HORT640 - Metabolic Plant Physiology
Secondary products derived from aromatic amino acids
General structures of flavonoids and anthocyanins
Anthocyanins are glycosylated anthocyanidins; sugars are attached to the 3-hydroxyl position of the anthocyanidin (sometimes to the 5 or 7 position). In many cases the sugar residues are acylated by p-coumaric, caffeic, ferulic, sinapic, p-hydroxybenzoic, malonic, oxalic, malic, succinic, or acetic acids.
Flavonoids are implicated in UV tolerance (as UV screening molecules), and can directly scavenge active oxygen species (superoxide, hydrogen peroxide, hydroxyl radical, singlet oxygen or peroxyl radical). Because flavonoids are largely located in the vacuole they are unlikely to be involved in direct scavenging of the highly reactive oxygen species generated in the chloroplast from photosynthetic electron transport. However, unlike other active oxygen species, hydrogen peroxide is stable and capable of diffusing across membranes. A flavonol-peroxidase system for detoxification of hydrogen peroxide in the vacuole has been proposed (Yamasaki et al, 1997):
2 FlavOH + H2O2 ---> 2 FlavO* + 2 H2O
The resulting flavonoid phenoxyl radical (FlavO*) can react with ascorbic acid (AsA) to generate the monodehydroascorbic acid radical (MDA*) :
2 FlavO* + 2 AsA ---> 2FlavOH + 2 MDA*
MDA* can in turn non-enzymatically generate AsA and DHA:
2 MDA* ---> AsA + DHA
The resulting DHA can be reduced back to AsA by cytosolic dehydroascorbate reductase (Yamasaki et al, 1997).
Yamasaki H, Sakihama Y, Ikehara N 1997 Flavonoid-peroxidase reaction as a detoxification mechanism of plant cells against H2O2. Plant Physiol. 115: 1405-1412.
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