α-Mangostin Competing the Menaquinone-Binding Sites of NDH-2 to Block the Electron Transfer at the Quinone Pool of Staphylococcus aureus

α-Mangostin, a natural product from Garcinia mangostana L, presents most antibacterial activity in plant flavonoids against Staphylococcus aureus so far. Recently, it was reported that the quinone pool is a key target of α-mangostin against Gram-positive bacteria. To further confirm this and investigate the detail of α-mangostin killing S. aureus, the interactions between α-mangostin and a key enzyme as type II NADH:quinone oxidoreductase (NDH-2), together with possible non-enzymatic mechanisms, were explored. Through the enzyme kinetic inhibition experiments, it was found that α-mangostin mainly competes with the menaquinone-binding sites of NDH-2, and the half-maximal inhibitory concentration (IC50) of α-mangostin on NDH-2 is 4.95 μM. Fluorescence analyses indicated that α-mangostin can spontaneously bind to NDH-2 to form an α-mangostin–NDH-2 complex. Subsequently, molecular simulation further indicated that α-mangostin can dock to the menaquinone-binding sites of NDH-2. Another, non-enzymatic mechanism showed that α-mangostin can cause membrane potential depolarization and disrupt the proton motive force balance, thereby promoting the cell-membrane destruction of S. aureus. These results suggest that α-mangostin mainly can interact with the amino acid residues at the menaquinone-binding pocket of NDH-2 to block the electron transfer at the quinone pool in the respiratory chain of S. aureus, and which will hinder the energy supply and promote its incidental effect on membrane disruption, ultimately leading to the death of S. aureus. This once again proves that the quinone pool is a key target of plant flavonoids against Gram-positive bacteria.