Proteins are composed of long chains of amino acids joined together via peptide bonds. They are produced through a two step process involving the transcription of deoxyribonucleic acids (DNA) and the subsequent translation of messenger ribonucleic acid (RNA). Although the same DNA blueprint that contains instructions for all the bodys tissue and organs is found in every cell, only certain proteins are expressed by specific cell types.
Each amino acid is composed of an amino group (NH2), a carboxylic acid group (COOH) and a functional group (R). There are twenty kinds of R groups that distinguish each different amino acid. All twenty amino acids are found in proteins, each contributing to the proteins overall structure or function. Some of these R groups or side chains form covalent or dipole-dipole interactions within the protein while others may form noncovalent interactions. As each segment of the protein is produced during translation, the polypeptide sequences begin to fold. The final protein conformation is achieved through a progressive stabilization of random intermediates.
Due to the diversity of their structures, proteins have many important functions in the body. Enzymes are a special class of proteins that catalyze biological reactions in plants and animals. They increase the speed of reactions by binding reversibly with their complementary substrate and stabilizing transition state intermediates. Other classes of proteins include membrane channels and pumps, functioning within the membranes of cells to regulate the flux of ions and small molecules. Proteins are also found in the immune system as antibodies that function in the recognition and destruction of foreign particles and antigens.
Finally, it is important to consider proteins in dietary terms. Nonessential amino acids can be synthesized by the liver through a process known as transanimation while essential amino acids must be obtained through the diet. Both nonessential and essential amino acids are used by the body for the production of proteins that constituent tissues like the skeletal muscle, heart and gastrointestinal tract. In adequate intakes of proteins or carbohydrates can lead to protein wasting, a situation where the proteins found in tissue are catabolized for energy. These degenerative conditions are seen mostly in people on low calorie or high protein and low carbohydrate diets.