Essential amino acid
Encyclopedia
An essential amino acid or indispensable amino acid is an amino acid
that cannot be synthesized de novo
by the organism (usually referring to humans), and therefore must be supplied in the diet.
(*) Essential only in certain cases.
(**) Truly unclassified. Added to sustain the 22 Numbers of Essential Amino Acids.
The amino acids regarded as essential for humans are phenylalanine
, valine
, threonine
, tryptophan
, isoleucine
, methionine
, leucine
, lysine
, and histidine
. Additionally, cysteine
(or sulphur-containing amino acids), tyrosine
(or aromatic amino acids), and arginine
are required by infants and growing children. Essential amino acids are "essential" not because they are more important to life than the others, but because the body does not synthesize them, making it essential to include them in one's diet in order to obtain them. In addition, the amino acids arginine
, cysteine
, glycine
, glutamine
, histidine
, proline
, serine
and tyrosine
are considered conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts. An example would be with the disease phenylketonuria
(PKU). Individuals living with PKU must keep their intake of phenylalanine extremely low to prevent mental retardation and other metabolic complications. However, they cannot synthesize tyrosine from phenylalanine, so tyrosine becomes essential in the diet of PKU patients.
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.
recommended daily amounts currently in use for essential amino acids in adult humans, 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.
, the cells of eukaryote
s require up to 21 different amino acids for protein synthesis. A shortfall of any one of these amino acids would thus be a limiting factor
in protein synthesis. However, eukaryotes can synthesize some of these amino acids from other substrates. Consequently, only a subset of the amino acids used in protein synthesis are essential nutrient
s. Whether a particular amino acid is essential depends upon the species and the stage of development.
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
. 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 is also essential for adult humans.
Because of the obvious difference in the nutritional value of zein versus casein in rat nutrition, 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 digestibility-corrected amino acid score. 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.
Although proteins from plant sources tend to have a relatively low biological value, in comparison to protein from eggs or milk, they are nevertheless "complete" in that they contain at least trace amounts of all of the amino acids that are essential in human nutrition. Eating various plant foods in combination can provide a protein of higher biological value; however, vegetarians do not need to intentionally combine different foods for this purpose necessarily.
. The experiments involved elemental diets to healthy male graduate students. These diets consisted of cornstarch, 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.
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.
Alternative mnemonics based on the amino acids' assigned single letters include LIFT HIM KIW(V)I and TV FILM HW(R)K.
Another method uses the first letter of each essential amino acid to begin each word in a phrase, such as: "Any Help In Learning These Little Molecules Proves Truly Valuable." This method begins with the two amino acids that need some qualifications as to their requirements.
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
that cannot be synthesized de novo
De novo synthesis
De novo synthesis refers to the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to their being recycled after partial degradation. For example, nucleotides are not needed in the diet as they can be constructed from small precursor molecules such as...
by the organism (usually referring to humans), and therefore must be supplied in the diet.
Essentiality vs. conditional essentiality in humans
| Essential | Nonessential |
|---|---|
| Isoleucine Isoleucine Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA.... |
Alanine Alanine Alanine is an α-amino acid with the chemical formula CH3CHCOOH. The L-isomer is one of the 20 amino acids encoded by the genetic code. Its codons are GCU, GCC, GCA, and GCG. It is classified as a nonpolar amino acid... |
| Leucine Leucine Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins... |
Arginine Arginine Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during... * |
| Lysine Lysine Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG.... |
Aspartate |
| Methionine Methionine Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein... |
Cysteine Cysteine Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid... * |
| Phenylalanine Phenylalanine Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form... |
Glutamate |
| Threonine Threonine Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar... |
Glutamine Glutamine Glutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders... * |
| Tryptophan Tryptophan Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG... |
Glycine Glycine Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid... * |
| Valine Valine Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar... |
Proline Proline Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary... * |
| Histidine Histidine Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans... |
Serine Serine Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:... * |
| Tyrosine Tyrosine Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group... * |
Asparagine Asparagine Asparagine is one of the 20 most common natural amino acids on Earth. It has carboxamide as the side-chain's functional group. It is not an essential amino acid... * |
| Selenocysteine Selenocysteine Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:... ** |
Pyrrolysine Pyrrolysine Pyrrolysine is a naturally occurring, genetically coded amino acid used by some methanogenic archaea and one known bacterium in enzymes that are part of their methane-producing metabolism. It is similar to lysine, but with an added pyrroline ring linked to the end of the lysine side chain... ** |
(*) Essential only in certain cases.
(**) Truly unclassified. Added to sustain the 22 Numbers of Essential Amino Acids.
The amino acids regarded as essential for humans are phenylalanine
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
, valine
Valine
Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar...
, threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
, tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
, isoleucine
Isoleucine
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....
, methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
, leucine
Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...
, lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
, and histidine
Histidine
Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
. Additionally, cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
(or sulphur-containing amino acids), tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
(or aromatic amino acids), and arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
are required by infants and growing children. Essential amino acids are "essential" not because they are more important to life than the others, but because the body does not synthesize them, making it essential to include them in one's diet in order to obtain them. In addition, the amino acids arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
, cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
, glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
, glutamine
Glutamine
Glutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders...
, histidine
Histidine
Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
, proline
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
, serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
and tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
are considered conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts. An example would be with the disease phenylketonuria
Phenylketonuria
Phenylketonuria is an autosomal recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...
(PKU). Individuals living with PKU must keep their intake of phenylalanine extremely low to prevent mental retardation and other metabolic complications. However, they cannot synthesize tyrosine from phenylalanine, so tyrosine becomes essential in the diet of PKU patients.
The distinction between essential and non-essential amino acids is somewhat unclear, as some amino acids can be produced from others. The sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-containing amino acids, methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
and homocysteine
Homocysteine
Homocysteine is a non-protein amino acid with the formula HSCH2CH2CHCO2H. It is a homologue of the amino acid cysteine, differing by an additional methylene group. It is biosynthesized from methionine by the removal of its terminal Cε methyl group...
, 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
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
and tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
. Likewise arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
, ornithine
Ornithine
Ornithine is an amino acid that plays a role in the urea cycle.-Role in urea cycle:L-Ornithine is one of the products of the action of the enzyme arginase on L-arginine, creating urea. Therefore, ornithine is a central part of the urea cycle, which allows for the disposal of excess nitrogen....
, and citrulline
Citrulline
The organic compound citrulline is an α-amino acid. Its name is derived from citrullus, the Latin word for watermelon, from which it was first isolated in 1930.It has the idealized formula H2NCNH3CHCO2H...
, which are interconvertible by the urea cycle
Urea cycle
The urea cycle is a cycle of biochemical reactions occurring in many animals that produces urea from ammonia . This cycle was the first metabolic cycle discovered , five years before the discovery of the TCA cycle...
, are considered a single group.
Recommended daily amounts
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 WHOWho
Who may refer to:* Who , an English-language pronoun* who , a Unix command* Who?, one of the Five Ws in journalism- Art and entertainment :* Who? , a 1958 novel by Algis Budrys...
recommended daily amounts currently in use for essential amino acids in adult humans, together with their standard one-letter abbreviations.
| Amino acid(s) | mg per kg body weight | mg per 70 kg | mg per 100 kg |
|---|---|---|---|
| I Isoleucine Isoleucine Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA.... |
20 | 1400 | 2000 |
| L Leucine Leucine Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins... |
39 | 2730 | 3900 |
| K Lysine Lysine Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG.... |
30 | 2100 | 3000 |
| M Methionine Methionine Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein... + C Cysteine Cysteine Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid... |
10.4 + 4.1 (15 total) | 1050 | 1500 |
| F Phenylalanine Phenylalanine Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form... + Y Tyrosine Tyrosine Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group... |
25 (total) | 1750 | 2500 |
| T Threonine Threonine Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar... |
15 | 1050 | 1500 |
| W Tryptophan Tryptophan Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG... |
4 | 280 | 400 |
| V Valine Valine Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar... |
26 | 1820 | 2600 |
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.
Use of essential amino acids
At the level of the ribosomeRibosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
, the cells of eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s require up to 21 different amino acids for protein synthesis. A shortfall of any one of these amino acids would thus be a limiting factor
Limiting factor
A limiting factor or limiting resource is a factor that controls a process, such as organism growth or species population, size, or distribution. The availability of food, predation pressure, or availability of shelter are examples of factors that could be limiting for an organism...
in protein synthesis. However, eukaryotes can synthesize some of these amino acids from other substrates. Consequently, only a subset of the amino acids used in protein synthesis are essential nutrient
Essential nutrient
An essential nutrient is a nutrient required for normal body functioning that either cannot be synthesized by the body at all, or cannot be synthesized in amounts adequate for good health , and thus must be obtained from a dietary source...
s. Whether a particular amino acid is essential depends upon the species and the stage of development.
Scientists had known since the early 20th century that rats could not survive on a diet whose only protein source was zein
Zein
Zein is a class of prolamine protein found in maize. It is usually manufactured as a powder from corn gluten meal.-Commercial uses:Zein is one of the best understood plant proteins and has a variety of industrial and food uses...
, which comes from maize
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
(corn), but recovered if they were fed casein
Casein
Casein is the name for a family of related phosphoprotein proteins . These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 60% and 65% of the proteins in human milk....
from cow's milk. This led William Cumming Rose
William Cumming Rose
William Cumming Rose was an American nutritionist whose research in the 1930s discovered the essential amino acid threonine....
to the discovery of the essential amino acid threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
. Through manipulation of rodent diets, Rose was able to show that ten amino acids are essential for rats: lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
, tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
, histidine
Histidine
Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
, phenylalanine
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
, leucine
Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...
, isoleucine
Isoleucine
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....
, methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
, valine
Valine
Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar...
, and arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
, 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 is also essential for adult humans.
Because of the obvious difference in the nutritional value of zein versus casein in rat nutrition, various attempts have been made to express the "quality" or "value" of various kinds of protein. Measures include the biological value
Biological Value
Biological value is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body. It summarises how readily the broken down protein can be used in protein synthesis in the cells of the organism. Proteins are the major source of...
, net protein utilization
Net protein utilization
The net protein utilization, or NPU, is the ratio of amino acid converted to proteins to the ratio of amino acids supplied. This figure is somewhat affected by the salvage of essential amino acids within the body, but is profoundly affected by the level of limiting amino acids within a...
, protein digestibility-corrected amino acid score. 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
Feed conversion ratio
In animal husbandry, feed conversion ratio , feed conversion rate, or feed conversion efficiency , is a measure of an animal's efficiency in converting feed mass into increased body mass....
. 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.
Although proteins from plant sources tend to have a relatively low biological value, in comparison to protein from eggs or milk, they are nevertheless "complete" in that they contain at least trace amounts of all of the amino acids that are essential in human nutrition. Eating various plant foods in combination can provide a protein of higher biological value; however, vegetarians do not need to intentionally combine different foods for this purpose necessarily.
Essential amino acid deficiency
The amino acids that are essential in the human diet were established in a series of experiments led by William Cumming RoseWilliam Cumming Rose
William Cumming Rose was an American nutritionist whose research in the 1930s discovered the essential amino acid threonine....
. The experiments involved elemental diets to healthy male graduate students. These diets consisted of cornstarch, 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
Nitrogen balance
Nitrogen balance is the measure of nitrogen output subtracted from nitrogen input.Blood urea nitrogen can be used in estimating nitrogen balance, as can the urea concentration in urine....
. 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.
Essential amino acid deficiency should be distinguished from protein-energy malnutrition
Protein-energy malnutrition
Protein-energy malnutrition refers to a form of malnutrition where there is inadequate protein intake.Types include:* Kwashiorkor...
, which can manifest as marasmus
Marasmus
Marasmus is a form of severe protein-energy malnutrition characterized by energy deficiency.A child with marasmus looks emaciated. Body weight may be reduced to less than 80% of the average weight that corresponds to the height . Marasmus occurrence increases prior to age 1, whereas kwashiorkor...
or kwashiorkor
Kwashiorkor
Kwashiorkor is an acute form of childhood protein-energy malnutrition characterized by edema, irritability, anorexia, ulcerating dermatoses, and an enlarged liver with fatty infiltrates. The presence of edema caused by poor nutrition defines 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.
Mnemonics
Using the one-letter designation shown above, mnemonic devices have been developed for use in memorizing the essential amino acids. Previous devices have utilized the first letter of the amino acids' names, and in general did not include arginine which is not always essential. Mnemonic devices in common use are PVT TIM HaLL and TT HALL V(ery) IMP(ortant).Alternative mnemonics based on the amino acids' assigned single letters include LIFT HIM KIW(V)I and TV FILM HW(R)K.
Another method uses the first letter of each essential amino acid to begin each word in a phrase, such as: "Any Help In Learning These Little Molecules Proves Truly Valuable." This method begins with the two amino acids that need some qualifications as to their requirements.
See also
- Biological Value (BV)Biological ValueBiological value is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body. It summarises how readily the broken down protein can be used in protein synthesis in the cells of the organism. Proteins are the major source of...
- Complete proteinComplete proteinA complete protein is a source of protein that contains an adequate proportion of all nine of the essential amino acids necessary for the dietary needs of humans or other animals...
- Essential nutrientEssential nutrientAn essential nutrient is a nutrient required for normal body functioning that either cannot be synthesized by the body at all, or cannot be synthesized in amounts adequate for good health , and thus must be obtained from a dietary source...
- Essential fatty acidEssential fatty acidEssential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them...
- Edible protein per unit area of landEdible protein per unit area of landEdible protein per unit area of land is a measure of agricultural productivity. This measure for various major foodstuffs is shown in the chart below. Values are expressed for one calendar year. Biological values and usable protein values have been added to the chart to show the true relative value...
- List of standard amino acids
- Orthomolecular medicineOrthomolecular medicineOrthomolecular medicine is a form of complementary and alternative medicine that seeks to maintain health and prevent or treat diseases by optimizing nutritional intake and/or prescribing supplements...
- Protein Digestibility Corrected Amino Acid ScorePDCAASProtein Digestibility Corrected Amino Acid Score is a method of evaluating the protein quality based on both the amino acid requirements of humans and their ability to digest it...
External links
- Amino acid content of some vegetarian foods - In a meal, make sure at least one food has a low value in each row of the "Amount to meet RDA" column, at veganhealth.org
- Amino Acid Profiles of Some Common Feeds at Virginia Tech
- Molecular Expressions: The Amino Acid Collection - Has detailed information and crystal photographs of each amino acid. at Florida State UniversityFlorida State UniversityThe Florida State University is a space-grant and sea-grant public university located in Tallahassee, Florida, United States. It is a comprehensive doctoral research university with medical programs and significant research activity as determined by the Carnegie Foundation...