Phenylalanine L-DOPA

FIGURE 27.5 Tyrosine is the biosynthetic precursor to a number of neurotransmitters. Each transformation is enzyme-catalyzed. Hydroxy-lation of the aromatic ring of tyrosine converts it to 3,4-dihydroxy phenylalanine (L-dopa), decarboxylation of which gives dopamine. Hy-droxylation of the benzylic carbon of dopamine converts it to norepinephrine (noradrenaline), and methy-lation of the amino group of norepinephrine yields epinephrine (adrenaline). 

Tyrosine hydroxylase is the rate-limiting enzyme for the biosynthesis of catecholamines. Tyrosine hydroxylase (TH) is found in all cells that synthesize catecholamines and is a mixed-function oxidase that uses molecular oxygen and tyrosine as its substrates and biopterin as its cofactor [1], TH is a homotetramer, each subunit of which has a molecular weight of approximately 60,000. It catalyzes the addition of a hydroxyl group to the meta position of tyrosine, thus forming 3,4-dihydroxy-L-phenylalanine (l-DOPA). 

The expression product of pG3TY was also isolated by PAGE at pH 9.4 gel [139], The activity band of tyrosinase was stained with dihydroxy-L-phenylalanine (l-DOPA) after acidifying at pH 3. Tyrosinase activity of the pG3YT product was determined to be 0.018 nkat/kg protein. Without acidifying treatment at pH 3.0, no tyrosinase activity was observed. It can be concluded that the expression product of the melO gene is the protyrosinase of A. oryzae. 

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