Quercetin, a Flavonoid with Great Pharmacological Capacity

Abstract

Quercetin is a flavonoid with a low molecular weight that belongs to the human diet's phenolic phytochemicals and nonenergy constituents. Quercetin has a potent antioxidant capacity, being able to capture reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive chlorine species (ROC), which act as reducing agents by chelating transition-metal ions. Its structure has five functional hydroxyl groups, which work as electron donors and are responsible for capturing free radicals. In addition to its antioxidant capacity, different pharmacological properties of quercetin have been described, such as carcinostatic properties; antiviral, antihypertensive, and anti-inflammatory properties; the ability to protect low-density lipoprotein (LDL) oxidation, and the ability to inhibit angiogenesis; these are developed in this review.

Keywords: antioxidant; free radicals; nanoparticles; pharmacological properties; quercetin.

Figure 1

Structural diversity of flavonoids. This figure presents a visual representation of the structural diversity of flavonoid molecules. Variations in the hydroxylation pattern and oxidation state of the central pyran ring lead to the formation of a wide spectrum of compounds (Eber J. Ca. Mtz. 2023. ChemDraw^®).

Figure 2

Chemical structure of quercetin. The figure shows the chemical characteristics that give it its antioxidant capacity, the most important of which is the catechol or dehydroxylated B-ring structure (yellow). Other important characteristics are the presence of unsaturation in the C-ring (red) and the presence of a 4-oxo in the C-ring (green); the oxygen in position 4 and the hydroxyl in position 5 allow it the capacity to chelate ions such as Cu⁺⁺ and Fe⁺⁺ (blue). On the other hand, in total, quercetin has 5 functional hydroxyl groups with the potential to be conjugated and that differ in their chemical reactivity following the order of reactivity 3 > 7 > 3′ > 4′ >> 5. The phenolic hydroxyl groups of quercetin act as donors of electrons, and these are responsible for the capture activity of free radicals, in particular the catechol structure with 2 hydroxyl groups in the neighboring positions, which is notably superior to other arrangements in electron donation (Eber J. Ca. Mtz. 2023. ChemDraw^®).

Figure 3

Metabolism of quercetin. The diagram illustrates the enzymatic transformations undergone by quercetin within the biological system. Key steps include deglycosylation by enterobacteria facilitating its absorption, and hydroxylation, glycosylation, and methylation processes, leading to the formation of various metabolites in the liver. The figure highlights the network of metabolic reactions involved in the biotransformation of quercetin, shedding light on its fate within the body (Eber J. Ca. Mtz. 2023. BioRender).

Figure 4

Effect of quercetin on the main signaling pathways involved in cancer. The figure shows some of the main signaling pathways involved in cancer. The increase in the activity of some of them results in an increase in cell proliferation, while the increase in some others increases cell apoptosis. In red are shown those molecules on which quercetin exerts an inhibitory effect, while in blue are those whose concentration and activity are increased when cells are exposed to quercetin (Eber J. Ca. Mtz. 2023. Bio-Render).