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Pathway Description
Tyrosine Metabolism
Bos taurus
Category:
Metabolite Pathway
Sub-Category:
Metabolic
Created: 2018-08-10
Last Updated: 2019-08-30
The tyrosine metabolism pathway describes the many ways in which tyrosine is catabolized or transformed to generate a wide variety of biologically important molecules. In particular, tyrosine can be metabolized to produce hormones such as thyroxine and triiodothyronine or it can be metabolized to produce neurotransmitters such as L-DOPA, dopamine, adrenaline, or noradrenaline. Tyrosine can also serve as a precursor of the pigment melanin and for the formation of Coenzyme Q10. Additionally, tyrosine can be catabolized all the way down into fumarate and acetoacetate. This particular pathway for tyrosine degradation starts with an alpha-ketoglutarate-dependent transamination reaction of tyrosine, which is mediated through the enzyme known as tyrosine transaminase. This process generates p-hydroxyphenylpyruvate. This aromatic acid is then acted upon by p-hydroxylphenylpyruvate-dioxygenase which generates the compound known as homogentisic acid or homogentisate (2,5-dihydroxyphenyl-1-acetate). In order to split the aromatic ring of homogentisate, a unique dioxygenase enzyme known as homogentisic acid 1,2-dioxygenase is required. Through this enzyme, maleylacetoacetate is created from the homogentisic acid precursor. The accumulation of excess homogentisic acid and its oxide (named alkapton) in the urine of afflicted individuals can lead to a condition known as alkaptonuria. This genetic condition, also known as an inborn error of metabolism or IEM, occurs if there are mutations in the homogentisic acid 1,2-dioxygenase gene. After the breakdown of homogentisate is achieved, maleylacetoacetate is then attacked by the enzyme known as maleylacetoacetate-cis-trans-isomerase, which generates fumarylacetate. This isomerase catalyzes the rotation of the carboxyl group created from the hydroxyl group via oxidation. This cis-trans-isomerase uses glutathione as a coenzyme or cofactor. The resulting product, fumarylacetoacetate, is then split into acetoactate and fumarate via the enzyme known as fumarylacetoacetate-hydrolase through the addition of a water molecule. Through this set of reactions fumarate and acetoacetate (3-ketobutyroate) are liberated. Acetoacetate is a ketone body, which is activated with succinyl-CoA, and thereafter it can be converted into acetyl-CoA, which in turn can be oxidized by the citric acid cycle (also known as the TCA cycle) or used for fatty acid synthesis. Other aspects of tyrosine metabolism include the generation of catecholamines. In this process, the enzyme known as tyrosine hydroxylase (or AAAH or TYH) catalyzes the conversion of tyrosine to L-DOPA. The L-DOPA can then be converted via the enzyme DOPA decarboxylase (DDC) to dopamine. Dopamine can then be converted to 3-methoxytyramine via the action of catechol-O-methyltransferase (COMT). Dopamine can also be converted to norepinephrine (noradrenaline) through the action of the enzyme known as dopamine beta hydroxylase (DBH). Norepinephrine can then be converted to epinephrine (adrenaline) through the action of phenyethanolamine N-methyltransferase (PNMT). Catecholamines such as L-DOPA, dopamine and methoxytyramine are produced mainly by the chromaffin cells of the adrenal medulla and by neuronal cells found in the brain. For example, dopamine, which acts as a neurotransmitter, is mostly produced in neuronal cell bodies in the ventral tegmental area and the substantia nigra while epinephrine is produced in neurons in the human brain that express PMNT. Catecholamines typically have a half-life of a few minutes in the blood. They are typically degraded via catechol-O-methyltransferases (COMT) or by deamination via monoamine oxidases (MAO). Another important aspect of tyrosine metabolism includes the production of melanin. Melanin is produced through a mechanism known as melanogenesis, a process that involves the oxidation of tyrosine followed by the polymerization of these oxidation by-products. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are three types of melanin: pheomelanin, eumelanin, and neuromelanin of which eumelanin is the most common. Melanogenesis, especially in the skin, is initiated through the exposure to UV light. Melanin is the primary pigment that determines skin color. Melanin is also found in hair and the pigmented tissue underlying the iris. The first step in the synthesis for both eumelanins and pheomelanins is the conversion of tyrosine to dopaquinone by the enzyme known as tyrosinase. The resulting dopaquinone can combine with cysteine to produce cysteinyldopa, which then polymerizes to form pheomelanin. Dopaquinone can also form lecuodopachrome, which then can be converted to dopachrome (a cyclization product) and this eventually becomes eumelanin. Tyrosine plays a critical role in the synthesis of thyroid hormones. Thyroid hormones are produced and released by the thyroid gland and include triiodothyronine (T3) and thyroxine (T4). These two hormones are responsible for regulating metabolism. Thyroxine was discovered and isolated by Edward Calvin Kendall in 1915. Thyroid hormones are produced by the follicular cells of the thyroid gland through the action of thyroperoxidase, which iodinates reactive tyrosine residues on thyroglobulin. Proteolysis of the thyroglobulin in cellular lysosomes releases the small molecule thyroid hormones. In mammals, tyrosine can be formed from dietary phenylalanine by the enzyme phenylalanine hydroxylase, found in large amounts in the liver. Phenylalanine is considered an essential amino acid, while tyrosine (which can be endogenously synthesized) is not. In plants and most microbes, tyrosine is produced via prephenate, an intermediate that is produced as part of the shikimate pathway.
References
Tyrosine Metabolism References
Harhay GP, Sonstegard TS, Keele JW, Heaton MP, Clawson ML, Snelling WM, Wiedmann RT, Van Tassell CP, Smith TP: Characterization of 954 bovine full-CDS cDNA sequences. BMC Genomics. 2005 Nov 23;6:166. doi: 10.1186/1471-2164-6-166.
Pubmed: 16305752
Aurilia V, Palmisano A, Ferrara L, Cubellis MV, Sannia G, Marino G: Cloning and sequence analysis of A cDNA encoding bovine cytosolic aspartate aminotransferase. Int J Biochem. 1993 Oct;25(10):1505-9.
Pubmed: 8224363
Zimin AV, Delcher AL, Florea L, Kelley DR, Schatz MC, Puiu D, Hanrahan F, Pertea G, Van Tassell CP, Sonstegard TS, Marcais G, Roberts M, Subramanian P, Yorke JA, Salzberg SL: A whole-genome assembly of the domestic cow, Bos taurus. Genome Biol. 2009;10(4):R42. doi: 10.1186/gb-2009-10-4-r42. Epub 2009 Apr 24.
Pubmed: 19393038
Kang UJ, Joh TH: Deduced amino acid sequence of bovine aromatic L-amino acid decarboxylase: homology to other decarboxylases. Brain Res Mol Brain Res. 1990 Jun;8(1):83-7. doi: 10.1016/0169-328x(90)90013-4.
Pubmed: 2166204
Schmutz SM, Berryere TG, Ciobanu DC, Mileham AJ, Schmidtz BH, Fredholm M: A form of albinism in cattle is caused by a tyrosinase frameshift mutation. Mamm Genome. 2004 Jan;15(1):62-7. doi: 10.1007/s00335-002-2249-5.
Pubmed: 14727143
Taljanidisz J, Stewart L, Smith AJ, Klinman JP: Structure of bovine adrenal dopamine beta-monooxygenase, as deduced from cDNA and protein sequencing: evidence that the membrane-bound form of the enzyme is anchored by an uncleaved signal peptide. Biochemistry. 1989 Dec 26;28(26):10054-61. doi: 10.1021/bi00452a026.
Pubmed: 2620060
Lewis EJ, Allison S, Fader D, Claflin V, Baizer L: Bovine dopamine beta-hydroxylase cDNA. Complete coding sequence and expression in mammalian cells with vaccinia virus vector. J Biol Chem. 1990 Jan 15;265(2):1021-8.
Pubmed: 1688549
Hawkins GA, Eggen A, Hayes H, Elduque C, Bishop MD: Tyrosinase-related protein-2 (DCT; TYRP2) maps to bovine chromosome 12. Mamm Genome. 1996 Jun;7(6):474-5.
Pubmed: 8662245
Baetge EE, Suh YH, Joh TH: Complete nucleotide and deduced amino acid sequence of bovine phenylethanolamine N-methyltransferase: partial amino acid homology with rat tyrosine hydroxylase. Proc Natl Acad Sci U S A. 1986 Aug;83(15):5454-8. doi: 10.1073/pnas.83.15.5454.
Pubmed: 2874553
Batter DK, D'Mello SR, Turzai LM, Hughes HB 3rd, Gioio AE, Kaplan BB: The complete nucleotide sequence and structure of the gene encoding bovine phenylethanolamine N-methyltransferase. J Neurosci Res. 1988 Mar;19(3):367-76. doi: 10.1002/jnr.490190313.
Pubmed: 3379652
Weisberg EP, Batter DK, Brown WE, Kaplan BB: Purification and partial amino acid sequence of bovine adrenal phenylethanolamine N-methyltransferase: a comparison of nucleic acid and protein sequence data. J Neurosci Res. 1988 Mar;19(3):377-82. doi: 10.1002/jnr.490190314.
Pubmed: 3379653
Powell JF, Hsu YP, Weyler W, Chen SA, Salach J, Andrikopoulos K, Mallet J, Breakefield XO: The primary structure of bovine monoamine oxidase type A. Comparison with peptide sequences of bovine monoamine oxidase type B and other flavoenzymes. Biochem J. 1989 Apr 15;259(2):407-13. doi: 10.1042/bj2590407.
Pubmed: 2719656
This pathway was propagated using PathWhiz -
Pon, A. et al. Pathways with PathWhiz (2015) Nucleic Acids Res. 43(Web Server issue): W552–W559.
Propagated from SMP0000006
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