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Pathway Description
Tyrosine Metabolism upload-662
Homo sapiens
Metabolic Pathway
Tyrosine is produced in cells by hydroxylating (via phenylalanine hydroxylase) the essential amino acid phenylalanine. Half of the phenylalanine required goes into the production of tyrosine; if the diet is rich in tyrosine itself, the requirements for phenylalanine are reduced by about 50%. Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced state by the NADH-dependent enzyme dihydropteridine reductase (DHPR). The catabolism of tyrosine starts with an α-ketoglutarate dependent transamination through the tyrosine transaminase, which generates p-hydroxyphenylpyruvate. The next oxidation step is catalyzed by p-hydroxylphenylpyruvate-dioxygenase and generates homogentisate (2,5-dihydroxyphenyl-1-acetate). In order to split the aromatic ring of homogentisate, a further dioxygenase, homogentistate-oxygenase is required. Through this reaction, maleylacetoacetate is created. Fumarylacetate is then generated by maleylacetoacetate-cis-trans-isomerase through rotation of the carboxyl group created from the hydroxyl group via oxidation. This cis-trans-isomerase contains glutathione as a coenzyme. Fumarylacetoacetate is finally split into acetoactate and fumarate via fumarylacetoacetate-hydrolase through the addition of a water molecule. Thereby fumarate (also a metabolite of the citric acid cycle) 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 or be used for fatty acid synthesis.
References
Tyrosine Metabolism upload-662 References
Lehninger, A.L. Lehninger principles of biochemistry (4th ed.) (2005). New York: W.H Freeman.
Salway, J.G. Metabolism at a glance (3rd ed.) (2004). Alden, Mass.: Blackwell Pub.
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