmetric distribution within the membrane. The carbohydrate chains of glycolipids, glycoproteins, and proteoglycans are localized exclusively on the non-cytosolic face of all membrane types. N-linked Oligosaccharides are attached to proteins via an asparagine residue, whereas O-linked oligosaccharides are attached via Serine and Threonine residues. The presence of carbohydrates on The Cell surface can be demonstrated using Lectins—proteins that specifically bind to particular sugar residue sequences. For instance, soybean lectin specifically binds α-galactose and N-acetyl-D-galactosamine, wheat germ agglutinin binds N-acetylglucosamine, and so on.

The carbohydrate-rich zone On the surface of eukaryotic Plasma Membranes is called the glycocalyx. However, the glycocalyx comprises not only the carbohydrate moieties of membrane glycolipids, glycoproteins, and proteoglycans, but also free glycoproteins and proteoglycans that are secreted by Cells and subsequently adsorbed onto the outer surface of the plasma membrane. Crucially, this high concentration of surface carbohydrates influences many—if not all—Membrane Functions, primarily cell-Cell Recognition, cell-Extracellular matrix interactions, and immune responses.

Water exists in the membrane in two states: as bulk (solvent) water and bound water. The physiological relevance of solvent water as a true membrane component remains debatable; it is localized mainly within the volumes of pores and Ion Channels. Bound water, however, is a genuine membrane component (Chapter 3). Because each H₂O molecule can act as both a donor and an acceptor of two Hydrogen Bonds, Membrane Lipids readily bind water (for example, the hydrophilic HEAD group of PE contains eight oxygen atoms that act as Hydrogen bond acceptors). Membrane proteins also possess water-binding sites, such as the oxygen atoms of carboxyl groups (H-bond acceptors) and amino groups (H-bond Donors). Quantifying the exact amount of bound water in a membrane is challenging. Importantly, bound water participates not only in shaping the "geometry" of protein and lipid molecules but also in forming delicate perimembrane layers (1-2 molecules thick). This means that bound water plays a vital role in transmembrane signaling.

The fluid-mosaic model of Membrane Structure assumes the existence of charged membrane surfaces, where negative charges are contributed by phosphoric acid esters and carboxyl groups. Such charges attract cations from the surrounding solution, whereas positive charges (quaternary amino groups) repel them. The membrane also contains cation-binding components featuring hydroxyl groups and ether oxygens. Because Na⁺, K⁺, and Ca²⁺ ions readily undergo dehydration, whereas Mg²⁺ does not, complexes formed with magnesium differ structurally from those formed with Na⁺, K⁺, and Ca²⁺. Furthermore, Ca²⁺ ions can form "bridge" bonds between ligands, which is particularly important in the case of Ligand heterogeneity. NH₄ can be adsorbed at K⁺-binding sites, forming hydrogen bonds involving oxygen atoms. This induces Conformational Changes in the cation-binding sites—a significant factor considering that cations predominantly select oxygen atoms as their nearest neighbors.

Thus, the Asymmetry of the Molecular Organization of the monolayers, the differential distribution of bound water between them, and the non-uniform distribution of cations (including the disruption of Hydration shells) endow the membrane with The properties of a dynamic structure capable of finely responding to stimuli of any nature and transforming the energy of these stimuli into the excitation energy of membrane component molecules.