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N Use By Plants
Nitrate Assimilation
Ammonia Assimilation
Glu, Gln, Asn, Gly, Ser
Aminotransferases
Asp, Ala, GABA
Val, Leu, Ileu, Thr, Lys
Pro, Arg, Orn
Polyamines
Non-protein AAs
Alkaloids
Sulfate Assimilation
Cys, Met, AdoMet, ACC
His, Phe, Tyr, Tryp
Secondary Products
Onium Compounds
Enzymes
Methods
Simulation
References
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):

Flavonoid Peroxidase

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).

References

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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David Rhodes
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Last Update: 10/01/09