Читать книгу Astrobiology - Charles S. Cockell - Страница 43

Covalent bonds play a central role in biology because they are the bonds that hold atoms together in the vast array of molecules from which life is constructed.

Covalent bonds are also found in specific situations where biologically important molecular structure is required. In analogy to the role of ionic bonds in holding charged amino acids together in protein chains, covalent bonds can form between sulfur-containing amino acids (cysteine and methionine). The sulfur atoms within the amino acids join together and form a disulfide bridge. These bonds anchor the structure of the protein, making sure that its three-dimensional shape is maintained. Figure 3.8 shows a schematic example of disulfide bridges made of covalent bonds holding a protein chain together in a loop.

Figure 3.8 As well as holding atoms together in molecules, covalent bonds link within molecules to provide structure. Here covalent bonds within two disulfide bridges hold a protein chain made of amino acids into a loop. The covalent bonds in this structure, as in many similar diagrams, are shown as solid black lines.

As disulfide bridges can add rigidity to a protein, life can use them to enhance the stability of proteins. For example, some microbes, such as those that live in hot springs in Yellowstone National Park (Figure 3.9), must cope with high temperatures, often well over 60 °C. Some proteins have been found to contain extra covalent bonds to enhance their thermostability. We return to examine adaptations to extreme environments later in the book. For now, the point to learn is that changes in bonding in molecules can be used by life as one of a repertoire of adaptations to extreme physical conditions.

Figure 3.9 An enhanced number of covalent bonds in proteins is used in some microbes to stabilize proteins against high temperatures. Enhanced stability of biological molecules is needed in such environments, for example in these volcanic pools in Yellowstone National Park, USA. In this image, the microbes live within the browns and yellows in the spring, colors caused by microbial pigments and minerals such as iron.

Source: Reproduced with permission of Public-domain-photos.com; Jon Sullivan.