Amino Acids

All peptides and proteins are polymers of alpha-amino acids. What is the structure of an amino acid? It is a molecule that contains both amino (NH2) and carboxyl (COOH) functional groups. Alanine is one of the standard amino acids:

AA exist in either D (dextro) or L (levo) form (stereoisomers). The D and L refer to the absolute confirmation of optically active compounds. With the exception of glycine, all other amino acids are mirror images that can not be superimposed. Most of the amino acids found in nature are of the L-type. Hence, eukaryotic proteins are always composed of L-amino acids although D-amino acids are found in bacterial cell walls and in some peptide antibiotics.

At least 300 amino acids have been described in nature but only twenty of these are typically found as components in human peptides and proteins.

Twenty standards AA are used by cells in peptide biosynthesis, and these are specified by the general genetic code. AA are classified into two groups: essential and nonessential.

An essential or indispensable amino acid cannot be made by the body and must be supplied by food. These include isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Another amino acid - histidine is considered semi-essential because the body does not always require dietary sources of it.

Nonessential amino acids are made by the body from the essential amino acids or normal breakdown of proteins. The nonessential amino acids are arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine.

Leucine, isoleucine, and valine are called branched-chain amino acid (BCAAs) because human beings cannot survive unless these amino acids are present in the diet. The combination of these three amino acids makes up approximately one-third of skeletal muscle in the human body.

The classification of amino acids as essential or nonessential does not reflect its importance, because all 20 amino acids are necessary for health.

In addition to their role in protein and enzyme synthesis, amino acids are extremely crucial for good health. Amino acids contribute significantly to the health of the nervous system, muscular structure, hormone production, vital organs and cellular structure. Some affects low levels of the essential amino acids result in hormonal imbalances, irritability, low concentration, and depression.

IUPAC recommendations on AA and Peptide Nomenclature is available for download in Adobe Acrobat format.