Amino acids are simple organic compounds made of carbon, hydrogen, oxygen, nitrogen, and, in a few cases, sulfur. Amino acids bond together to form protein molecules, the basic building blocks of all living things. Amino acids can vary widely. Only about 20 amino acids are common in humans and animals, with 2 additional ones present in a few animal species. There are over 100 lesser known amino acids found in other living organisms, particularly plants.
The first few amino acids were discovered in the early 1800s. Although scientists determined that amino acids were unique compounds, they were unsure of their exact significance. Scientists did not understand their importance in the formation of proteins—chemical compounds responsible for the structure and function of all cells—until the first part of the twentieth century.
An important characteristic of amino acids is their ability to join together in chains. The chains may contain as few as 2 or as many as 3,000 amino acid units. Amino acids become proteins when 50 or more are bonded together in a chain.
All the millions of different proteins in living things are formed by the bonding of only 20 amino acids. Like the 26 letters of the alphabet that join together to form different words, the 20 amino acids join together in different combinations and sequences to form a large variety of proteins. But whereas most words are formed by about 10 or fewer letters, proteins are formed by 50 to more than 3,000 amino acids. Because each amino acid can be used many times along the chain and because there are no restrictions on the length of the chain, the number of possible combinations for the formation of protein is truly enormous.
The order of amino acids in the chain, however, is extremely important. Just as not all combinations of letters make sense, not all combinations of amino acids make functioning proteins. Some amino acid combinations can cause serious problems. Sickle-cell anemia is a serious, sometimes fatal disease caused by a single amino acid being replaced by a different one at the sixth position from the end of the protein chain in the hemoglobin molecule, the oxygen-carrying particle in red blood cells.
The 20 amino acids required by humans for making protein are necessary for the growth and repair of tissue, red blood cells, enzymes, and other materials in the body. Twelve of these amino acids, called non-essential amino acids, can be made within the body. The other eight, called the essential amino acids, cannot be made by the body and must be obtained from the diet. Proteins from animal sources—meat, eggs, milk, cheese—contain all the essential amino acids. Except for soybeans, vegetable proteins do not have all the essential amino acids. Combinations of different vegetables, however, form a complete source of essential amino acids.
Essential amino acid: Amino acid that cannot be produced by the human body and must be obtained from the diet.
Non-essential amino acid: Amino acid that is produced within the human body.
Proteins: Organic substances consisting of amino acids and other elements that form the basis of living tissues.