Figure 2.

Potential mechanisms underlying the lethal effect of PCA on gram-negative bacteria. Autoxidation of PCA and semiquinone oxidation through the inhibition of NADH-quinone oxidoreductase (NQR) and succinate-quinone oxidoreductase (SQR) can cause ATP depletion and produce •O²⁻, which attacks the polyunsaturated fatty acid components of the membrane to cause lipid peroxidation and attacks the thiol group of protein to cause protein oxidation. Although SOD converts •O^2- to H₂O₂, which can be finally changed to H₂O and O₂ by catalase, excessive H₂O₂ produces •OH during the Fenton reaction (Fe²⁺→Fe³⁺), and •OH attacks DNA bases to cause DNA fragmentation. Ultimately, lipid peroxidation, protein oxidation, DNA fragmentation, and ATP depletion induce bacterial death. PCA, protocatechuic acid; SOD, superoxide dismutase.