Flavonoids are polyphenolic compounds widely distributed in plants. Their benefits to human health are attributed to a wide range of biological activities which include antimutagenic, anticarcinogenic, immune-stimulating, antiinflammatory and artheriosclerosis inhibiting effects. The polyphenols are able to act as antioxidants, inhibiting lipid peroxidation and scavenge superoxide or hydroxyl radicals [1].

Another mechanism for the endogenous defense against free radicals (oxidative stress) has been proposed via the modification of signal transduction pathways.

It was proved that quercetin, binding to the active site of the enzyme, inhibits protein kinase Hck which plays significant role in signal transduction in the immune system [2]. Additionally, flavonoids have antiinflammatory properties and could interact with cyclooxygenase (COX-2) as selective inhibitors [3].

Taxifolin, a natural dihydroflavonol (found in grapefruit, orange, the wood of the Siberian and Dahurian larches), quercetin (present in onions, apples and red wine) and epicatechin (common in black and green tea) are structurally similar with the same 5,7,3',4' hydroxylation pattern.

We have investigated selected flavonoids: quercetin, taxifolin and epicatechin in order to find structural features that govern the specific binding to the Hck and COX-2. These compounds differ in the structure of C-ring; the double bond between C2 and C3 in the ring C may play a dominant role on their biological activities. Molecular modeling confirmed that the geometry of C-ring is an important factor for intermolecular interactions with these enzymes.

1. Williams, R.J.; Spencer, J.P.E.; Rice-Evans, C. Flavonoids: antioxidants or signaling molecules? Free. Rad. Biol. & Med. 36 (2004) 838-849.
2. Sicheri, F.; Moarefi, I.; Kuriyan, J. Crystal structure of the Src family tyrosine kinase Hck. Nature 385 (1997) 602-609.
3. Cho, H., et al., Modulation of the activity of pro-inflammatory enzymes, COX-2 and iNOS, by chrysin derivatives. Pharmacol. Res. 49 (2004) 37-43.

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