Amino Acids (2003)

Abstract Amino acids are the main components of proteins. Approximately 20 amino acids are common constituents of proteins and are called protein amino acids, or primary protein amino acids. Hydroxylated amino acids (eg, 4‐hydroxyproline, 5‐hydroxylysine) and N ‐methylated amino acids (eg, N ‐methylhistidine) are obtained by the acid hydrolysis of proteins. γ‐Carboxyglutamic acid occurs as a component of some sections of protein molecules; it decarboxylates spontaneously to L ‐glutamate at low pH. These examples are often called secondary protein amino acids. The presence of many nonprotein amino acids has been reported in various living metabolites, such as in antibiotics, some other microbial products, and in nonproteinaceous substance of animals and plants. Amino acids are important components of the elementary nutrients of living organisms. For humans, ten amino acids are essential for existence and must be ingested in food. The nutritional value of a protein can be improved by the addition of amino acids of low abundance in that protein. Thus the fortification of plant proteins such as wheat, corn, and soybean with L ‐lysine, DL ‐methionine, or other essential amino acids ( L ‐tryptophan and L ‐threonine) is expected to alleviate some food problems. Such fortification has been widespread in the feedstuff of domestic animals. Proteins are metabolized continuously by all living organisms, and are in dynamic equilibrium in living cells. Most of the amino acids absorbed through the digestion of proteins are used to replace body proteins. The remaining portion is metabolized into various bioactive substances such as hormones, or is consumed as an energy source. All of the protein amino acids are currently available commercially and their uses are growing. In the food industries a number of amino acids have been widely used as flavor enhancers and flavor modifiers. For example, an enormous quantity of monosodium L ‐glutamate is now used in various food applications. Tricholomic acid and ibotenic acid, nonprotein amino acids found in mushrooms, have 4 to 25 times stronger umami taste than L ‐glutamic acid. Umami taste, which is typically represented by L ‐glutamic acid salt, makes food more palatable and is recognized as a basic taste, independent of the four other classical basic tastes of sweet, sour, salty, and bitter. Some peptides have special tastes. L ‐Aspartyl phenylalanine methyl ester is very sweet and is used as an artificial sweetener. Amino acids are also used in medicine. All α‐amino acids contain at least one asymmetric carbon atom and may be characterized by their ability to rotate light to the right ( ) or to the left ( ), depending on the solvent and the degree of ionization. In all instances there are at least two polar groups acting synergistically on the solubility in water. The solubility of amino acids having additional polar groups is even more enhanced. Many methods for chemical synthesis of α‐amino acids have been established. The Strecker synthesis and the Bucherer synthesis are popular for α‐amino acid synthesis and are used in the industrial production of some amino acids. In many cases only the racemic mixtures of α‐amino acids can be obtained through chemical synthesis. Therefore, optical resolution is indispensable to get the optically active L ‐ or D ‐forms in the production of expensive or uncommon amino acids. The optical resolution of amino acids can be done by physical or chemical methods and biological or enzymatic methods. Asymmetric synthesis, classified as either enantioselective or diastereoselective, is a method for direct synthesis of optically active amino acids. The chemical reactions of amino acids can be classified according to their carboxyl, amino, and side‐chain groups. Protein amino acids, which are not synthesized by the body and should be supplied as nutrients to maintain life, are called essential amino acids. For humans L ‐arginine, L ‐histidine, L ‐isoleucine, L ‐leucine, L ‐lysine, L ‐methionine, L ‐phenylalanine, L ‐valine, L ‐threonine, and L ‐tryptophan are essential amino acids. However, in adults, L ‐arginine and L ‐histidine are somewhat synthesized in cells. Recent advances in nutritional studies of amino acids have led to development of amino acid transfusion. The human body is maintained by a continuous equilibrium between the biosynthesis of proteins and their degradative metabolism, where the nitrogen is lost as urea and other nitrogen compounds. The amino acids are metabolized, principally in the liver, to a variety of physiologically important metabolites, eg, creatine (creatinine), purines, pyrimidines, hormones, lipids, amino sugars, urea, ammonia, carbon dioxide, and energy sources. Several amino acids serve as specialized neurotransmitters. Protein kinases modify physiologically important proteins. Consequently, various cellular functions, cell growth, and cell differentiation are seriously affected. Methods have been developed for analysis or determination of free amino acids in blood, food, and feedstocks. DL ‐Alanine is the first amino acid which was synthesized chemically. Glycine and DL ‐methionine have also been supplied by this method. However, amino acids formed by the chemical method are racemic. The use of mutant derivatives of coryneform bacteria offered a fermentation process for the production of many other kinds of amino acids, mostly of the L ‐form. Rapid development of fermentative production and enzymatic production have contributed to the lower costs of many protein amino acids. Amino acids are used in feeds, food, parenteral and enteral nutrition, medicine, cosmetics, and as raw materials for the chemical industry.