Читать книгу Astrobiology - Charles S. Cockell - Страница 78

The first major class of molecules that play a substantial role in building living things is proteins. Proteins are involved in many functions. They can act as catalysts, carrying out chemical reactions. These proteins are called enzymes. Proteins are also used in cell membranes to transport molecules, and they are found as structural materials and electron transfer molecules in energy acquisition in the cell.

Proteins are composed of chains of amino acids. Amino acids are molecules made up of a central carbon atom called the alpha carbon that has four side groups attached to it. Two of the groups are an amine (–NH₂) and a carboxyl (–COOH) group. There is a hydrogen atom attached as the third side group. The fourth side group is sometimes called the “–R” group and it can be a variety of different combinations of atoms. It is this side group that is altered in the wide variety of amino acids found in biology (Figure 4.3).

Figure 4.3 The 20 common amino acids found in life. The figure shows the different chemical structure of the side groups (in blue). Also shown in brackets is the three-letter designation of each amino acid.

Source: Reproduced with permission of wikicommons.

Although there are various amino acids in nature, with over 500 known, only 20 of these compounds are commonly used in life (Figure 4.3), with two others more rarely used (selenocysteine and pyrrolysine). Life therefore uses a very select number. We don't really know why this is the case. It might be like asking why someone building a house doesn't use all the wonderful variety of bricks that are available from their local garden store. It makes no sense to use all of them because that would result in incompatibility between brick types and too much complexity to get the job done. If 20 amino acids allow for a life form to come into existence and reproduce, then there is no evolutionary selection pressure to use more amino acids. Another reason could be that they are the best of all possible amino acids that could have been used by life. Their biochemical characteristics may have favored the use of these particular 20 amino acids in the earliest types of cells. You might like to read the following Discussion Point, which investigates this idea further.