Biosynthesis of Betalains

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... 

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.

Fischer, N. and Dreiding, A.S. (1972) Biosynthesis of betalaines. On the cleavage of the aromatic ring during the enzymatic transformation of dopa into betalamic acid. Helv. Chim. Acta, 55, 649-58. 

The biosynthesis of betalains provides a rare example of the cleavage of an aromatic ring in higher plants. The conversion of alkaloids of the type (103) into (106) provides another these results do not allow definition of the manner in which ring cleavage occurs since both intra-diol (C-15—C-16) and extra-diol (C-16—C-17) cleavage may lead to loss of C-16 and retention of tritium at C-17 as observed. 

Biosynthesis of Betalain Pigments via Qxidative Cleavage of Dopa 616... 

Figure 37 Biosynthesis of betalain pigments via oxidative cleavage of dopa.

Fischer, N., and A.S. Dreiding Biosynthesis of Betalaines. On the Cleavage of the Aromatic Ring During the Enzymatic Transformation of Dopa into Betalamic Acid. Helv. Chim. Acta 55, 649 (1972), and references therein. 

---