This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. 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|
Some taxa known to possess this pathway include : Amanita muscaria
Betaxanthins represent a subclass of betalains, the nitrogen-containing compounds that replace the anthocyanins in the order of Caryophyllales and some higher fungi [Strack03] [Cai05]. 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 (λ max 470-486 nm) [Christinet04a].
Pigments such as anthocyanins and carotenoids are important ecological means to attract pollinators. It could be demonstrated that betalains are equally developed pigments to fill out this physiological function. Betaxanthins are able to fluoresce by absorbing blue light and emitting green light [GandiaHerrero05b]. That implies a complex light-filtering system to create a contrasting fluorescent pattern as violet betacyanins absorb those emitted wavelengths of the yellow betaxanthins. Those findings about fluorescent flowers have been discussed as a new possibility for pollinator perception [GandiaHerrero05c].
About This Pathway
This pathway demonstrates the formation of betaxanthins such as portulacaxanthin II and dopaxanthin by means of non-enzymatic condensation from the amino acids L-tyrosine and L-DOPA, respectively. Tyrosinases have been described as capable to use those betaxanthins [GandiaHerrero05a] 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 [GandiaHerrero05]. 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 [GandiaHerrero05d]. This article also describes miraxanthin III (tyramine as amine moiety) as an alternative substrate for the tyrosinase to form dopamine-betaxanthin, usually derived from the precursor dopamine.
Superpathways: superpathway of betalain biosynthesis
Christinet04a: Christinet L (2004). "Characterization and functional identification of a novel plant extradiol 4,5-dioxygenase involved in betalain pigment biosynthesis in Portulaca grandiflora." PhD thesis, Universite de Lausanne, Département de Biologie Moléculaire Végétale.
GandiaHerrero05: Gandia-Herrero F, Escribano J, Garcia-Carmona F (2005). "Betaxanthins as substrates for tyrosinase. An approach to the role of tyrosinase in the biosynthetic pathway of betalains." Plant Physiol 138(1);421-32. PMID: 15805475
GandiaHerrero05a: Gandia-Herrero F, Escribano J, Garcia-Carmona F (2005). "Characterization of the activity of tyrosinase on betaxanthins derived from (R)-amino acids." J Agric Food Chem 53(23);9207-12. PMID: 16277424
GandiaHerrero05d: Gandia-Herrero F, Escribano J, Garcia-Carmona F (2005). "Characterization of the monophenolase activity of tyrosinase on betaxanthins: the tyramine-betaxanthin/dopamine-betaxanthin pair." Planta 222(2);307-18. PMID: 15968512
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