Essential amino acid

Last updated

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]

Contents

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

EssentialConditionally essential [5] [6] Non-essential
Histidine (H) Arginine (R) Alanine (A)
Isoleucine (I) Cysteine (C) Aspartic acid (D)
Leucine (L) Glutamine (Q) Asparagine (N)
Lysine (K) Glycine (G) Glutamic acid (E)
Methionine (M) Proline (P) Serine (S)
Phenylalanine (F) Tyrosine (Y) Selenocysteine (U)
Threonine (T)
Pyrrolysine* (O)
Tryptophan (W)
Valine (V)

(*) 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.

Eukaryotes can synthesize some of the amino acids from other substrates. Consequently, only a subset of the amino acids used in protein synthesis are essential nutrients.

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. Glutamate dehydrogenase catalyzes the reductive amination of α-ketoglutarate to glutamate. A transamination reaction takes place in the synthesis of most amino acids. At this step, the chirality of the amino acid is established. Alanine and aspartate are synthesized by the transamination of pyruvate and oxaloacetate, respectively. Glutamine is synthesized from NH4+ and glutamate, and asparagine is synthesized similarly. Proline and arginine are both derived from glutamate. Serine, formed from 3-phosphoglycerate, which comes from glycolysis, is the precursor of glycine and cysteine. Tyrosine is synthesized by the hydroxylation of phenylalanine, which is an essential amino acid.

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.

Essential (+ conditional) amino acid(s)Daily intake in mg per kg body mass
WHO [11] US NAM [12] FAO (2018) young children catch-up growth [13]
Histidine (H)101466
Isoleucine (I)201995
Leucine (L)3942198
Lysine (K)3038183
Methionine (M)
+ Cysteine (C)
10.4 + 4.1
(14.5 total)
19 total88
Phenylalanine (F)
+ Tyrosine (Y)
25 (total)33 total177
Threonine (T)1520103
Tryptophan (W)4529
Valine (V)2624130

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 nitrogen intake and nitrogen excreted in the liquid and solid wastes, because proteins represent the largest nitrogen content in a body. A positive balance occurs when more nitrogen is consumed than is excreted, which indicates that some of the nitrogen is being used by the body to build proteins. A negative nitrogen balance occurs when more nitrogen is excreted than is consumed, which indicates that there is insufficient intake for the body to maintain its health. Graduate students at the University of Illinois were fed an artificial diet so that there was a slightly positive nitrogen balance. Then one amino acid was omitted and the nitrogen balance recorded. If a positive balance continued, then that amino acid was deemed not essential. If a negative balance occurred, then that amino acid was slowly restored until a slightly positive nitrogen balance stabilized and the minimum amount recorded. [17] [18]

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 ion exchange chromatography to determine the actual amino acid content of foods. The USDA used this technique in their own labs to determine the content of 7793 foods across 28 categories. The USDA published the final database in 2018 to the public. [19]

The limiting amino acid depends on the human requirements and there are currently two sets of human requirements from authoritative sources: one published by WHO [11] and the other published by USDA. [12]

This table displays the number of Items in each Category with the same limiting Essential Amino Acid [11] [12] [19]
Based on WHO RequirementsBased on USDA Requirements
Category
Tryptophan
Threonine
Isoleucine
Leucine
Lysine
Methionine+Cystine
Phenylalanine+Tyrosine
Valine
Histidine
Tryptophan
Threonine
Isoleucine
Leucine
Lysine
Methionine+Cystine
Phenylalanine+Tyrosine
Valine
Histidine
American Indian/Alaska Native Foods400104001507203650010
Baby Foods2107358011151053413007
Baked Products0105338105101003392009
Beef Products276062000649228910176630001594
Beverages00021150110001125002
Breakfast Cereals00114010000011401000
Cereal Grains and Pasta0009143003121041480001
Dairy and Egg Products196421161220123191900111220032
Fast Foods43093980621640108215018
Fats and Oils000444000000024006
Finfish and Shellfish Products330150002280530174000067
Fruits and Fruit Juices1509541231331415174011353128
Lamb10052543201550100220791120287
Legumes and Legume Products0001261540220000127175000
Meals100115001402202240010
Nut and Seed Products001249680000011310310002
Pork Products1101540202490200019707301512
Poultry Products6126581002870362201675809933
Restaurant Foods0931424304111250933120015
Sausages and Luncheon Meats501310207801411168111047
Snacks20068360412106819003
Soups0027102807000012131001
Spices and Herbs30061130113203124001
Sweets0103174701201011747005
Vegetables and Vegetable Products70823811419901819132801121442460258

Protein quality

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 the 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). [20] For comparison, there are 2.8 grams of protein in a serving of raw broccoli (100 grams) or 82 mg of protein per calorie (34 calories total), or the Daily Value of 47.67g of protein after eating 1,690g of raw broccoli a day at 574 cal. [21] An egg contains 12.5g of protein per 100g, but 4 mg more protein per calorie, or the protein DV after 381g of egg, which is 545 cal. [22] The ratio of essential amino acids (the quality of protein) is not taken into account, one would actually need to eat more than 3 kg of broccoli a day to have a healthy protein profile, and almost 6 kg to get enough calories. [21] It is recommended that adult humans obtain between 10–35% of their 2000 calories a day as protein. [23]

Complete proteins in non-human animals

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 sulfur-containing amino acids, methionine and homocysteine, can be converted into each other but neither can be synthesized de novo in humans. Likewise, cysteine can be made from homocysteine but cannot be synthesized on its own. So, for convenience, sulfur-containing amino acids are sometimes considered a single pool of nutritionally equivalent amino acids as are the aromatic amino acid pair, phenylalanine and tyrosine. Likewise arginine, ornithine, and citrulline, which are interconvertible by the urea cycle, are considered a single group.[ citation needed ]

Effects of deficiency

If one of the essential amino acids is not available in the required quantities, protein synthesis will be inhibited, irrespective of the availability of the other amino acids. [2] Protein deficiency has been shown to affect all of the body's organs and many of its systems, for example affecting brain development in infants and young children; inhibiting upkeep of the immune system, increasing risk of infection; affecting gut mucosal function and permeability, thereby reducing absorption and increasing vulnerability to systemic disease; and impacting kidney function. [2] The physical signs of protein deficiency include edema, failure to thrive in infants and children, poor musculature, dull skin, and thin and fragile hair. Biochemical changes reflecting protein deficiency include low serum albumin and low serum transferrin. [2]

The amino acids that are essential in the human diet were established in a series of experiments led by William Cumming Rose. The experiments involved elemental diets to healthy male graduate students. These diets consisted of corn starch, sucrose, butterfat without protein, corn oil, inorganic salts, the known vitamins, a large brown "candy" made of liver extract flavored with peppermint oil (to supply any unknown vitamins), and mixtures of highly purified individual amino acids. The main outcome measure was nitrogen balance. Rose noted that the symptoms of nervousness, exhaustion, and dizziness were encountered to a greater or lesser extent whenever human subjects were deprived of an essential amino acid. [17]

Essential amino acid deficiency should be distinguished from protein-energy malnutrition, which can manifest as marasmus or kwashiorkor. Kwashiorkor was once attributed to pure protein deficiency in individuals who were consuming enough calories ("sugar baby syndrome"). However, this theory has been challenged by the finding that there is no difference in the diets of children developing marasmus as opposed to kwashiorkor. [26] Still, for instance in Dietary Reference Intakes (DRI) maintained by the USDA, lack of one or more of the essential amino acids is described as protein-energy malnutrition. [2]

See also

Related Research Articles

<span class="mw-page-title-main">Amino acid</span> Organic compounds containing amine and carboxylic groups

Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 appear in the genetic code of life.

<span class="mw-page-title-main">Methionine</span> Sulfur-containing amino acid

Methionine is an essential amino acid in humans.

<span class="mw-page-title-main">Isoleucine</span> Chemical compound

Isoleucine (symbol Ile or I) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+3 form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO form under biological conditions), and a hydrocarbon side chain with a branch (a central carbon atom bound to three other carbon atoms). It is classified as a non-polar, uncharged (at physiological pH), branched-chain, aliphatic amino acid. It is essential in humans, meaning the body cannot synthesize it. Essential amino acids are necessary in the human diet. In plants isoleucine can be synthesized from threonine and methionine. In plants and bacteria, isoleucine is synthesized from pyruvate employing leucine biosynthesis enzymes. It is encoded by the codons AUU, AUC, and AUA.

A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excreted by cells to create non-cellular structures, such as hair, scales, feathers, or exoskeletons. Some nutrients can be metabolically converted to smaller molecules in the process of releasing energy, such as for carbohydrates, lipids, proteins, and fermentation products, leading to end-products of water and carbon dioxide. All organisms require water. Essential nutrients for animals are the energy sources, some of the amino acids that are combined to create proteins, a subset of fatty acids, vitamins and certain minerals. Plants require more diverse minerals absorbed through roots, plus carbon dioxide and oxygen absorbed through leaves. Fungi live on dead or living organic matter and meet nutrient needs from their host.

<span class="mw-page-title-main">Proteinogenic amino acid</span> Amino acid that is incorporated biosynthetically into proteins during translation

Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation. The word "proteinogenic" means "protein creating". Throughout known life, there are 22 genetically encoded (proteinogenic) amino acids, 20 in the standard genetic code and an additional 2 that can be incorporated by special translation mechanisms.

A low-protein diet is a diet in which people decrease their intake of protein. A low-protein diet is used as a therapy for inherited metabolic disorders, such as phenylketonuria and homocystinuria, and can also be used to treat kidney or liver disease. Low protein consumption appears to reduce the risk of bone breakage, presumably through changes in calcium homeostasis. Consequently, there is no uniform definition of what constitutes low-protein, because the amount and composition of protein for an individual with phenylketonuria would differ substantially from one with homocystinuria or tyrosinemia.

Protein digestibility-corrected amino acid score (PDCAAS) is a method of evaluating the quality of a protein based on both the amino acid requirements of humans and their ability to digest it.

Ham's tissue culture medium is a growth medium for mammalian cells.

Biological value (BV) is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body. It captures how readily the digested protein can be used in protein synthesis in the cells of the organism. Proteins are the major source of nitrogen in food. BV assumes protein is the only source of nitrogen and measures the amount of nitrogen ingested in relation to the amount which is subsequently excreted. The remainder must have been incorporated into the proteins of the organisms body. A ratio of nitrogen incorporated into the body over nitrogen absorbed gives a measure of protein "usability" – the BV.

<span class="mw-page-title-main">Protein (nutrient)</span> Nutrient for the human body

Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue and can also serve as a fuel source. As a fuel, proteins provide as much energy density as carbohydrates: 4 kcal per gram; in contrast, lipids provide 9 kcal per gram. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition.

Protein combining or protein complementing is a dietary theory for protein nutrition that purports to optimize the biological value of protein intake. According to the theory, vegetarian and vegan diets may provide an insufficient amount of some essential amino acids, making protein combining with multiple foods necessary to obtain a complete protein food. The terms complete and incomplete are outdated in relation to plant protein. In fact, all plant foods contain all 20 amino acids including the 9 essential amino acids in varying amounts.

<span class="mw-page-title-main">Amino acid synthesis</span> The set of biochemical processes by which amino acids are produced

Amino acid biosynthesis is the set of biochemical processes by which the amino acids are produced. The substrates for these processes are various compounds in the organism's diet or growth media. Not all organisms are able to synthesize all amino acids. For example, humans can synthesize 11 of the 20 standard amino acids. These 11 are called the non-essential amino acids).

<span class="mw-page-title-main">Aromatic amino acid</span> Amino acid having an aromatic ring

An aromatic amino acid is an amino acid that includes an aromatic ring.

<span class="mw-page-title-main">Glucogenic amino acid</span> Type of amino acid

A glucogenic amino acid is an amino acid that can be converted into glucose through gluconeogenesis. This is in contrast to the ketogenic amino acids, which are converted into ketone bodies.

<span class="mw-page-title-main">Cecile Hoover Edwards</span> African American nutritional researcher

Cecile Hoover Edwards was an American nutritional researcher whose career focused on improving the nutrition and well-being of disadvantaged people. Her scientific focus was on finding low-cost foods with an optimal amino acid composition, with a special interest in methionine metabolism. She was also a university administrator, serving as dean of several schools within Howard University between 1974 and 1990.

The ciliate, dasycladacean and Hexamita nuclear code is a genetic code used by certain ciliate, dasycladacean and Hexamita species.

The Mesodinium nuclear code is a genetic code used by the nuclear genome of the ciliates Mesodinium and Myrionecta.

Essential amino acids(EAAs) are amino acids that are necessary to build proteins in an organism. The source of complete EAAs are both animal and plant-based food.

References

  1. Young VR (1994). "Adult amino acid requirements: the case for a major revision in current recommendations". J. Nutr. 124 (8 Suppl): 1517S–1523S. doi: 10.1093/jn/124.suppl_8.1517S . PMID   8064412.
  2. 1 2 3 4 5 6 7 Otten, Jennifer J.; Hellwig, Jennifer Pitzi; Meyers, Linda D., eds. (2006) [1943]. Dietary Reference Intakes: The Essential Guide to Nutrient Requirements (Technical report). doi:10.17226/11537. ISBN   978-0-309-15742-1.
  3. 1 2 Lopez, Michael J.; Mohluddin, Shamim S. (18 March 2022). Biochemistry, Essential Amino Acids (Technical report).
  4. "Limiting Amino Acids". National Agricultural Library. 30 November 2012. Retrieved 19 September 2022.
  5. Fürst P, Stehle P (1 June 2004). "What are the essential elements needed for the determination of amino acid requirements in humans?". Journal of Nutrition. 134 (6 Suppl): 1558S–1565S. doi: 10.1093/jn/134.6.1558S . PMID   15173430.
  6. Reeds PJ (1 July 2000). "Dispensable and indispensable amino acids for humans". J. Nutr. 130 (7): 1835S–40S. doi: 10.1093/jn/130.7.1835S . PMID   10867060.
  7. Richard Cammack. "Newsletter 2009, Biochemical Nomenclature Committee of IUPAC and NC-IUBMB". Archived from the original on 12 September 2017. Retrieved 16 July 2012.
  8. "PROTEIN AND AMINO ACID REQUIREMENTS IN HUMAN NUTRITION". 2007. p. 29.
  9. Fürst P, Stehle P (1 June 2004). "What are the essential elements needed for the determination of amino acid requirements in humans?". J. Nutr. 134 (6 Suppl): 1558S–1565S. doi: 10.1093/jn/134.6.1558S . PMID   15173430.
  10. Reeds PJ (1 July 2000). "Dispensable and indispensable amino acids for humans". J. Nutr. 130 (7): 1835S–40S. doi: 10.1093/jn/130.7.1835S . PMID   10867060.
  11. 1 2 3 4 FAO/WHO/UNU (2007). Protein and amino acid requirements in human nutrition: report of a joint FAO/WHO/UNU expert consultation (PDF). WHO Press. ISBN   978-9241209359., page 150
  12. 1 2 3 Institute of Medicine (2002). "Protein and Amino Acids". Dietary Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. p. 680. doi:10.17226/10490. ISBN   978-0-309-08525-0.
  13. "Publication card | FAO | Food and Agriculture Organization of the United Nations". www.fao.org. p. x, Table 1 - Protein and amino acid requirement and amino acid reference pattern proposed for FUF-YC (1–2 year) and for RUTF (target weight gain value of 10 g/kg/d), in infants and children, 6 months to 5 years. Retrieved 6 March 2023.
  14. Imura K, Okada A (1998). "Amino acid metabolism in pediatric patients". Nutrition. 14 (1): 143–8. doi:10.1016/S0899-9007(97)00230-X. PMID   9437700.
  15. "Methionine and Cysteine metabolism". National Center for Biotechnology Information PubChem. 18 May 2022. Retrieved 21 September 2022.
  16. "Phenylalanine and Tyrosine Metabolism". National Center for Biotechnology Information PubChem. 18 May 2022. Retrieved 21 September 2022.
  17. 1 2 Rose, WC; Haines, WJ; Warner, DT (1951). "The amino acid requirements of man. III. The role of isoleucine; additional evidence concerning histidine" (PDF). J Biol Chem. 193 (2): 605–612. doi: 10.1016/S0021-9258(18)50916-9 . PMID   14907749. Archived (PDF) from the original on 15 June 2016. Retrieved 15 December 2012.
  18. 1 2 McGilvery, Robert W. Ph.D.; et al. (Gerald Goldstein M.D.) (1979) [1970]. "Chapter 41 Nutrition: The Nitrogen Economy". Biochemistry, a Functional Approach (2nd ed.). W. B. Saunders Company. pp. 785–796. ISBN   0-7216-5912-8.
  19. 1 2 "FoodData Central Standard Reference (SR) Legacy Foods". U.S. Department of Agriculture, Agricultural Research Service. April 2019.
  20. SDA National Nutrient Database for Standard Reference (Release 21 ed.). U.S. Department of Agriculture, Agricultural Research Service. 2008.
  21. 1 2 Vanovschi, Vitalii. "Broccoli, raw: nutritional value and analysis". www.nutritionvalue.org. Retrieved 4 November 2019.
  22. Vanovschi, Vitalii. "Egg, poached, cooked, whole: nutritional value and analysis". www.nutritionvalue.org. Retrieved 4 November 2019.
  23. "Web MD Protein: Are You Getting Enough?". webmd.com. 5 September 2014. Retrieved 31 March 2015.
  24. Rose WC, Haines WJ, Warner DT, Johnson JE (1951). "The amino acid requirements of man. II. The role of threonine and histidine". The Journal of Biological Chemistry. 188 (1): 49–58. doi: 10.1016/S0021-9258(18)56144-5 . PMID   14814112.
  25. Kopple JD, Swendseid ME (May 1975). "Evidence that histidine is an essential amino acid in normal and chronically uremic man". J Clin Invest. 55 (5): 881–891. doi:10.1172/JCI108016. PMC   301830 . PMID   1123426.
  26. Ahmed T, Rahman S, Cravioto A (2009). "Oedematous malnutrition". The Indian Journal of Medical Research. 130 (5): 651–4. PMID   20090122.