82-452: See text The Mormyridae , sometimes called "elephantfish" (more properly freshwater elephantfish ), are a superfamily of weakly electric fish in the order Osteoglossiformes native to Africa . It is by far the largest family in the order, with around 200 species. Members of the family can be popular, if challenging, aquarium species. These fish have a large brain size and unusually high intelligence . They are not to be confused with
164-488: A beat with a frequency equal to the difference between the discharge frequencies of the two fish. The jamming avoidance response comes into play when fish are exposed to a slow beat. If the neighbour's frequency is higher, the fish lowers its frequency, and vice versa. A similar jamming avoidance response was discovered in the distantly related Gymnarchus niloticus , the African knifefish, by Walter Heiligenberg in 1975, in
246-472: A Brønsted acid. Histidine under these conditions can act both as a Brønsted acid and a base. For amino acids with uncharged side-chains the zwitterion predominates at pH values between the two p K a values, but coexists in equilibrium with small amounts of net negative and net positive ions. At the midpoint between the two p K a values, the trace amount of net negative and trace of net positive ions balance, so that average net charge of all forms present
328-518: A discharge that is typically less than one volt. These are too weak to stun prey and instead are used for navigation , electrolocation in conjunction with electroreceptors in their skin, and electrocommunication with other electric fish. The major groups of weakly electric fish are the Osteoglossiformes , which include the Mormyridae (elephantfishes) and the African knifefish Gymnarchus , and
410-418: A further example of convergent evolution between the electric fishes of Africa and South America. Both the neural computational mechanisms and the behavioural responses are nearly identical in the two groups. Amino acid 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
492-543: A hydrogen atom. With the exception of glycine, for which the side chain is also a hydrogen atom, the α–carbon is stereogenic . All chiral proteogenic amino acids have the L configuration. They are "left-handed" enantiomers , which refers to the stereoisomers of the alpha carbon. A few D -amino acids ("right-handed") have been found in nature, e.g., in bacterial envelopes , as a neuromodulator ( D - serine ), and in some antibiotics . Rarely, D -amino acid residues are found in proteins, and are converted from
574-448: A nearly identical mechanism. All fish, indeed all vertebrates , use electrical signals in their nerves and muscles. Cartilaginous fishes and some other basal groups use passive electrolocation with sensors that detect electric fields; the platypus and echidna have separately evolved this ability. The knifefishes and elephantfishes actively electrolocate, generating weak electric fields to find prey. Finally, fish in several groups have
656-439: A pK a of 6.0, and is only around 10% protonated at neutral pH. Because histidine is easily found in its basic and conjugate acid forms it often participates in catalytic proton transfers in enzyme reactions. The polar, uncharged amino acids serine (Ser, S), threonine (Thr, T), asparagine (Asn, N) and glutamine (Gln, Q) readily form hydrogen bonds with water and other amino acids. They do not ionize in normal conditions,
738-454: A patch of hydrophobic amino acids on their surface that sticks to the membrane. In a similar fashion, proteins that have to bind to positively charged molecules have surfaces rich in negatively charged amino acids such as glutamate and aspartate , while proteins binding to negatively charged molecules have surfaces rich in positively charged amino acids like lysine and arginine . For example, lysine and arginine are present in large amounts in
820-461: A prominent exception being the catalytic serine in serine proteases . This is an example of severe perturbation, and is not characteristic of serine residues in general. Threonine has two chiral centers, not only the L (2 S ) chiral center at the α-carbon shared by all amino acids apart from achiral glycine, but also (3 R ) at the β-carbon. The full stereochemical specification is (2 S ,3 R )- L - threonine . Nonpolar amino acid interactions are
902-402: A signal from the nervous system. Neurons release the neurotransmitter acetylcholine ; this triggers acetylcholine receptors to open and sodium ions to flow into the electrocytes. The influx of positively charged sodium ions causes the cell membrane to depolarize slightly. This in turn causes the gated sodium channels at the anterior end of the cell to open, and a flood of sodium ions enters
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#1732847887263984-693: A small minority of all fishes, include both oceanic and freshwater species, and both cartilaginous and bony fishes. Electric fish produce their electrical fields from an electric organ . This is made up of electrocytes, modified muscle or nerve cells, specialized for producing strong electric fields, used to locate prey, for defence against predators , and for signalling , such as in courtship. Electric organ discharges are two types, pulse and wave, and vary both by species and by function. Electric fish have evolved many specialised behaviours. The predatory African sharptooth catfish eavesdrops on its weakly electric mormyrid prey to locate it when hunting, driving
1066-451: A way unique among amino acids. Selenocysteine (Sec, U) is a rare amino acid not directly encoded by DNA, but is incorporated into proteins via the ribosome. Selenocysteine has a lower redox potential compared to the similar cysteine, and participates in several unique enzymatic reactions. Pyrrolysine (Pyl, O) is another amino acid not encoded in DNA, but synthesized into protein by ribosomes. It
1148-482: Is Pyz –Phe–boroLeu, and MG132 is Z –Leu–Leu–Leu–al. To aid in the analysis of protein structure, photo-reactive amino acid analogs are available. These include photoleucine ( pLeu ) and photomethionine ( pMet ). Amino acids are the precursors to proteins. They join by condensation reactions to form short polymer chains called peptides or longer chains called either polypeptides or proteins. These chains are linear and unbranched, with each amino acid residue within
1230-505: Is a clade . Most electric organs evolved from myogenic tissue (which forms muscle), however, one group of Gymnotiformes , the Apteronotidae , derived their electric organ from neurogenic tissue (which forms nerves). In Gymnarchus niloticus (the African knifefish), the tail, trunk, hypobranchial, and eye muscles are incorporated into the organ, most likely to provide rigid fixation for the electrodes while swimming. In some other species,
1312-421: Is found in archaeal species where it participates in the catalytic activity of several methyltransferases. Amino acids with the structure NH + 3 −CXY−CXY−CO − 2 , such as β-alanine , a component of carnosine and a few other peptides, are β-amino acids. Ones with the structure NH + 3 −CXY−CXY−CXY−CO − 2 are γ-amino acids, and so on, where X and Y are two substituents (one of which
1394-404: Is greatly enlarged, enabling them to interpret complex bio-electrical signals, and to the large size of the valve. Secondly, an auditory vesicle (a small bladder) is present inside the labyrinth of the left and right inner ears . This vesicle, together with a bag with an otolith (sacculum containing the otolith sagitta), itself communicating to the lagena (containing the otolith asteriscus),
1476-573: Is in fact unique among vertebrates, completely independent of the other organs ; it is neither connected to the labyrinth to which only one otolith bag (the utriculus containing the otolith lapillus ) is attached, nor is it connected to the swim bladder (except in embryos ) of which it has the same histological structure , nor is it therefore related to the pharynx . Some species possess modifications of their mouthparts to facilitate electrolocating and feeding on small invertebrates buried in muddy substrates. The shape and structure of these leads to
1558-933: Is indeed what has driven the evolution of the electric organs in the two groups. Actively electrolocating fish are marked on the phylogenetic tree with a small yellow lightning flash [REDACTED] . Fish able to deliver electric shocks are marked with a red lightning flash [REDACTED] . Non-electric and purely passively electrolocating species are not shown. Torpediniformes (electric rays) (69 spp) [REDACTED] [REDACTED] [REDACTED] Rajiformes (skates) (~200 spp) [REDACTED] [REDACTED] elephantfishes (~200 spp) [REDACTED] [REDACTED] African knifefish (1 sp) [REDACTED] [REDACTED] (>100 spp) [REDACTED] [REDACTED] (3 spp) [REDACTED] [REDACTED] [REDACTED] (11 spp) [REDACTED] [REDACTED] [REDACTED] Stargazers (50 spp) [REDACTED] [REDACTED] Weakly electric fish generate
1640-681: Is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isoelectric point, and some amino acids (in particular, with nonpolar side chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point. The 20 canonical amino acids can be classified according to their properties. Important factors are charge, hydrophilicity or hydrophobicity , size, and functional groups. These properties influence protein structure and protein–protein interactions . The water-soluble proteins tend to have their hydrophobic residues ( Leu , Ile , Val , Phe , and Trp ) buried in
1722-510: Is normally H). The common natural forms of amino acids have a zwitterionic structure, with −NH + 3 ( −NH + 2 − in the case of proline) and −CO − 2 functional groups attached to the same C atom, and are thus α-amino acids, and are the only ones found in proteins during translation in the ribosome. In aqueous solution at pH close to neutrality, amino acids exist as zwitterions , i.e. as dipolar ions with both NH + 3 and CO − 2 in charged states, so
SECTION 20
#17328478872631804-503: Is perforated with small pores leading to electroreceptors. The retina is called a "grouped retina", an eye structure seen in mormyrids and a few other fishes. Instead of being smooth, their retina is composed of tiny cups, acting like parabolic mirrors. Because of the murky waters they inhabit, the cones in their eyes have adapted to see only red light. The cups are made of four layers of light-reflecting proteins, funneling red light to areas of cones, intensifying its brightness 10-fold, while
1886-403: Is rare. For example, 25 human proteins include selenocysteine in their primary structure, and the structurally characterized enzymes (selenoenzymes) employ selenocysteine as the catalytic moiety in their active sites. Pyrrolysine and selenocysteine are encoded via variant codons. For example, selenocysteine is encoded by stop codon and SECIS element . N -formylmethionine (which is often
1968-460: Is similar to the low voltage electrolocative discharge of the electric eel . This is thought to be a form of bluffing Batesian mimicry of the powerfully protected electric eel. Fish that prey on electrolocating fish may "eavesdrop" on the discharges of their prey to detect them. The electroreceptive African sharptooth catfish ( Clarias gariepinus ) may hunt the weakly electric mormyrid, Marcusenius macrolepidotus in this way. This has driven
2050-527: Is similar to the use of abbreviation codes for degenerate bases . Unk is sometimes used instead of Xaa , but is less standard. Ter or * (from termination) is used in notation for mutations in proteins when a stop codon occurs. It corresponds to no amino acid at all. In addition, many nonstandard amino acids have a specific code. For example, several peptide drugs, such as Bortezomib and MG132 , are artificially synthesized and retain their protecting groups , which have specific codes. Bortezomib
2132-517: Is so brief that the discharges fuse together to form a wave. The electric discharge is produced from an electric organ that evolved from muscle , as can also be seen in gymnotiform electric fish, electric rays, and skates. The convergent evolution between the South American gymnotiforms and the African Mormyridae is remarkable, with the electric organ being produced by the substitution of
2214-474: Is synthesised from proline . Another example is selenomethionine ). Non-proteinogenic amino acids that are found in proteins are formed by post-translational modification . Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a phospholipid membrane. Examples: Some non-proteinogenic amino acids are not found in proteins. Examples include 2-aminoisobutyric acid and
2296-438: Is these 22 compounds that combine to give a vast array of peptides and proteins assembled by ribosomes . Non-proteinogenic or modified amino acids may arise from post-translational modification or during nonribosomal peptide synthesis. The carbon atom next to the carboxyl group is called the α–carbon . In proteinogenic amino acids, it bears the amine and the R group or side chain specific to each amino acid, as well as
2378-439: Is used in plants and microorganisms in the synthesis of pantothenic acid (vitamin B 5 ), a component of coenzyme A . Amino acids are not typical component of food: animals eat proteins. The protein is broken down into amino acids in the process of digestion. They are then used to synthesize new proteins, other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy. The oxidation pathway starts with
2460-440: Is useful to avoid various nomenclatural problems but should not be taken to imply that these structures represent an appreciable fraction of the amino-acid molecules. The first few amino acids were discovered in the early 1800s. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound from asparagus that was subsequently named asparagine , the first amino acid to be discovered. Cystine
2542-409: Is zero. This pH is known as the isoelectric point p I , so p I = 1 / 2 (p K a1 + p K a2 ). For amino acids with charged side chains, the p K a of the side chain is involved. Thus for aspartate or glutamate with negative side chains, the terminal amino group is essentially entirely in the charged form −NH + 3 , but this positive charge needs to be balanced by
Mormyridae - Misplaced Pages Continue
2624-887: The L -amino acid as a post-translational modification . Five amino acids possess a charge at neutral pH. Often these side chains appear at the surfaces on proteins to enable their solubility in water, and side chains with opposite charges form important electrostatic contacts called salt bridges that maintain structures within a single protein or between interfacing proteins. Many proteins bind metal into their structures specifically, and these interactions are commonly mediated by charged side chains such as aspartate , glutamate and histidine . Under certain conditions, each ion-forming group can be charged, forming double salts. The two negatively charged amino acids at neutral pH are aspartate (Asp, D) and glutamate (Glu, E). The anionic carboxylate groups behave as Brønsted bases in most circumstances. Enzymes in very low pH environments, like
2706-408: The 22 α-amino acids incorporated into proteins . Only these 22 appear in the genetic code of life. Amino acids can be classified according to the locations of the core structural functional groups ( alpha- (α-) , beta- (β-) , gamma- (γ-) amino acids, etc.); other categories relate to polarity , ionization , and side-chain group type ( aliphatic , acyclic , aromatic , polar , etc.). In
2788-689: The Gymnotiformes (South American knifefishes). These two groups have evolved convergently , with similar behaviour and abilities but different types of electroreceptors and differently sited electric organs. Strongly electric fish, namely the electric eels , the electric catfishes , the electric rays , and the stargazers , have an electric organ discharge powerful enough to stun prey or be used for defence , and navigation . The electric eel, even when very small in size, can deliver substantial electric power, and enough current to exceed many species' pain threshold . Electric eels sometimes leap out of
2870-523: The IUPAC - IUBMB Joint Commission on Biochemical Nomenclature in terms of the fictitious "neutral" structure shown in the illustration. For example, the systematic name of alanine is 2-aminopropanoic acid, based on the formula CH 3 −CH(NH 2 )−COOH . The Commission justified this approach as follows: The systematic names and formulas given refer to hypothetical forms in which amino groups are unprotonated and carboxyl groups are undissociated. This convention
2952-520: The Mormyrinae and Petrocephalinae . The latter has only a single genus: Petrocephalus Myomyrus Mormyrops Brienomyrus Isichthys Mormyrus Pollimyrus Stomatorhinus Paramormyrops Cryptomyrus Boulengeromyrus Ivindomyrus Hyperopisus Brevimyrus Hippopotamyrus Campylomormyrus Gnathonemus Genyomyrus Marcusenius Eschmeyer's Catalog of Fishes classifies
3034-403: The brown ghost knifefish ( Apteronotus leptorhynchus ), the electric organ produces distinct signals to be received by individuals of the same or other species. The electric organ fires to produce a discharge with a certain frequency , along with short modulations termed "chirps" and "gradual frequency rises", both varying widely between species and differing between the sexes. For example, in
3116-473: The glass knifefish genus Eigenmannia , females produce a nearly pure sine wave with few harmonics, males produce a far sharper non-sinusoidal waveform with strong harmonics . Male bluntnose knifefishes ( Brachyhypopomus ) produce a continuous electric "hum" to attract females; this consumes 11–22% of their total energy budget, whereas female electrocommunication consumes only 3%. Large males produced signals of larger amplitude, and these are preferred by
3198-888: The human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food. In addition, cysteine, tyrosine , and arginine are considered semiessential amino acids, and taurine a semi-essential aminosulfonic acid in children. Some amino acids are conditionally essential for certain ages or medical conditions. Essential amino acids may also vary from species to species. The metabolic pathways that synthesize these monomers are not fully developed. Many proteinogenic and non-proteinogenic amino acids have biological functions beyond being precursors to proteins and peptides.In humans, amino acids also have important roles in diverse biosynthetic pathways. Defenses against herbivores in plants sometimes employ amino acids. Examples: Amino acids are sometimes added to animal feed because some of
3280-481: The low-complexity regions of nucleic-acid binding proteins. There are various hydrophobicity scales of amino acid residues. Some amino acids have special properties. Cysteine can form covalent disulfide bonds to other cysteine residues. Proline forms a cycle to the polypeptide backbone, and glycine is more flexible than other amino acids. Glycine and proline are strongly present within low complexity regions of both eukaryotic and prokaryotic proteins, whereas
3362-622: The Mormyridae, or with waves, as in the Torpediniformes and Gymnarchus , the African knifefish. Many electric fishes also use EODs for communication, while strongly electric species use them for hunting or defence. Their electric signals are often simple and stereotyped, the same on every occasion. Weakly electric fish can communicate by modulating the electrical waveform they generate. They may use this to attract mates and in territorial displays. In sexually dimorphic signalling, as in
Mormyridae - Misplaced Pages Continue
3444-461: The UGA codon to encode selenocysteine instead of a stop codon. Pyrrolysine is used by some methanogenic archaea in enzymes that they use to produce methane . It is coded for with the codon UAG, which is normally a stop codon in other organisms. Several independent evolutionary studies have suggested that Gly, Ala, Asp, Val, Ser, Pro, Glu, Leu, Thr may belong to a group of amino acids that constituted
3526-399: The ability to deliver electric shocks powerful enough to stun their prey or repel predators . Among these, only the stargazers, a group of marine bony fish, do not also use electrolocation. In vertebrates , electroreception is an ancestral trait , meaning that it was present in their last common ancestor. This form of ancestral electroreception is called ampullary electroreception, from
3608-401: The amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed " peptide ". 2- , alpha- , or α-amino acids have the generic formula H 2 NCHRCOOH in most cases, where R is an organic substituent known as a " side chain ". Of the many hundreds of described amino acids, 22 are proteinogenic ("protein-building"). It
3690-423: The aspartic protease pepsin in mammalian stomachs, may have catalytic aspartate or glutamate residues that act as Brønsted acids. There are three amino acids with side chains that are cations at neutral pH: arginine (Arg, R), lysine (Lys, K) and histidine (His, H). Arginine has a charged guanidino group and lysine a charged alkyl amino group, and are fully protonated at pH 7. Histidine's imidazole group has
3772-479: The cell. Consequently, the anterior end of the electrocyte becomes highly positive, while the posterior end, which continues to pump out sodium ions, remains negative. This sets up a potential difference (a voltage ) between the ends of the cell. After the voltage is released, the cell membranes go back to their resting potentials until they are triggered again. Electric organ discharges (EODs) need to vary with time for electrolocation , whether with pulses, as in
3854-420: The chain attached to two neighboring amino acids. In nature, the process of making proteins encoded by RNA genetic material is called translation and involves the step-by-step addition of amino acids to a growing protein chain by a ribozyme that is called a ribosome . The order in which the amino acids are added is read through the genetic code from an mRNA template, which is an RNA derived from one of
3936-536: The characteristics of hydrophobic amino acids well. Several side chains are not described well by the charged, polar and hydrophobic categories. Glycine (Gly, G) could be considered a polar amino acid since its small size means that its solubility is largely determined by the amino and carboxylate groups. However, the lack of any side chain provides glycine with a unique flexibility among amino acids with large ramifications to protein folding. Cysteine (Cys, C) can also form hydrogen bonds readily, which would place it in
4018-576: The chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. The first use of the term "amino acid" in the English language dates from 1898, while the German term, Aminosäure , was used earlier. Proteins were found to yield amino acids after enzymatic digestion or acid hydrolysis . In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between
4100-571: The early genetic code, whereas Cys, Met, Tyr, Trp, His, Phe may belong to a group of amino acids that constituted later additions of the genetic code. The 20 amino acids that are encoded directly by the codons of the universal genetic code are called standard or canonical amino acids. A modified form of methionine ( N -formylmethionine ) is often incorporated in place of methionine as the initial amino acid of proteins in bacteria, mitochondria and plastids (including chloroplasts). Other amino acids are called nonstandard or non-canonical . Most of
4182-420: The elephantfishes; or it may be in the head, as in the electric rays and the stargazers. Electric organs are made up of electrocytes, large, flat cells that create and store electrical energy, awaiting discharge. The anterior ends of these cells react to stimuli from the nervous system and contain sodium channels . The posterior ends contain sodium–potassium pumps . Electrocytes become polar when triggered by
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#17328478872634264-523: The family as follows Family Mormyridae The Medjed was a sacred fish in Ancient Egypt. At the city of Per-Medjed, better known as Oxyrhynchus , whose name means "sharp-nosed" after the fish, archaeologists have found fishes depicted as bronze figurines, mural paintings, or wooden coffins in the shape of fishes with downturned snouts, with horned sun-disc crowns like those of the goddess Hathor . The depictions have been described as resembling members of
4346-501: The females. The cost to males is reduced by a circadian rhythm , with more activity coinciding with night-time courtship and spawning, and less at other times. Electric catfish ( Malapteruridae ) frequently use their electric discharges to ward off other species from their shelter sites, whereas with their own species they have ritualized fights with open-mouth displays and sometimes bites, but rarely use electric organ discharges. The electric discharge pattern of bluntnose knifefishes
4428-423: The form of proteins, amino-acid residues form the second-largest component ( water being the largest) of human muscles and other tissues . Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis . It is thought that they played a key role in enabling life on Earth and its emergence . Amino acids are formally named by
4510-412: The genus Mormyrus . [REDACTED] [REDACTED] Electric fish An electric fish is any fish that can generate electric fields , whether to sense things around them, for defence, or to stun prey. Most fish able to produce shocks are also electroreceptive, meaning that they can sense electric fields. The only exception is the stargazer family (Uranoscopidae). Electric fish, although
4592-673: The initial amino acid of proteins in bacteria, mitochondria , and chloroplasts ) is generally considered as a form of methionine rather than as a separate proteinogenic amino acid. Codon– tRNA combinations not found in nature can also be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating non-proteinogenic amino acids . Aside from the 22 proteinogenic amino acids , many non-proteinogenic amino acids are known. Those either are not found in proteins (for example carnitine , GABA , levothyroxine ) or are not produced directly and in isolation by standard cellular machinery. For example, hydroxyproline ,
4674-412: The marine and brackish-water callorhinchid elephantfish (family Callorhinchidae) of Southern Hemisphere oceans. The elephantfish are a diverse family, with a wide range of different sizes and shapes. The smallest are just 5 cm (2.0 in) in adult length, while the largest reach up to 1.5 m (4.9 ft). They do, however, have a number of unique features in common. Firstly, their cerebellum
4756-414: The middle of the protein, whereas hydrophilic side chains are exposed to the aqueous solvent. (In biochemistry , a residue refers to a specific monomer within the polymeric chain of a polysaccharide , protein or nucleic acid .) The integral membrane proteins tend to have outer rings of exposed hydrophobic amino acids that anchor them in the lipid bilayer . Some peripheral membrane proteins have
4838-798: The name of the receptive organs involved, ampullae of Lorenzini . These evolved from the mechanical sensors of the lateral line , and exist in cartilaginous fishes ( sharks , rays , and chimaeras ), lungfishes , bichirs , coelacanths , sturgeons , paddlefish , aquatic salamanders , and caecilians . Ampullae of Lorenzini were lost early in the evolution of bony fishes and tetrapods . Where electroreception does occur in these groups, it has secondarily been acquired in evolution, using organs other than and not homologous with ampullae of Lorenzini. Most common bony fish are non-electric. There are some 350 species of electric fish. Electric organs have evolved eight times, four of these being organs powerful enough to deliver an electric shock. Each such group
4920-409: The neurotransmitter gamma-aminobutyric acid . Non-proteinogenic amino acids often occur as intermediates in the metabolic pathways for standard amino acids – for example, ornithine and citrulline occur in the urea cycle , part of amino acid catabolism (see below). A rare exception to the dominance of α-amino acids in biology is the β-amino acid beta alanine (3-aminopropanoic acid), which
5002-573: The nonstandard amino acids are also non-proteinogenic (i.e. they cannot be incorporated into proteins during translation), but two of them are proteinogenic, as they can be incorporated translationally into proteins by exploiting information not encoded in the universal genetic code. The two nonstandard proteinogenic amino acids are selenocysteine (present in many non-eukaryotes as well as most eukaryotes, but not coded directly by DNA) and pyrrolysine (found only in some archaea and at least one bacterium ). The incorporation of these nonstandard amino acids
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#17328478872635084-438: The only one that is useful for chemistry in aqueous solution is that of Brønsted : an acid is a species that can donate a proton to another species, and a base is one that can accept a proton. This criterion is used to label the groups in the above illustration. The carboxylate side chains of aspartate and glutamate residues are the principal Brønsted bases in proteins. Likewise, lysine, tyrosine and cysteine will typically act as
5166-433: The opposite is the case with cysteine, phenylalanine, tryptophan, methionine, valine, leucine, isoleucine, which are highly reactive, or complex, or hydrophobic. Many proteins undergo a range of posttranslational modifications , whereby additional chemical groups are attached to the amino acid residue side chains sometimes producing lipoproteins (that are hydrophobic), or glycoproteins (that are hydrophilic) allowing
5248-424: The organism's genes . Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or natural amino acids. Of these, 20 are encoded by the universal genetic code. The remaining 2, selenocysteine and pyrrolysine , are incorporated into proteins by unique synthetic mechanisms. Selenocysteine is incorporated when the mRNA being translated includes a SECIS element , which causes
5330-415: The overall structure is NH + 3 −CHR−CO − 2 . At physiological pH the so-called "neutral forms" −NH 2 −CHR−CO 2 H are not present to any measurable degree. Although the two charges in the zwitterion structure add up to zero it is misleading to call a species with a net charge of zero "uncharged". In strongly acidic conditions (pH below 3), the carboxylate group becomes protonated and
5412-536: The polar amino acid category, though it can often be found in protein structures forming covalent bonds, called disulphide bonds , with other cysteines. These bonds influence the folding and stability of proteins, and are essential in the formation of antibodies . Proline (Pro, P) has an alkyl side chain and could be considered hydrophobic, but because the side chain joins back onto the alpha amino group it becomes particularly inflexible when incorporated into proteins. Similar to glycine this influences protein structure in
5494-402: The popular name "elephant-nosed fish" for those species with particularly prominent mouth extensions. The extensions to the mouthparts usually consist of a fleshy elongation attached to the lower jaw. They are flexible, and equipped with touch, and possibly taste, sensors. The mouth is not protrusible, and the head (including the eyes), the dorsum, and belly are covered by a thin layer of skin that
5576-417: The presence of prey or other objects of different conductivities . This allows them to sense their environment in turbid waters where vision is impaired by suspended matter. Electric fish can be classified into two types: pulse fish or wave fish. Pulse-type discharges are characterized by long intervals between electric discharges, whereas wave-type discharges occur when the interval between consecutive pulses
5658-432: The prey fish to develop electric signals that are harder to detect. Bluntnose knifefishes produce an electric discharge pattern similar to the electrolocation pattern of the dangerous electric eel, probably a form of Batesian mimicry to dissuade predators. Glass knifefish that are using similar frequencies move their frequencies up or down in a jamming avoidance response ; African knifefish have convergently evolved
5740-496: The prey, in an evolutionary arms race , to develop more complex or higher frequency signals that are harder to detect. It had been theorized as early as the 1950s that electric fish near each other might experience some type of interference. In 1963, Akira Watanabe and Kimihisa Takeda discovered the jamming avoidance response in Eigenmannia . When two fish are approaching one another, their electric fields interfere. This sets up
5822-480: The primary driving force behind the processes that fold proteins into their functional three dimensional structures. None of these amino acids' side chains ionize easily, and therefore do not have pK a s, with the exception of tyrosine (Tyr, Y). The hydroxyl of tyrosine can deprotonate at high pH forming the negatively charged phenolate. Because of this one could place tyrosine into the polar, uncharged amino acid category, but its very low solubility in water matches
5904-455: The protein to attach temporarily to a membrane. For example, a signaling protein can attach and then detach from a cell membrane, because it contains cysteine residues that can have the fatty acid palmitic acid added to them and subsequently removed. Although one-letter symbols are included in the table, IUPAC–IUBMB recommend that "Use of the one-letter symbols should be restricted to the comparison of long sequences". The one-letter notation
5986-431: The removal of the amino group by a transaminase ; the amino group is then fed into the urea cycle . The other product of transamidation is a keto acid that enters the citric acid cycle . Glucogenic amino acids can also be converted into glucose, through gluconeogenesis . Of the 20 standard amino acids, nine ( His , Ile , Leu , Lys , Met , Phe , Thr , Trp and Val ) are called essential amino acids because
6068-509: The rods are hit by light from other wavelengths. Only a single gonad is present, located on the left side of their body. The Mormyridae and the closely related genus Gymnarchus are also unique in being the only vertebrates where the male sperm cell does not have a flagellum . Elephantfish possess electric organs that generate weak electric fields , and electroreceptors ( ampullae of Lorenzini , knollenorgans , and Mormyromasts) that detect small variations in these electric fields caused by
6150-490: The same amino acid in the same voltage-gated sodium channel despite the two groups of fish being on different continents and the evolution of the electric sense organ being separated in time by around 60 million years. Convergent changes to other key transcription factors and regulatory pathways in both Gymnotiforms and Mormyridae also contributed to the evolution of the electric sense organ. The roughly 221 species of elephantfish which are sometimes grouped into two subfamilies,
6232-580: The state with just one C-terminal carboxylate group is negatively charged. This occurs halfway between the two carboxylate p K a values: p I = 1 / 2 (p K a1 + p K a(R) ), where p K a(R) is the side chain p K a . Similar considerations apply to other amino acids with ionizable side-chains, including not only glutamate (similar to aspartate), but also cysteine, histidine, lysine, tyrosine and arginine with positive side chains. Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour
6314-509: The structure becomes an ammonio carboxylic acid, NH + 3 −CHR−CO 2 H . This is relevant for enzymes like pepsin that are active in acidic environments such as the mammalian stomach and lysosomes , but does not significantly apply to intracellular enzymes. In highly basic conditions (pH greater than 10, not normally seen in physiological conditions), the ammonio group is deprotonated to give NH 2 −CHR−CO − 2 . Although various definitions of acids and bases are used in chemistry,
6396-428: The tail fin is lost or reduced. This may reduce lateral bending while swimming, allowing the electric field to remain stable for electrolocation. There has been convergent evolution in these features among the mormyrids and gymnotids. Electric fish species that live in habitats with few obstructions, such as some bottom-living fish, display these features less prominently. This implies that convergence for electrolocation
6478-401: The water must be matched : Electric organs vary widely among electric fish groups. They evolved from excitable, electrically active tissues that make use of action potentials for their function: most derive from muscle tissue, but in some groups the organ derives from nerve tissue. The organ may lie along the body's axis, as in the electric eel and Gymnarchus ; it may be in the tail, as in
6560-405: The water to electrify possible predators directly, as has been tested with a human arm. The amplitude of the electrical output from these fish can range from 10 to 860 volts with a current of up to 1 ampere , according to the surroundings, for example different conductances of salt and freshwater. To maximize the power delivered to the surroundings, the impedances of the electric organ and
6642-512: Was chosen by IUPAC-IUB based on the following rules: Two additional amino acids are in some species coded for by codons that are usually interpreted as stop codons : In addition to the specific amino acid codes, placeholders are used in cases where chemical or crystallographic analysis of a peptide or protein cannot conclusively determine the identity of a residue. They are also used to summarize conserved protein sequence motifs. The use of single letters to indicate sets of similar residues
6724-402: Was discovered in 1810, although its monomer, cysteine , remained undiscovered until 1884. Glycine and leucine were discovered in 1820. The last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose , who also determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth. The unity of
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