Dopa 4,5-dioxygenase

Figure 2.23 Biosynthesis of betalains, involving two early enzymes, the tyrosinase (1A, hydroxylating activity 1 B, oxidizing activity) and the Dopa 4,5-dioxygenase (2), and one late enzyme activity, glucosylating cyc/o-Dopa and/or betanidin (3). Reactions 4 and 5 are considered to proceed spontaneously.

Detection of Dopa 4,5-dioxygenase in higher plants and clarification of the level of glucosylation, at betanidin and/or ci/do-Dopa, are the last two steps in betalain biosynthesis to be confirmed. Molecular studies are still needed to elucidate the evolutionary mechanisms of the mutual exclusion of the two pathways (Stafford, 1994), one leading to the ubiquitously occurring anthocyanins and the other to the rare betalains. 

Girod, P.-A. and Zryd, J.-P. (1991) Biogenesis of betalains purification and partial characterization of Dopa 4,5-dioxygenase from Amanitamuscaria. Phytochemistry, 30,169-74. 

Mueller, L.A., Hinz, U. and Zryd, J.-P. (1997a) The formation of betalamic acid and muscafiavin by recombinant Dopa-dioxygenase from Amanita. Phytochemistry, 44, 567-69. 

One of the most important applications of PPO, although rarely reported, is its role in synthetic processes, such as the biosynthesis of betalains. Several researchers reported the hydroxylation of tyrosine to dopa, which can then be oxidized to dopaquinone, through a PPO from Portulaca grandiflora and from Beta vulgaris. Thus, a dioxygenase activity complements the constitutive PPO activity and the initiation of this dioxygenase... 

The distal extradiol cleavage of L-dopa 12, catalyzed by an iron-dependent dioxygenase, gives an alanyl muconic semialdehyde derivative 18 which, on cyclization and lactol oxidation, yields stizolobic acid 16. The pyrone ring is then ammonolyzed24 to give 3-(6-carboxy-2-oxo-4-pyridyl)alanine 17 (Scheme 3). 

Phenylalanine hydroxylase [39] catalyzes the first step of phenylalanine degradation in mammals. Phenylalanine is converted into tyrosine (Scheme XI.9). The enzyme has one tightly bound, non-heme iron atom per subunit. Tyrosine hydroxylase catalyzes [40] the hydroxylation of tyrosine to produce dihydroxy-phenylalanine (DOPA), the first step in the biosynthesis of catechol-amine neurotransmitters (Scheme XI.9). This enzyme also contains one ferrous iron atom per subunit. These two enzymes, together with tryptophane hydroxylase (Scheme XI.9) [41], constitute a family of tetrahydropterin-dependent aromatic acid hydroxylases (monooxygenases) [42], Other dioxygenases catalyze the hydroxylation ofarenes [43],... 

The alternative extradiol cleavage which breaks bond b in L-DOPA (172) can leaci via the intermediate (183) to muscaflavin (180) 688) and to stizolobinic acid (185), a known constituent of A. pantherina 168). The incorporation of radioactively labelled L-DOPA into stizolobinic acid in A. pantherina 553) and the co-occurrence of L-DOPA with muscaflavin in fruit bodies of Hygrocybe conica 619) add support to this 2,3-dioxygenase pathway. 

Fig. 291. Recyclization of l-DOPA after oxidative ring cleavage 1 Dioxygenases, e.g., stizolobate synthase and stizolobinate synthase...

Dioxygenases (C 2.5) splitt off the aromatic ring of l-DOPA adjacent to the both phenolic groups (Fig. 291). Cleavage of the ring proceeds... 

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