Amino acids are the building blocks of which protein molecules are made and thus are the basic structural material of all living matter.
Although there are many millions of different kinds of proteins, only 20 amino acids are commonly used in protein synthesis. To form proteins, amino acid molecules are linked together in a large number of different ways and often in huge numbers. For example, hemoglobin consists of the protein globin, which is formed of 574 amino acid molecules, and a complex organic compound, hem, which gives blood its characteristic red color. The information necessary for the correct sequencing of amino acids is contained in deoxyribonucleic acid (DNA).
Animals constantly use up protein through processes such as muscle growth and enzyme production. As a result, they need a constant supply of amino acids that can be used where required for energy and for building into protein. They get this supply by taking in protein, hydrolyzing it (breaking it down) into separate amino acids through the action of enzymes in the digestive tract, and rebuilding them into the proteins that are needed. Enzymes are themselves proteins and function like catalysts in the body, speeding up the various biochemical reactions.
The 20 amino acids are valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, lysine, histidine, glycine, alanine, serine, cysteine, tyrosine, aspartic acid, asparagine, glutamic acid, glutamine, arginine, and proline. Of these 20 amino acids, only the first 9 are essential in the human diet; the rest are synthesized from other sources. A lack of any of them in the diet causes malnutrition.
The general amino acid formula is NH2·CHR·COOH, with R standing for either a hydrogen atom (in the case of the simplest amino acid, glycine) or a more complex organic radical or group of atoms. The formula contains both the acidic or carboxyl group, -COOH, and the alkaline or amino group, -NH2. When an amino acid molecule dissolves in water, it ionizes, but because the carboxyl group has a negative charge and the amino group a positive charge, the resulting ion is electrically neutral and called a zwitterion.
If a positively charged ion, such as a hydrogen ion (H+) from an acid, is introduced into an amino acid solution, it is attracted to the negatively charged portion of the amino acid zwitterion, neutralizing it and leaving it with only its positive charge. Similarly, a negative ion, such as a hydroxyl ion (OH-) from an alkali, is attracted to the positively charged portion of the amino acid molecule.
This ability of amino acids to collect any stray positive and negative (and therefore acidic or alkaline) ions allows them to act as buffers in living cells, maintaining the delicately balanced pH (a measure of acidity or alkalinity) that the cells must have in order to function.
