This classification of amino acids has little to do with the structure of amino acids. Nutritionists divide amino acids into two groups - essential amino acids (must be in the diet because cells can’t synthesize them) and non-essential amino acids (can be made by cells). Pyrrolysine-containing proteins are much rarer and are mostly confined to archaea. Enzymes containing selenocysteine, for example, include glutathione peroxidases, tetraiodothyronine 5' deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, and selenophosphate synthetase. When this happens, these unusual amino acids can be incorporated into proteins. The others, selenocysteine and pyrrolysine use tRNAs that are able to base pair with stop codons in the mRNA during translation. There are 22 amino acids that are found in proteins and of these, only 20 are specified by the universal genetic code. Figure 2.1 - General amino acid structure With only very minor exceptions, every amino acid found in cells and in proteins is in the L configuration. (A minor exception to this structure is that of proline, in which the end of the R-group is attached to the α-amine.) With the exception of glycine, which has an R-group consisting of a hydrogen atom, all of the amino acids in proteins have four different groups attached to them and consequently can exist in two mirror image forms, L and D. The α carbon, carboxyl, and amino groups are common to all amino acids, so the R-group is the only unique feature in each amino acid. At the “center” of each amino acid is a carbon called the α carbon and attached to it are four groups - a hydrogen, an α- carboxyl group, an α-amine group, and an R-group, sometimes referred to as a side chain. that the twenty amino acids and the four bases, are, with minor reservations, the same throughout Nature." - Francis CrickĪll amino acids have the same basic structure, which is shown in Figure 2.1. "It is one of the more striking generalizations of biochemistry. Linked together in long chains called polypeptides, amino acids are the building blocks for the vast assortment of proteins found in all living cells. The entire textbook is available for free from the authors at Īll of the proteins on the face of the earth are made up of the same 20 amino acids.
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