5.2Lipids and Membrane Proteins in Biological Membranes
Characteristics of a Lipid Bilayer
Lipid is a collective term for materials that do not dissolve readily in water but do so easily in organic solvents. The lipids that constitute biological membranes are made of hydrocarbon chains with hydrophilic heads and hydrophobic tails. The most abundant constituents of biological membranes are phospholipids (Fig. 5-2A), and other constituents include sterols and glycolipids.
When placed in water, the hydrophilic heads of phospholipids face the water, while the hydrophobic tails line up against one another, thus forming a bylayer or a ball-shaped structure (i.e., a micelle) (Fig. 5-2B).
Lipid molecules in the bilayer continually move in a horizontal direction, but do not move between the inner and outer sides of the membrane except under special conditions. As shown in Figure. 5-2C, lipid composition differs between the inner and outer sides of the membrane, and in animal cells glycolipids are known to be found more on the outside.
In cells, the plasma membrane - which has a lipid bilayer as its basic framework - forms a ball-shaped structure and separates the cytoplasm from the aqueous system outside the cell. Electron microscopy shows that the plasma membrane consists of a lipid bilayer to which numerous membrane proteins are attached.
Membrane proteins take various forms; as shown in Figure. 5-3, single- or multipass transmembrane proteins, tunnel-shaped proteins similar to ion channels, and proteins that attach to one side of the lipid membrane.
For transmembrane proteins, two major polypeptide structures are known. One of these is the α-helix structure (Fig. 5-3A). Its highly hydrophobic surface, which has many amino acids attached that have hydrophobic side chains (e.g., leucine and isoleucine), faces the lipids. Multiple α-helices can form a path that penetrates the plasma membrane. In this case, as shown in Fig. 5-3B, many hydrophobic amino acids are located on the outside facing the lipids, whereas many hydrophilic amino acids are found on the inside facing the aqueous solution.
Membrane proteins can also form a hole that penetrates the plasma membrane using a β-sheet structure (Fig. 5-3C). In this case, hydrophobic amino acids are concentrated on the outside facing the lipid bilayer, and hydrophilic amino acids are located on the inside of the tunnel. This β-sheet structure is called a β-barrel because of its barrel-like shape.
These membrane proteins are involved in the transport of ions and chemical compounds through the membrane. They also stabilize the membrane by undercoating it, act as receptors that pass extracellular information to the cell (see Chapter 9), and bind to the cytoskeleton by connecting to specific lipids in the plasma membrane (see Chapter 6).
Some proteins, as shown in Figure. 5-3E, do not have a structure that penetrates the membrane; rather, they bind to it via fatty acids (see the Column in 5.3). Many proteins that form complexes with membrane proteins also accumulate on the membrane (Fig. 5-3F).
Proteins located on the outside of the plasma membrane of eukaryotic cells often have oligosaccharide chains attached. Since extracellular fluid, such as blood, contains many proteases, the attachment of sugar chains makes it difficult for proteases to digest the proteins, thus stabilizing the proteins outside the cell. Sugar chains often found in the proteins located on the cell surface are also used as cell markings, and play an important role when cells recognize and attach themselves to each other.