The complete biosynthetic pathway has been explained by Strack et al and Han et al.
The starting point is an amino acid tyrosine but in some betalains, tyramine is the starting point as well.
The Tyr pathway: The tyrosine pathway begins with hydroxylation of Tyr to L-DOPA by the action of tyrosinase (EC 126.96.36.199). tyrosinase belongs to a group of copper-containing bifunctional. These enzymes are involved in the hydroxylation of phenols to o-diphenols.
DOPA can be converted in three ways: it can be oxidized by the same enzyme (EC188.8.131.52) to dopaquinone, cleaved and opened into 4,5-seco-DOPA in a reaction catalyzed by DOPA 4,5--dioxygenase (DOD), or decarboxylation to form dopamine, catalyzed by DOPA-decarboxylase.
Dopaquinone forms cyclo-DOPA, while betalamic acid is yielded by cyclizationof 4,5-seco-DOPA . DOD catalyzes this step.
DOD is one of the most important enzymes in…show more content… Betanidin is glycosylated on the DOPA side with the aid of betanidin-5--glucosyltransferase, to form betanin (16). Alternatively, cyclo--DOPA can be glycosylated by cyclo-DOPA-5-glucosyltransferase to form cyclo-DOPA glucoside. The product can condense with betalamic acid to form betanin directly without passing through the betanidin intermediate (47,48). spontaneous condensation of betalamic acid with an amino acid or amine, involving the formation of an aldimine bond, generates betaxanthins like indicaxanthin or vulgaxanthin (49). A combination of betalamic acid with 2-descarboxy-cyclo-DOPA will result in the formation of 2-descarboxy-betanidin, a minor betacyanin found in the yellow beet (50).
If betalamic acid is back-condensed with tyrosine, the starting molecule of the pathway, tyrosine-betaxanthin (portulacaxanthin II) is generated. This compound is found in Portulaca sp. and other species