Proteins are some of the most interesting and complex molecules in plants and animals. In essence, they have been the entities that have been selected for and against during the course of evolution, their structures and functions shaped and perfected by natural selection. The evolution and mutation of proteins can be realized through changes in deoxyribonucleic acid (DNA), the blueprint for all the proteins that the entire body produces. DNA is translated to proteins via ribonucleic acid (RNA). Although every cell contains an identical copy of DNA with complete instructions for all types of body tissues, only certain proteins are produced by each cell type. In this way, cells of different tissues can perform diverse tasks through the production of unique proteins.
Proteins are composed of long chains of amino acids. Eleven of the twenty amino acids cannot be synthesized by the body and must be obtained through the diet. Since these amino acids are necessary for protein biosyntheses, they are called essential amino acids and include histidine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The other amino acids can be synthesized by the liver and are called nonessential amino acids.
Each amino acid contains a carboxylic acid group (COOH), an NH2 amino group and one of twenty functional (R) groups. The NH2 group is very basic and accepts protons (H+) from solution, while the carboxylic acid group is acidic and donates protons to the solution. Amino acids in this form are called zwitterions. Because the pH of the body is relatively neutral (pH 7), zwitterions are abundant. If the pH were too low, the NH2 group would be ionized but the carboxylic acid group would be surrounded by protons and could not ionize to a great extent. Therefore, zwitterions are most abundant at neutral pHs.
The distinguishing feature of amino acids are their side chains or R groups. R groups can be either acidic, basic, polar or neutral depending on their structure and formula.
Acidic and Basic Amino Acids
Like zwitterions, acidic and basic side chains can ionize depending upon the pH of the surrounding solution. The amino acids that form charged side chains in solution are lysine, arginine, histidine, aspartic acid, and glutamic acid.