An Introduction to Molecular Biotechnology. Группа авторовЧитать онлайн книгу.
keratin sulfate. Heparin, involved in the control of blood coagulation, also falls into this structural group. These polysaccharides are charged molecules under physiological conditions and can therefore interact with cellular macromolecules, such as proteins and nucleic acid by form hydrogen bridges and ion bonds.
2.2 Structure of Membrane Lipids
Biological membranes consist of a lipid bilayer (Figure 2.2). They are formed from phospholipids, glycolipids, and sterols (e.g. in animal membranes, cholesterol), which have lipophilic (fat loving, water repelling) and hydrophilic (water loving, fat repelling) structural elements. The lipid composition differs between cell types and compartments. Furthermore, biomembranes carry a diversity of membrane proteins (see Chapter 3). Biomembranes generate a diffusion barrier and enclose all cells and in eukaryotes enclose all internal organelles (mitochondria, plastids) and compartments (see Chapter 3).
Figure 2.2 Structure of the cytoplasmic membrane. Schematic diagram of the lipid bilayer containing phospholipids, cholesterol, and membrane proteins.
Figure 2.3 describes the structure of phospholipids. Of the three hydroxyl groups of the alcohol glycerol, two are linked to fatty acids (length usually 16 or 18 carbon atoms; Table 2.3), and the third is linked by an ester bond to a phosphate residue. An additional ester bond links the negatively charged phosphate residue to either an amino alcohol (choline or ethanolamine), the amino acid serine, or the sugar alcohol inositol. In the case of phosphatidylcholine (lecithin), the nitrogen atom is present as a quaternary amine and is therefore always positively charged. Phosphatidylinositol is a precursor for inositol‐1,4,5‐triphosphate (IP3) – an important signaling molecule in signal transduction pathways of the cell (see Section 3.1.1.3).
Figure 2.3 Structures of important phospholipids. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and sphingomyelin (a ceramide).
Table 2.3 Important fatty acids in membrane lipids.
| Trivial name | Abbreviation | Melting temperature Tm (°C) | Structure |
|---|---|---|---|
| Saturated fatty acids | |||
| Myristic acid | 14 : 0 | 52.0 | CH3(CH2)12COOH |
| Palmitic acid | 16 : 0 | 63.1 | CH3(CH2)14COOH |
| Stearic acid | 18 : 0 | 69.1 | CH3(CH2)16COOH |
| Unsaturated fatty acids | |||
| Palmitoleic acid | 16 : 1 | −0.1 | CH3(CH2)5CH=CH(CH2)7COOH |
| Oleic acid | 18 : 1 | 13.4 | CH3(CH2)7CH=CH(CH2)7COOH |
| Linoleic acid | 18 : 2 | −9.0 | CH3(CH2)4(CH=CHCH2)2(CH2)6COOH |
| γ‐Linolenic acid | 18 : 3 | −17.0 | CH3(CH2)4(CH=CHCH2)3(CH2)3COOH |
| Arachidonic acid | 20 : 4 | −49.5 | CH3(CH2)4(CH=CHCH2)4(CH2)2COOH |
Phospholipids are amphiphilic molecules; their fatty acid residues are strongly lipophilic, while their charged head group is hydrophilic. Of the two fatty acids, one is generally unsaturated (i.e. one or more double bonds are present). As the single phospholipids constantly rotate, the fatty acid, which is kinked due to the inflexible double bond, has a significantly greater radius than that of two saturated fatty acids. This increases the fluidity of the biomembrane, and the formation of paracrystalline structures is avoided. In bacterial or yeast cells that are exposed to different temperatures, the fluidity is constantly adjusted according to the surrounding temperatures by incorporation of phospholipids with different lengths of fatty acid residues, with or without double bonds. Also fishes, living in cold waters, have a higher content of unsaturated fatty acids than those living in warm tropical waters.
In addition to the membrane lipids that are derivatives of glycerol, animal cells contain additional lipids and phospholipids. These have the amino alcohol sphingosine as a base and are referred to as sphingolipids. The N‐acyl fatty acid derivatives of sphingosine are termed ceramides. Sphingomyelin, one of the most important of the sphingolipids, has a structure analogous to that of phosphatidylcholine (Figure 2.3). It is very common in the myelin sheaths found around the axons of neurons.
If the sphingomyelin head group is substituted with a sugar residue (e.g. galactose or glucose), a cerebroside results. These membrane lipids are missing the phosphate residue and are therefore uncharged. Cerebrosides are common in the brain, where they are oriented toward the cell exterior. Gangliosides are sphingolipids with an especially complex structure. They contain oligosaccharides and at least one sialic acid unit (Figure 2.4). In the brain, 6% of lipids are present in the form of gangliosides. Sphingolipid storage diseases (e.g. Tay–Sachs disease), which result in early neurological deterioration, are of great medical importance.
Figure 2.4 Chemical structure of cerebrosides (glycolipids). (a)