Essential amino acid

An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms, the nine amino acids humans cannot synthesize are valine, isoleucine, leucine, methionine, phenylalanine, tryptophan, threonine, histidine, and lysine.^[1][2]

Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress.^[2] These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine. Six amino acids are non-essential (dispensable) in humans, meaning they can be synthesized in sufficient quantities in the body. These six are alanine, aspartic acid, asparagine, glutamic acid, serine,^[2] and selenocysteine (considered the 21st amino acid). Pyrrolysine (considered the 22nd amino acid),^[3] which is proteinogenic only in certain microorganisms, is not used by and therefore non-essential for most organisms, including humans.

The limiting amino acid is the essential amino acid which is furthest from meeting nutritional requirements.^[4] This concept is important when determining the selection, number, and amount of foods to consume because even when total protein and all other essential amino acids are satisfied if the limiting amino acid is not satisfied then the meal is considered to be nutritionally limited by that amino acid.^[3]

Overview

(*) Pyrrolysine, sometimes considered the "22nd amino acid", is not used by the human body.^[7]

Essentiality in humans

Of the twenty amino acids common to all life forms (not counting selenocysteine), humans cannot synthesize nine: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Additionally, the amino acids arginine, cysteine, glutamine, glycine, proline and tyrosine are considered conditionally essential,^[8] which means that specific populations who do not synthesize it in adequate amounts, such as newborn infants and people with diseased livers who are unable to synthesize cysteine, must obtain one or more of these conditionally essential amino acids from their diet.^[9][10] For example, enough arginine is synthesized by the urea cycle to meet the needs of an adult but perhaps not those of a growing child. Amino acids that must be obtained from the diet are called essential amino acids.

From intermediates of the citric acid cycle and other pathways

Nonessential amino acids are produced in the body. The pathways for the synthesis of nonessential amino acids come from basic metabolic pathways.

Estimating the daily requirement for the indispensable amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years. The following table lists the recommended daily amounts currently in use for essential amino acids in adult humans (unless specified otherwise), together with their standard one-letter abbreviations.

The recommended daily intakes for children aged three years and older is 10% to 20% higher than adult levels and those for infants can be as much as 150% higher in the first year of life. Cysteine (or sulfur-containing amino acids), tyrosine (or aromatic amino acids), and arginine are always required by infants and growing children.^[11][14] Methionine and cysteine are grouped together because one of them can be synthesized from the other using the enzyme methionine S-methyltransferase and the catalyst methionine synthase.^[15] Phenylalanine and tyrosine are grouped together because one of them can be synthesized from the other using the enzyme phenylalanine/tyrosine ammonia-lyase.^[16]

Amino acid requirements and the amino acid content of food

Historically, amino acid requirements were determined by calculating the balance between dietary

A similar method was used to determine the protein content of foods. Test subjects were fed a diet containing no protein and the nitrogen losses recorded. During the first week or more there is a rapid loss of labile proteins. Once the nitrogen losses stabilize, this baseline is determined to be the minimum required for maintenance. Then the test subjects were fed a measured amount of the food being tested. The difference between the nitrogen in that food and the nitrogen losses above baseline was the amount the body retained to rebuild proteins. The amount of nitrogen retained divided by the total nitrogen intake is called net protein utilization. The amount of nitrogen retained divided by the (nitrogen intake minus nitrogen loss above baseline) is called biological value and is usually given as a percentage.^[18]

Modern techniques make use of

Various attempts have been made to express the "quality" or "value" of various kinds of protein. Measures include the biological value, net protein utilization, protein efficiency ratio, protein digestibility-corrected amino acid score and complete proteins concept. These concepts are important in the livestock industry, because the relative lack of one or more of the essential amino acids in animal feeds would have a limiting effect on growth and thus on feed conversion ratio. Thus, various feedstuffs may be fed in combination to increase net protein utilization, or a supplement of an individual amino acid (methionine, lysine, threonine, or tryptophan) can be added to the feed.

Protein per calorie

Protein content in foods is often measured in protein per serving rather than protein per calorie. For instance, the USDA lists 6 grams of protein per large whole egg (a 50-gram serving) rather than 84 mg of protein per calorie (71 calories total).

Scientists had known since the early 20th century that rats could not survive on a diet whose only protein source was zein, which comes from maize (corn), but recovered if they were fed casein from cow's milk. This led William Cumming Rose to the discovery of the essential amino acid threonine.^[24] Through manipulation of rodent diets, Rose was able to show that ten amino acids are essential for rats: lysine, tryptophan, histidine, phenylalanine, leucine, isoleucine, methionine, valine, and arginine, in addition to threonine. Rose's later work showed that eight amino acids are essential for adult human beings, with histidine also being essential for infants. Longer-term studies established histidine as also essential for adult humans.^[25]

Interchangeability

The distinction between essential and non-essential amino acids is somewhat unclear, as some amino acids can be produced from others. The

• Biological Value (BV)
• Essential fatty acid
• Essential genes
• List of standard amino acids
• Low-protein diet
• Orthomolecular medicine
• Protein Digestibility Corrected Amino Acid Score
• Ketogenic amino acid
• Glucogenic amino acid

References

External links

Wikimedia Commons has media related to Essential amino acids.

• Amino acid content of some vegetarian foods at veganhealth.org.
• Amino Acid Profiles of Some Common Feeds at Virginia Tech.
• Molecular Expressions: The Amino Acid Collection at Florida State University. Features detailed information and crystal photographs of each amino acid.
• vProtein, an online software tool to analyze the essential amino acid profiles of single and pairs of plant based foods based on human requirements.