Which aromatic amino acid is non essential

After this stage, 3-hydroxyanthranilate is further cleaved by a non-heme iron enzyme, 3-hydroxyanthranilate 3,4-dioxygenase EC 1. This compound represents a branching point: it can be either catabolized via 2-aminomuconate Martynowski et al.

This section covers AAA catabolism by microflora resident in animal guts, pathogens which infect animals, as well microbes that utilize AAA for the biosynthesis of antibiotics. While some prokaryotes share the 3-hydroxykynurenine version of the kynurenine pathway with eukaryotes including animals as discussed in the previous section, in bacteria belonging to the genus Pseudomonas , kynureninase preferentially acts directly on kynurenine, hydrolyzing it to anthranilate and alanine Hayaishi and Stanier, The pseudomonads subsequently convert anthranilate to catechol by the action of an NADPH-dependent non-heme iron enzyme called anthranilate-1,2-dioxygenase EC 1.

Nevertheless, the broad distribution and inducible nature of the enzymes of this pathway among bacteria and fungi suggests roles in catabolism and secondary metabolism. Several antibiotics can be derived from the kynurenine pathway, including sibirimycin, which arises from a modified pathway involving a methylation step Giessen et al.

The kynurenine pathway is involved in the interplay between host and pathogens during infections. The host imposes tryptophan limitation on invading microorganisms by degrading it via the kynurenine pathway, whereas the pathogens use the pathway to synthesize compounds necessary for their growth and metabolism. Anthranilate is a key molecule which participates in many of these interactions. While anthranilate can be derived from many pathways including tryptophan biosynthesis, when P.

Some pathogens, such as Chlamydia psittaci , have evolved to evade host-imposed tryptophan depletion by replacing the anthranilate synthase in the tryptophan biosynthetic operon by a kynureninase Wood et al. Infection with Toxoplasma gondii has been speculated as a factor in increasing incidence of schizophrenia; in mouse models, stimulation of the kynurenine pathway was observed and the levels of several pathway metabolites including 3-hydroxykynurenine, quinolinic acid, and kynurenic acid are elevated Notarangelo et al.

During Helicobacter pylori infection in the human gastric mucosa, a specific up-regulation of IDO expression is observed, which regulates multiple helper T-cell lines, resulting in lowered gastric inflammation Larussa et al.

The obligate intracellular pathogen, Anaplasma phagocytophilum , which causes one of the most common tick-borne diseases, has been shown to up-regulate the expression of a specific organic anion uptake protein and a KAT enzyme enhancing its survival in the arthropod vector Taank et al. Recent studies have demonstrated that the pathway branching from kynurenine to kynurenic acid involving KAT is induced in bacterial meningitis and the metabolites thus produced contribute directly to the pathology of the disease Coutinho et al.

Aerobic, microaerophilic and strictly anaerobic microorganisms occupy the gut or gastro-intestinal GI tracts of animals including humans. Prominent GI tract bacteria which utilize AAA as substrates include lactobacilli, enteric bacteria and strict anaerobes mostly Firmicutes including the Clostridia and related genera.

Microbial AAA degradation commonly involves enzymes such as aminotransferases, dehydrogenases and decarboxylases and produces products including the corresponding aromatic metabolites such as arylpyruvate, arylpropionate, aryllactate, arylacrylate and arylacetate. The levels of phenylacetate, 3-phenylpropionate and 3-phenyllactate derived from phenylalanine catabolism, and 4-hydroxyphenyl lactate from tyrosine catabolism are elevated in intestinal diseases and sepsis Fedotcheva et al.

Multiple enteric bacteria including E. For example, Lactococci catabolize tryptophan via a tryptophan aminotransferase EC 2. Decarboxylases produce the corresponding primary aromatic amines from AAA Nakazawa et al.

Phenylalanine is converted into phenylpyruvate by the action of an aryllactate dehydrogenase EC 1. Certain unusual reactions such as the cleavage of IPy into indole and pyruvate, the conversion of tyrosine to p -cresol via the decarboxylation of p -hydroxyphenylacetate and the formation of phenol from tyrosine via the elimination of ammonia and acetate, were also detected in intestinal anaerobic bacteria Smith and Macfarlane, The conversion of arylpyruvates into arylaldehydes is known; lactobacilli convert Ipy derived from tryptophan into indolealdehyde I3A and phenylalanine-derived phenylpyruvate into benzaldehyde Nierop Groot and de Bont, Perhaps the most distinctive pathway in all of AAA aerobic catabolism in the gut is found in several bacteria including E.

In Stickland fermentation, one AA donates electrons while another accepts them, thereby generating ATP and reducing power. Stickland electron donors include the branched chain AA, acidic AA and sulfur-containing AA as well as alanine, serine, histidine and phenylalanine; electron acceptors include glycine, proline, hydroxyproline, arginine, ornithine derived from arginine and tryptophan.

The AAA can all be fermented as single AAs via the 2-hydroxyacid pathway, which has been studied extensively by Buckel and coworkers Kim et al. The pathway variant for AAA fermentation is termed the 3-aryllactate pathway and is discussed in detail in Radical dehydratases and the 3-aryllactate pathway.

The end products of AAA reduction were known for some decades Elsden et al. However, all the intermediates involved in both AAA oxidation and reduction in Clostridium sporogenes were identified much later, and the enzymes responsible for the fermentation of phenylalanine Dickert et al. Initially, the issue of whether all the AAA were catabolized via the same radical dehydratase enzymes or different ones was unresolved Li, , but later work with mutants showed that only one dehydratase was active in the degradation of all the AAA via this pathway Dodd et al.

The conversion of tryptophan to 3-indolepyruvate, oxidizing it to 3-indoleacetate, and reducing it via R indolelactate and E indoleacrylate to IPA, according to this pathway is depicted in Figure 5.

Apart from C. Interest in AAA catabolism in the gut had been earlier stimulated by the fact that one of the end products of tryptophan degradation, IPA, passes through the blood-brain barrier, scavenging reactive oxygen species ROS in the brain by the formation of kynuric acid Bendheim, , and thereby protecting it from Alzheimer's disease Chyan et al. Indeed, gut bacteria were shown to exert a large influence on the production of mammalian blood metabolites such as indoxyl sulfate and IPA; specifically IPA production was microflora-dependent and could be induced by the colonization of C.

Out of the 12 end products derived from the degradation of all the AAA via the reductive branch of this pathway, nine were detected recently in the host plasma Dodd et al. The same authors also proposed the genetic engineering of gut bacteria involved in producing major metabolites such as IPA, as a way to influence host health.

Protozoa also occur frequently in ruminants and AAA catabolism by bacteria alone or bacteria in the presence of protozoa not only have differing utilization, but also different end products.

For example, while rumen bacteria alone produced skatole, p -cresol and IPA as the end products, mixed bacterial-protozoan catabolism produced IAA, indolelactate and indole Mohammed et al. Tyrosine is converted by mixed cultures of bacteria and protozoa into p -hydroxyphenylacetic acid and further into p -cresol Mohammed et al. AAA catabolism also plays a role in bloodstream infections caused by protozoa. In Trypanosoma brucei , the conversion of phenylalanine into phenylpyruvate and tyrosine into p -hydroxyphenyllactate has been reported Stibbs and Seed, a.

In the same organism, tryptophan was converted into indolelactate, IAA and tryptophol Stibbs and Seed, b. The existence of AAA-related dehydrogenases, transaminases and decarboxylases was surmised from these results. Since the levels of AAA catabolites shown to originate from the pathogen were elevated in infected animals, AAA catabolism might be important for the development of sleeping sickness Stibbs and Seed, c.

While the oxidation of AAA catabolic pathways in aerobes and anaerobes are often similar, the reductive branches of AAA fermentations are interesting due to the unique enzymes involved. The 3-aryllactate pathway is a type of 2-hydroxyacid pathway as mentioned earlier. Phenylalanine is fermented via 3-phenyllactate, tyrosine via 4-hydroxyphenyllactate and tryptophan via ILA, with all three fermentations involving catalysis by a common set of enzymes.

Then, the arylpyruvate is oxidatively decarboxylated by a pathway-specific 2-ketoacid:ferredoxin-oxidoreductase with Coenzyme A CoA thioesterification to the corresponding acyl-CoA arylacetyl-CoA which is one carbon atom shorter than the arylpyruvate. Subsequently, substrate level phosphorylation SLP occurs with the formation of the oxidized end product, namely the arylacetate. In the reductive branch, the 2-ketoacid is reduced by NADH-dependent Re face -stereospecific dehydrogenase Berk et al.

Therefore, a special enzyme of the 2-hydroxyacyl-CoA dehydratase 2-HADH family whose members share a common biochemical mechanism, the aryllactyl-CoA dehydratase, becomes necessary. The ketyl radical generated by one-electron reduction of the substrate eliminates water via proton transfer to a conserved glutamate residue Knauer et al. Finally, 3-arylacrylate is reduced to the corresponding 3-arylpropionate. The reduced end product 3-arylpropionate has the same chain length as the respective parent AAA.

One-electron reduction of a 2-HADH by its activator involves a large conformational change of the helix-[4Fe-4S] cluster-helix motif of the latter coupled to the hydrolysis of 2 ATP molecules Knauer et al. Once the electron transfer is complete, the activator dissociates from the 2-HADH, as deduced from chelation experiments Kim J. Therefore, electron transfer involving [4Fe-4S] clusters as the sole cofactors in the 2-HADH-activator system facilitates radical dehydrations with minimal ATP hydrolysis.

The mechanism of the reaction is shown in Figure 6. Figure 6. Reaction mechanism of 3-aryllactyl-CoA dehydratase. The formation of the ketyl radical anion leads to the elimination of water via proton transfer to a conserved glutamate residue and the pK a of the beta-proton in the enoxy radcal is reduced from approximately 40 to about 15, a value which is further reduce by interactions with the active site residues.

These highly negative redox potentials could be supplied by ferredoxin- or flavodoxin-oxidoreductases which oxidize arylpyruvates coupled to the addition of CoA, yielding arylacetyl-CoA. ATP formation is coupled to the reduction of 3-arylacrylates in the 3-aryllactate pathways of C. Recent studies suggest that there is additional energy conservation in C. It can be seen from the preceding section that CoA-transferases are essential for AA degradation through the 2-hydroxyacid pathways.

The arylacrylyl-CoA is then converted into acrylacrylate by FldA. Unlike other CoA-transferases, catalysis by the Type III family entails a ternary complex mechanism without any intermediates covalently bound to the enzyme. Acid-R2 and the CoA-donor-R1 first bind non-covalently to the enzyme to form an anhydride whereby the released CoAthiolate stays at the enzyme.

The presence of a CoA-ligase gene in many gene clusters containing Type III transferases indicates that catalytic amounts of CoA-thioesters are required to start the reaction and prevent depletion of the CoA-thioester pool by unspecific hydrolysis.

The three AAA, their biosynthetic precursors, as well as modified non-protein AAA are important in the synthesis of a variety of antibiotics by bacteria and fungi. Phenylalanine is incorporated into the several antibiotics, for example the bacterial cell wall biosynthesis inhibiting mureidomycins Bugg et al. The biosynthesis of chlorobiocin also involves 3-dimethylallylhydroxybenzoic acid, which is derived from phenylalanine by prenylation and retro-aldol condensation Pojer et al.

Tyrosine is the precursor for the biosynthesis of novobiocin, whose ring B is derived from the AAA via a coumarin intermediate Chen and Walsh, ; Pacholec et al. Examples of tryptophan-derived antibiotics include actinomycin, which later became well-known as a cancer drug Hollstein, Tryptophan-rich peptides such as indolicidin and tritrpticin, belong to a newer class of antimicrobial peptides Chan et al.

Combining tryptophan residues with cationic AA like arginine generates antimicrobials able to penetrate bacterial cells effectively. Recently, researchers have developed lipopeptide analogs of polymyxin B often used in multi drug resistant cases that incorporate tryptophan Grau-Campistany et al. The shikimate pathway was believed to lead to the synthesis of the intermediate amino hydroxy benzoic acid AHBA , a compound which feeds into the biosynthesis of polyketide antibiotics of the ansamycin class, among which the anti-tubercular rifamycins are the most well-known Sensi et al.

Candicidin is an aromatic polyene macrolide antifungal molecule containing a 4-aminoacetophenone moiety Martin and McDaniel, ; Martin, , derived from chorismate via 4-aminobenzoic acid PABA by means of an aminotransferase reaction, with glutamine acting as the amino donor Gibson et al. Chorismate is also transformed into 4-hydroxybenzoic acid in bacteria by the action of chorismate lyase which belongs to the ubiquinone biosynthetic pathway Poon et al.

Prephenate is the metabolic precursor of bacilysin produced by Bacillus subtilis , as elucidated by studies of mutants of phenylalanine and tyrosine biosynthesis Hilton et al.

Dihydrophenylalanine, a non-protein AA and antibiotic produced by Photorhabdus luminescens , is generated via rerouting of prephenate by the action of an unusual non-aromatizing prephenate decarboxylase, followed by a transaminase Crawford et al. Anthranilate formed from tryptophan degradation; Figure 5 inhibits biofilm formation by Pseudomonas aeruginosa, Vibrio vulnificus, Bacillus subtilis, Salmonella enterica serovar Typhimurium, and Staphylococcus aureus , and disrupted biofilms already formed by these bacteria via multiple mechanisms Li et al.

Therefore, anthranilate could potentially be used as a broad-spectrum biofilm inhibitor. AAA biosynthetic precursors as well as the AAA themselves are often rerouted into the production of non-canonical AAA analogs, which form parts of antibiotic scaffolds.

Obafluorin, produced by Pseudomonas fluorescens , is biosynthesized via the key intermediate L-aminophenylalanine Herbert and Knaggs, The AAA biosynthesis intermediate prephenate is the starting point for the synthesis of Hpg, which involves the four enzymes described hence Hubbard et al.

Prephenate dehydrogenase Pdh converts prephenate to p -hydroxyphenylpyruvate, followed by 4-hydroxymandelate synthase HmaS , which transforms p -hydroxyphenylpyruvate into L- p -hydroxymandelate and hydroxymandelate oxidase Hmo , which oxidizes L- p -hydroxymandelate to p -hydroxylbenzoylformate.

Finally, transamination of the penultimate compound by p -hydroxyphenylglycine transaminase Pgat , yields Hpg. Vancomycin biosynthesis involves similar enzymes. Tyrosine is first activated to a thiol ester and attached to one of the modular thioesterease enzyme domains for antibiotic synthesis. The metabolism of the AAA pathways offer a rich source for the understanding of many aspects of plant, animal and microbial metabolism. Research in this area affords the opportunity to further investigate the involvement of the AAA and their metabolite derivatives in a variety of functions and roles related to the health of plants and animals.

There are many aspects pertaining to the regulation, role, and function of enzymes involved in the anabolism or catabolism of compounds derived from the AAA. In addition, there are many structural aspects of key enzymes involved in these pathways that have yet to be elucidated. All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Al Hafid, N. Phenylketonuria: a review of current and future treatments.

Altendorf, K. Biosynthesis of 4-aminobenzoic acid in Aerobacter aerogenes. FEBS Lett. Alves-Ferreira, M. Global expression profiling applied to the analysis of Arabidopsis stamen development. Plant Physiol. Amireault, P. ACS Chem. Anterola, A. Phytochemistry 61, — Arnao, M. Functions of melatonin in plants: a review. Pineal Res. Aroca, P. Regulation of mammalian melanogenesis I: partial purification and characterization of a dopachrome converting factor: Dopachrome tautomerase.

A new spectrophotometric assay for dopachrome tautomerase. Ashenafi, M. The fused anthranilate synthase from Streptomyces venezuelae functions as a monomer. Aso, Y. Further studies on dopa quinone imine conversion factor from the cuticles of Manduca sexta L.

Insect Biochem. Properties of tyrosinase and dopa quinone imine conversion factor from pharate pupal cuticle of Manduca sexta. Axe, J. Bader, J. ATP formation is coupled to the hydrogenation of 2-enoates in Clostridium sporogenes. FEMS Microbiol. Baldwin, E. Effect of volatiles and their concentration on perception of tomato descriptors.

Food Sci. Barker, J. Microbial synthesis of p-hydroxybenzoic acid from glucose. Bartel, B. Inputs to the active indoleacetic acid pool: de novo synthesis, conjugate hydrolysis, and indolebutyric acid b-oxidation. Plant Growth Regul. Bashiri, G. Structure and inhibition of subunit I of the anthranilate synthase complex of Mycobacterium tuberculosis and expression of the active complex.

Acta Crystallogr. D Biol. Basset, G. Folate synthesis in plants: the p-aminobenzoate branch is initiated by a bifunctional PabA—PabB protein that is targeted to plastids. Natl Acad. Ben Zvi, M. Interlinking showy traits: co-engineering of scent and colour biosynthesis in flowers. Plant Biotechnol. Bender, D. Amino Acid Metabolism, 3rd Edn. Chichester: Wiley. Bendheim, P. PubMed Abstract Google Scholar.

Bendrat, K. Identification of the gene encoding the activator of R hydroxyglutaryl-CoA dehydratase from Acidaminococcus fermentans by gene expression in Escherichia coli. FEBS J. Benesova, M. Chorismate mutase isoforms from seeds and seedlings of Papaver somniferum. Phytochemistry , 31, — Local, efflux-dependent auxin gradients as a common module for plant organ formation. Cell , — Berger, M. The expanded biology of serotonin.

Berk, H. Bertin, C. Grass roots chemistry: meta-tyrosine, an herbicidal nonprotein amino acid. Bickel, H. Incorporation of shikimate and other precursors into aromatic amino acids and preylquinones of isolated spinach chloroplasts. Phytochemistry 17, — Blackmore, N. Three sites and you are out: ternary synergistic allostery controls aromatic amino acid biosynthesis in Mycobacterium tuberculosis. Blanco, B. Mycobacterium tuberculosis shikimate kinase inhibitors: design and simulation studies of the catalytic turnover.

Blomberg, L. Theoretical study of the reaction mechanism of Streptomyces coelicolor type II dehydroquinase. Theory Comput. Plant Cell. Boudet, A. Evolution and current status of phenolic compounds.

Brannon, D. Biosynthesis of dithiadiketopiperazine antibiotics: comparison of possible aromatic amino acid precursors. Broadley, K. The vascular effects of trace amines and amphetamines. Brown, D. Incorporation of monofluorotryptophans into protein during the growth of Escherichia coli. Brunke, S.

Histidine degradation via an aminotransferase increases the nutritional flexibility of Candida glabrata. Cell 13, — Buchanan, B. Biochemistry and Molecular Biology of Plants. Google Scholar. Buckel, W. Unusual enzymes involved in five pathways of glutamate fermentation. One-electron redox reactions of CoASH esters in anaerobic bacteria - a mechanistic proposal. Enzyme catalyzed radical dehydrations of hydroxy acids.

Acta , — Bugg, T. Phospho-MurNAc-pentapeptide translocase MraY as a target for antibacterial agents and antibacterial proteins.

Infect Disord Drug Targets. Bulloch, E. Identification of 4-aminodeoxychorimsate synthase as the molecular target for the antimicrobial action of, 6S fluoroshikimate. Campbell, S.

Evidence for the shikimate pathway in apicomplexan parasites. Casati, P. Differential accumulation of maysin and rhamnosylisoorientin in leaves of high-altitude landraces of maize after UV-B exposure. Plant Cell Environ. Castell, A. The substrate capture mechanism of Mycobacterium tuberculosis anthranilate phosphoribosyltransferase provides a mode for inhibition. Biochemistry 52, — Chan, D. Tryptophan- and arginine-rich antimicrobial peptides: structures and mechanisms of action.

Chang, Z. Chatterjee, A. Transaminases of Leishmania donovani , the causative organism of Kala-azar. Nature Chen, H. Formation of beta-hydroxy histidine in the biosynthesis of nikkomycin antibiotics. Glycopeptide antibiotic biosynthesis: enzymatic assembly of the dedicated amino acid monomer S -3,5-dihydroxyphenylglycine.

Chen, J. The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword. Chyan, Y. Potent neuroprotective properties against the Alzheimer beta-amyloid by an endogenous melatonin-related indole structure, indolepropionic acid.

Colabroy, K. The pyridine ring of NAD is formed by a nonenzymatic pericyclic reaction. Cole, S. Deciphering the biology of Mycobacterium tuberculosis from the complete genome sequence. Nature , — Colombari, E. Role of the medulla oblongata in hypertension.

Hypertension 38, — Cookson, T. Alternative substrates reveal catalytic cycle and key binding events in the reaction catalysed by anthranilate phosphoribosyltransferase from Mycobacterium tuberculosis. Structures of Mycobacterium tuberculosis anthranilate phosphoribosyltransferase variants reveal the conformational changes that facilitate delivery of the substrate to the active site.

Biochemistry 54, — Coquoz, J. The biosynthesis of salicylic acid in potato plants. Coracini, J. Shikimate kinase, a protein target for drug design. Coutinho, L. The kynurenine pathway is involved in bacterial meningitis. Crawford, J. Dihydrophenylalanine: a prephenate-derived Photorhabdus luminescens antibiotic and intermediate in dihydrostilbene biosynthesis.

Chem Biol. Cross, P. Engineering allosteric control to an unregulated enzyme by transfer of a regulatory domain. Tyrosine latching of a regulatory gate affords allosteric control of aromatic amino acid biosynthesis. Culbertson, J. Conversion of aminodeoxychorismate synthase into anthranilate synthase with Janus mutations: mechanism of pyruvate elimination catalyzed by chorismate enzymes. Czekster, C. Steady-state kinetics of indoleglycerol phosphate synthase from Mycobacterium tuberculosis.

Dai, X. An amino acid that was discovered from asparagus. Both asparagine and Aspartate are positioned close to the tricarboxylic acid TCA cycle that produces energy. A non-essential amino acid that is made in the body. Glycine is plentiful in the body. It acts as a transmitter in the central nervous system and helps regulate body functions such as locomotion and sensory perception. Glycine makes up one-third of collagen.

Tyrosine is used to make many types of useful amines. Tyrosine is grouped as an aromatic amino acid together with phenylalanine and tryptophan. Amino acids are essential compounds common to all living things, from microbes to humans. All living bodies contain the same 20 types of amino acids. What is These days we hear a lot about amino acids.

But many of us probably do not understand how they work or their link to human Proteins are made up of hundreds of An essential amino acid that is used to make many types of useful amines. An essential amino acid that is used to make many different substances needed in the body. What are Amino Acids?

Some amino acids are precursors of important compounds in the body. Examples include epinephrine, thyroid hormones, Ldopa, and dopamine all from tyrosine , serotonin from tryptophan , and histamine from histidine. Although amino acids serve other functions in cells, their most important role is as constituents of proteins. Proteins, as we noted earlier, are polymers of amino acids. Amino acids are linked to each other by peptide bonds, in which the carboxyl group of one amino acid is joined to the amino group of the next, with the loss of a molecule of water.

Additional amino acids are added in the same way, by formation of peptide bonds between the free carboxyl on the end of the growing chain and the amino group of the next amino acid in the sequence. The end of the peptide that has a free amino group is called the N-terminus for NH2 , while the end with the free carboxyl is termed the C-terminus for carboxyl.

The folding of polypeptides into their functional forms is the topic of the next section. R-group chemistry Table 2. A D-form of the amino acid is also found in bacterial cell walls. Alanine is non-essential, being readily synthesized from pyruvate. As a result, glycine is the only amino acid that is not chiral. Its small side chain allows it to readily fit into both hydrophobic and hydrophilic environments.

It is nonessential to humans. It has a hydrophobic side chain and is also chiral in its side chain. Leucine is the only dietary amino acid reported to directly stimulate protein synthesis in muscle, but caution is in order, as 1 there are conflicting studies and 2 leucine toxicity is dangerous, resulting in "the four D's": diarrhea, dermatitis, dementia and death. Methionine is non-polar and encoded solely by the AUG codon. In prokaryotic cells, the first methionine in a protein is formylated.

It is the least flexible of the protein amino acids and thus gives conformational rigidity when present in a protein. Proline hydroxylation of hypoxia-inducible factor HIF serves as a sensor of oxygen levels and targets HIF for destruction when oxygen is plentiful.

It is noteworthy in hemoglobin, for when it replaces glutamic acid at position number six, it causes hemoglobin to aggregate abnormally under low oxygen conditions, resulting in sickle cell disease. It is readily produced by transamination of oxaloacetate.

With a pKa of 3. Premature infants cannot synthesize arginine. In addition, surgical trauma, sepsis, and burns increase demand for arginine. Most people, however, do not need arginine supplements.

It is an essential amino acid in humans and other mammals. With a side chain pKa of 6, it can easily have its charge changed by a slight change in pH. Protonation of the ring results in two NH structures which can be drawn as two equally important resonant structures. It can also be ubiquitinated, sumoylated, neddylated, biotinylated, carboxylated, and pupylated, and. O-Glycosylation of hydroxylysine is used to flag proteins for export from the cell.

This is mainly due to its thiol group. The thiol made up of a sulphur and hydrogen atom is very susceptible to oxidation , allowing cysteine to form disulphide bonds with other molecules including other cysteines. The resulting product of two linked cysteines is called cystine. When bound to other cysteines, the disulphide bond greatly increases the stability of the protein. However, as this is an oxidation reaction, it is exclusive to extracellular proteins with a few exceptions.

This is because the inside of the cell is highly reducing making the disulphide bond highly unstable. Aromatic Amino acids are the largest of the amino acids and include; phenylalanine F , tyrosine Y and tryptophan W.

They can all absorb ultra-violet light however some can absorb more than others, tyrosine and tryptophan absorb more than phenylalanine meaning that tryptophan is the main molecule which absorbs light in the protein. Aromatic amino acids are also hydrophobic so are located in the core of the protein ensuring they are not near water.

Humans cannot synthesize phenylalanine or tryptophan, and can only make tyrosine from phenylalanine, this means that aromatic amino acids are a vital component of our diet as we require them in certain proteins but do not synthesize them ourselves. Aromatic amino acids contain an aromatic ring [17].

Phenylalanine deficiency can cause confusion, depression, lack of energy and decreased alertness. It can be bought in a tablet form to supplement any deficiency [18]. An inability to break down excess phenylalanine is called Phenylketonuria. To combat this a low phenylalanine diet is used and avoids aspartame sweeteners which resemble phenylalanine and can break down to produce it.

Jump to: navigation , search. Personal tools Log in. Namespaces Page Discussion. Views Read View source View history.