Which are Natural Amino - Acids?

Proteinogenic amino acids are those 22 amino acids that are found in proteins and require cellular machinery coded for in the genetic code [1] of any organism for their isolated production. Humans can synthesize 10 of them (by interconversions) from each other or from other molecules of intermediary metabolism, but the other 10 (essential amino acids: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) must be consumed in the diet. Proteinogenic literally means protein building. Proteinogenic amino acids can be assembled into a polypeptide (the subunit of a protein) through a process known as translation (the second stage of protein biosynthesis, part of the overall process of gene expression).

An example of a Proteinogenic amino acid is an arginine.

Arginine is a popular medication for sexual dysfunctions and used to increase libido in both men and women. Arginine is needed by the body to make nitric oxide, a compound that works to relax blood vessels and allow more blood to flow through the sexual organs to improve clitoral sensation.

Arginine can be found in dairy products such as cottage cheese, ricotta, milk, yogurt and whey protein drinks. It can also be found in beef, pork, poultry and other seafoods (e.g. halibut, lobster, salmon, shrimp, snails, tuna).

Some plant sources of arginine are wheat germ and flour, buckwheat, chickpeas, cooked soybeans and seeds (pumpkin, sesame, sunflower).

Non-proteinogenic amino acids are either not found in proteins (like carnitine, GABA, or L-DOPA), or are not produced directly and in isolation by standard cellular machinery. (like hydroxyproline and selenomethionine). The latter often results from posttranslational modification of proteins.

An example of a Non-Proteinogenic amino acid is a Selenomethionin.

Selenomethionine (Se-met) along with other seleno amino acidswas suspected already in the mid 1930s to be one of the toxiccomponents of seleniferous plants, but suggestive experimentalevidence for its presence in seleniferous wheat protein hydrolyzateswas obtained only in 1949. Se-met was definitely identifiedin plant proteins in the 1950s–1960s and was concurrentlyalso shown to be produced by strains of Saccharomyces cerevisiea, Candidaalbicans, Escherichia coli, rumen bacteria and marine algae,when these were grown in Se-containing media.  In 1962,75Se-met became available and was introduced as a pancreaticradioimaging agent. In the mid-1970s, metabolic studiesindicated that Se-met is well absorbed and retained, suggestingits use for nutritional Se supplementation. At about thesame time, high Se-yeast was introduced as an economical foodsource of Se-met. By 1984, synthetic L-Se-met was also beginning tobe produced at a cost comparable to that of Se-yeast on a per-Sebasis. Numerous experimental studies have since established thatSe-met and Se-yeast are suitable for nutritional Se-supplementation. However, concerns have also been raised that Se-metmight, under some conditions, accumulate in, or be releasedfrom body stores to toxic levels. This review addressesthese concerns within a more general description of its occurrence,metabolism and toxicity.