If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Nitrogen-Containing Secondary Compounds Biosynthesis → Alkaloids Biosynthesis → Betalaine Alkaloids Biosynthesis|
Betalains are water-soluble, chromo-alkaloid pigments that replace the anthocyanins in most families of the Caryophyllales order [Cai05]. The two subclasses of betalains, i.e. betacyanins and betaxanthins dominate that order and provide violet and yellow hues to flowers, fruits and vegetative tissues, respectively [Strack03, Grotewold06].
The red to violet betacyanins are the result of the condensation of betalamic acid (betalamic acid biosynthesis) and cyclo-DOPA to form the aglycon betanidin from which the big majority of natural betacyanins derive (compare betacyanin biosynthesis (via dopamine)). The violet color of betacyanins is based on the observed shift to the corresponding absorbance maximum (λmax 534-554 nm) of the aromatic structure after that condensation [Christinet04a]. In general, the majority of betacyanins are derived from cyclo-DOPA (betacyanin biosynthesis). However, there are few exceptions that have demonstrated the alternative route of betacyanins derived from the catecholamine dopamine such as 2-descarboxybetanidin and derivatives thereof [Piattelli70, Schliemann99].
Amaranthin belongs to and is the eponym for a major subgroup of betacyanins (see betacyanin biosynthesis) that have mostly been identified in the amaranth family. Only betacyanins contain a glucose moiety that may be further furnished with other molecules such as malonylate and hydroxycinnamic acids.
Betaxanthins represent another subclass of betalains. These pigments are biosynthesized by the condensation of betalamic acid (betalamic acid biosynthesis) and amino acids/amines forming a Schiff-base that causes the yellow to orange colors (λmax 470-486 nm) [Christinet04a].
Those pigments have also been identified in a restricted number of basidiomycetes such as in the genera Amanita and Hygrocybe. No betacyanin pigments have been found in those fungi but the biosynthesis of several betaxanthin compounds along with other derivatives of betalamic acid such as muscaflavin and muscapurpurin has been confirmed [Mueller97d]. However, their physiological role in fungi remains completely unknown [Strack03].
About This Pathway
The central intermediate for both betacyanin (betacyanin biosynthesis) and betaxanthin biosynthesis (betaxanthin biosynthesis (via dopaxanthin), betaxanthin biosynthesis (via dopamine), betaxanthin biosynthesis) is betalamic acid which derives from 3,4-dihydroxy-L-phenylalanine (DOPA). The key-enzyme, i.e. the extradiol ring-opening DOPA-4,5-dioxygenase (DODA) catalyzes the formation of 4,5-seco-DOPA in plants which is, in a consecutive reaction, spontaneously converted to the chromophore betalamic acid [Christinet04]. The decisive step in both subclasses to form the corresponding betacyanin- and betaxanthin compounds is a non-enzymatic condensation of betalamic acid with either cyclo-DOPA and derivatives or amino acids/amines, respectively [Schliemann99].
DOPA may be synthesized through two different pathways from L-tyrosine involving either a bifunctional tyrosinase [Steiner99] or a tyrosine hydroxylase that requires pteridin cofactors for its activity, the latter representing a new enzyme reported for this reaction [Yamamoto01b].
The observed diversity of betacyanin structures is due to the frequently observed acylation and glucosylation of the aglycone betanidin. The glucosylation of betanidin takes place in a regiospecific way resulting in either the production of betanin (via betanidin 5-O-glucosyltransferase) [Vogt99] or e.g. gomphrenin I (via betanidin 6-O-glucosyltransferase) [Vogt02]. One crucial intermediate in this biosynthesis is cyclo-DOPA that is also a precursor for the further furnishing of betalains. The glucosylated cyclo-DOPA is further metabolized by a rather specific UDP-glucuronic acid: cyclo-DOPA 5-glucoside glucuronosyltransferase that adds glucuronic acid to the structure thus forming amaranthin [Sasaki05a].
It has been demonstrated that the conjugation of a broad variety of amino acids (see betaxanthin biosynthesis (via dopaxanthin)) and amino acid derivatives (e.g. 3-methoxytyramine) with betalamic acid results in the corresponding betaxanthins [Trezzini91]. The first enzymatic step in the pathway is the decarboxylation of aromatic amino acids such as tyrosine or DOPA to form the corresponding amines tyramine and dopamine, respectively [Facchini94a, Facchini95a].
Tyrosinases have also been described as capable to use certain betaxanthins [GandiaHerrero05b] as substrates for further metabolization. From those results, an alternative pathway has been proposed for the formation of betanidin which involves a tyrosinase activity that converts dopaxanthin into dopaxanthinquinone. Dopaxanthinquinone is than spontaneously cyclisized into betanidin [GandiaHerrero05a]. A similar approach has been undertaken to delineate alternative metabolism for the formation of 2-descarboxy-betanidin (compare betacyanin biosynthesis (via dopamine)) by acting of those tyrosinases on the amine-derived intermediates such as dopamine-betaxanthin to produce the corresponding quinone [GandiaHerrero05].
Subpathways: betalamic acid biosynthesis , betacyanin biosynthesis , amaranthin biosynthesis , betaxanthin biosynthesis (via dopaxanthin) , betaxanthin biosynthesis (via dopamine) , betaxanthin biosynthesis , betacyanin biosynthesis (via dopamine)
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