Tyrosine is one of the 22 protein-forming amino acids. In Danio rerio, it is an essential amino acid, meaning it must be obtained from dietary sources, as the body either cannot synthesize any of it, or cannot produce enough to satisfy the demand. Phenylalanine, another essential amino acid, can be metabolized to form tyrosine, which is then important in the formation of melanin, as well as dopamine, epinephrine and other related compounds. Tyrosine can be converted to tyramine by aromatic-L-amino-acid decarboxylase, which removes a carbon dioxide molecule from it. Then, in the mitochondria, either monoamine oxidase or an amine oxidase can convert it to 4-hycroxyphenylacetaldehyde. From there, aldehyde dehydrogenase can convert it to and from p-hydroxyphenylacetic acid. Tyrosine can also interact with iodine peroxidase, which adds an iodine and removes a hydrogen to form iodotyrosine. Iodotyrosine can then have another iodine added and hydrogen removed by iodine peroxidase, forming 3,5-diiodo-L-tyrosine. This can undergo two reactions, both catalyzed by iodine peroxidase, forming either thyroxine or liothyronine, with the latter also using iodotyrosine as one of the reactants. Another metabolism of tyrosine can be facilitated by either aspartate aminotransferase or tyrosine transaminase in the mitchondria, or amine oxidase in other locations in the cell. The aminotransferases can convert L-tyrosine to and from 4-hydroxyphenylpyruvic acid, while the amine oxidase can only convert it to 4-hydroxyphenylpyruvic acid in a non-reversible reaction. From here, phenylpyruvate tautomerase can catalyze the reversible tautomerization of 4-hydroxyphenylpyruvic acid to 2-hydroxy-3-(4-hydroxyphenyl)propenoic acid. Alternately, 4-hydroxyphenylpyruvate dioxygenase can catalyze the conversion of 4-hydroxyphenylpyruvic acid to homogentisic acid. Homogenistic acid can then form genistate aldehyde via an oxidoreductase of which the protein is currently unknown in Danio rerio, and following this reaction, aldehyde oxidase can catalyze the formation of gentistic acid from the aldehyde. If it does not interact with the oxidoreductase, homogentisic acid can instead interact with homogentisate 1,2-dioxygenase, which adds an oxygen molecule and breaks the aromatic ring, forming maleylacetoacetic acid. This can then form 4-fumarylacetoacetic acid via catalysis by maleylacetoacetate isomerase. 4-fumarylacetoacetic acid can then be converted to fumaric acid by fumarylacetoacetase. Fumaric acid can also be formed from 3-fumarylpyruvate, catalyzed by acylpyruvate hydrolase, which also forms pyruvic acid. The pyruvic acid can be used in pyruvate metabolism, while the fumaric acid from either source is used in the citrate cycle. One final path of metabolism that tyrosine can undergo is its catalysis by tyrosinase to form either dopaquinone or L-dopa. If it forms dopaquinone, this can, without enzymes, combine with L-cysteine to form cysteinlydopa, or a ring can close in the structure spontaneously, forming leucodopachrome. Dopaquinone, together with leucodopachrome can form two molecules of L-dopachrome, which is then used in the biosynthesis of melanin. They can also combine to form L-dopa. L-dopa, whether from tyrosinase or this reaction, reacts with aromatic-L-amino-acid decarboxylase to remove a carbon dioxide molecule, forming dopamine. There are then multiple pathways dopamine can go through. First, it can interact with catechol O-methyltransfearse A to form 3-methoxytyramine, which then interacts with monoamine oxidase in the mitochondria to form homovanillin. Finally, homovanillin can interact with aldehyde dehydrogenase to form homovanillic acid. Alternatively, dopamine can interact directly with either monoamine oxidase or amine oxidase in the mitochondria, forming 3,4-dihydroxyphenylacetaldehyde, which interacts with aldehyde dehydrogenase to form 3,4-dihydroxybenzeneacetic acid. Finally, 3,40dihydroxybenzeneacetic acid interacts with catechol O-methyltransferase A to once again form homovanillic acid. Dopamine can also interact with dopamine beta-hydroxylase to form norepinephrine. Norepinephrine can once again interact with monoamine oxidase in the mitochondria, forming 3,4-dihydroxymandelaldehyde. This can also be converted to and from 3,4-dihydroxyphenylglycol by S-(hydroxymethyl)glutathione dehydrogenase. 3,4-dihdyroxyphenylglycol can then form vanylglycol following catalysis by catechol O-methyltransferase A. 3,4-dihydroxymandelaldehyde can also be converted to and from 3,4-dihydroxymandelic acid by an aldehyde dehydrogenase, and 3,4-dihydroxymandelic acid can be converted to and from vanillylmandelic acid by catechol O-methyltransferase A. Norepinephrine can interact with an uncharacterized protein that forms a phenylethanolamine N-methyltransferase, in a reaction that forms epinephrine. Following this, epinephrine can interact with catechol O-methyltransferase A to form metanephrine. Then, in the mitochondria, it can interact with monoamine oxidase to form 3-methoxy-4-hydroxyphenylglycolaldehyde. Alternatively, norepinephrine can interact directly with catechol O-methyltransferase A to form normetanephrine, which then interacts with monoamine oxidase in the mitochondria to again form 3-methoxy-4-hydroxyphenylglycolaldehyde. Regardless of which set of reactions creates it, this can then interact with aldehyde dehydrogenase, forming vanillylmandelic acid.

Tyrosine Metabolism References