Astrobiology. Charles S. Cockell

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       Figure 4.8 Different chiralities of life? Gertie the aardvark is a rescued aardvark. She is made of L-amino acids and D-sugars. Is it theoretically possible for Gertie to exist in the three other chiral combinations of L- and D-amino acids or L- and D-sugars?

      Now an interesting thing about life on Earth is that almost all the amino acids used in life are of the L form. The D form is very rare, although D-amino acids are found in the cell membranes of bacteria. They reappear in the next chapter. The reason why life predominantly uses one form of amino acids is not fully understood, but it might have something to do with molecular recognition. As many biochemical interactions involve one molecule slotting into another to carry out chemical reactions, a system of life that was a mix of L and D forms would cause great complexity because we would need proteins with active sites that could recognize either the L or D form of molecules. It seems plausible to speculate that once a biochemical architecture emerged that had a preponderance to use either the L or D form, then that choice would have been perpetuated and amplified. Life on Earth developed a biochemistry based on the L form. Later in the textbook, we explore the possibility that the preference for the L form in life was caused by a pre-existing enantiomeric excess of L forms of amino acids in the prebiotic compounds from which life originated.

4.7 Carbohydrates (Sugars)

Other important molecules in life form chains. Carbohydrates are used in structural support and as energy- storage molecules (which is why people on diets are interested in how much carbohydrate they eat). Carbohydrates are hydrated carbon atoms with the generic formula CH₂O and multiples thereof. They are made up of chains of individual sugars such as glucose, C₆H₁₂O₆, or fructose, which has the same chemical formula as glucose, but a different structure (it is an isomer; Figure 4.9).

       Figure 4.9 The molecular structure of the sugars: glucose, fructose, and ribose. The figure shows the numbering convention on the carbon atoms. Note that the carbon atoms within the rings are not indicated with the letter “C,” but occur where the numbers are shown.

      A sugar with six carbon atoms is called a hexose (hence despite their different structures, glucose and fructose are hexose sugars, as they both contain six carbon atoms). A sugar with five carbon atoms is called a pentose. Ribose is such an example (Figure 4.9). This sugar is found in the structure of ribonucleic acid (RNA) as part of the repeating ribose–phosphate backbone.

      Sugars join through a glycosidic bond to form chains, analogous to the peptide bonds in proteins. O-glycosidic bonds are oxygen-bridged links between sugar molecules (Figure 4.10). Figure 4.10 shows the example of maltose formed by a link between the –OH bond of one glucose (the 4 carbon position) and the hemiacetal group of another glucose (that is the carbon with an –OH group and a link to the oxygen atom in the sugar ring – the 1 carbon position in glucose). This explains why they are sometimes called 1,4 glycosidic linkages. Alternatively, a link between the 1 carbon of a glucose molecule and the 2 carbon of a fructose molecule (the hemiacetal carbon) produces the sugar sucrose (a 1,2 glycosidic link). Like the formation of the peptide bond in proteins, glycosidic linkages are condensation reactions in which a water molecule is lost during the reaction.

       Figure 4.10 Glycosidic bonds allow sugar molecules to be linked together. In this case, two glucose molecules have linked together to form the two-sugar molecule maltose. By adding further units, we can produce polysaccharides (carbohydrates).

Carbohydrate polymers can be produced by linking many sugar molecules together with glycosidic bonds. These carbohydrates are sometimes called polysaccharides.

      A further subtlety, but one crucial for life, is the way in which the glycosidic links are formed. For example, the 1,4 glycosidic link can exist in a 1,4 alpha or 1,4 beta form. The 1,4 alpha form occurs when the –OH group is below the plane of the glucose ring (alpha glucose), while the beta form occurs when the –OH is above the ring (beta glucose) (Figure 4.11). The differences between these two forms of glucose may seem a trivial point of fact, but the consequences of the links formed between them are immense. In the former case, the linking together of many alpha glucose molecules results in the polysaccharide molecule starch. This material is used in energy storage in many organisms, and in the human diet it is found in potatoes, wheat, rice, and other foods. The linkage of beta glucose into a polysaccharide produces the material cellulose, an important structural component of plants and the most abundant polymer in the biosphere. Cellulose is indigestible to humans, since we lack the enzymes to break it down. Some organisms, such as termites, can break it down, as they possess microbes in their guts (symbionts) capable of carrying out the enzymatic degradation of the material. Thus, subtle chemical differences in bonds within sugar molecules СКАЧАТЬ