Amino Acids: The Building Blocks of Proteins

Amino acids are the monomeric building blocks from which every protein is constructed. Each of the twenty standard amino acids found in proteins shares a common general structure: a central carbon atom, called the alpha-carbon, bonded to four different groups -- an amino group (-NH2), a carboxyl group (-COOH), a single hydrogen atom, and a fourth, variable side chain known as the R group. It is this R group alone that differs from one amino acid to the next, and its particular chemical nature -- size, shape, charge and reactivity -- gives each amino acid its distinctive chemical personality and ultimately determines how the finished protein folds and behaves.

Because an amino acid carries both a basic amino group and an acidic carboxyl group on the same molecule, it is amphoteric -- able to act as either an acid or a base. At physiological pH, both groups are ionised simultaneously, the amino group carrying a positive charge and the carboxyl group a negative charge, so that the molecule as a whole exists as a dipolar ion, or zwitterion, with no net charge.

Amino acids are commonly classified by the chemical nature of their R group: non-polar or hydrophobic (such as glycine, alanine and valine), polar but uncharged (such as serine and threonine), acidic, carrying a negatively charged carboxyl side chain (such as aspartic acid and glutamic acid), and basic, carrying a positively charged side chain (such as lysine, arginine and histidine). A separate and nutritionally important classification distinguishes essential amino acids, which the human body cannot synthesise in adequate amounts and must obtain from the diet (such as leucine, lysine, valine, threonine, methionine, phenylalanine, tryptophan and isoleucine), from non-essential amino acids, which the body is able to synthesise for itself from other metabolic intermediates (such as glycine, alanine, serine and glutamic acid). …