There are 20 different amino acids in nature. Some, called essential amino acids, you must get from foods in your diet because your body cannot make them. Others, called nonessential amino acids, are capable of being synthesized. Amino acids link together in long chains to form proteins. Each unique protein has a different amino acid sequence and chain length. It is this configuration that determines the protein's physiologic function.
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Amino Acid Structure
Structurally, each amino acid has a central carbon atom with four bonding sites for other atoms. Three of these attached atoms are common to each amino acid -- one hydrogen atom; one acid group, consisting of one carbon atom, one hydrogen atom and two oxygen atoms; and an amine group, consisting of one nitrogen atom and two hydrogen atoms. The fourth binding site, known as a side chain, varies and is what makes each of the 20 amino acids found in nature unique. Amino means "nitrogen-containing."
Essential Amino Acids
Your body cannot make essential amino acids, so you must get them from the foods you eat. There are nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. If any of these is missing from your diet on a regular basis, you cannot synthesize the proteins that require it. Animal proteins are complete proteins because they supply all essential amino acids. If you are a vegetarian, you can get the essential amino acids you need by combining plant-based foods together. Taking individual amino acids as supplements can upset the balance of amino acids available to build different proteins.
Nonessential Amino Acids
Eleven nonessential amino acids exist in nature. Your body is able to synthesize these on its own, provided there is a sufficient supply of nitrogen and atoms from carbohydrate and fat molecules. The nonessential amino acids are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine.
Conditionally Essential Amino Acid
Under certain circumstances a nonessential amino acid can become essential. For example, a newborn baby does not have fully developed metabolic pathways for the synthesis of some amino acids. So for babies, only five amino acids are nonessential; babies must get the rest from formula and milk until these pathways mature. Another example is when insufficient phenylalanine exists. Normally tyrosine is made from phenylalanine; when it is not available, tyrosine must come from foods in the diet. Tyrosine is also considered a conditionally essential amino acid in phenylketonuria, a condition characterized by a lack of the enzyme necessary for the phenylalanine-to-tyrosine conversion.