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 isomeric forms. The α carbon, carboxylic acid, and amino groups are common to all amino acids, so the R-group is the only variable feature.
At the center of each amino acid is a carbon called the α carbon and attached to it are four groups – a hydrogen, a carboxylic acid group, an amine group, and an R-group, sometimes referred to as a variable group or side chain. “It is one of the more striking generalizations of biochemistry …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, shown in Figure 2.1.
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