Mechanism of Action

Mechanism of ActionThe mechanism by which copper kills microorganisms is multifaceted and not yet fully understood. Literature suggests that copper interacts with microorganisms on different cellular levels, all resulting in cell death, including cell membrane permeabilization, membrane lipid peroxidation, protein alteration, and denaturation of nucleic acids.

Copper ions released from the surface are transported through the cell membrane via an energy-dependent transport system. A decreased barrier function of the plasma membrane occurs upon reaching a crucial intracellular concentration of copper ions. The membrane becomes increasingly permeable, leading to leakage of vital cell content such as nucleotides, amino acids, and potassium; Warnes et al. demonstrated rapid membrane depolarization and loss of membrane integrity upon exposure to metallic copper surfaces, leading to cytoplasmic accumulation of free radicals.

Another mechanism by which copper damages the cell membrane is via the formation of reactive oxygen species (ROS). Copper ions are able to donate and accept electrons and thereby can change between Cu⁺ and Cu²⁺ via oxidation. This catalyzes the formation of ROS such as hydroxyl radicals (HO) and superoxide anions (O₂⁻); causes oxidative stress to membrane lipids (lipid peroxidation), amino acids in proteins, and nucleic acids; and causes oxidative deactivation of enzymes.

Essential metals are required for proper protein function through a process called coordination chemistry. They are needed as cofactors and to catalyze cytotoxic reactions. Proteins controlling the activity of intracellular metal ions include membrane transporters, metalloregulating proteins, and metallochaperoning proteins. Metal ions are able to bind to certain atoms of protein complexes, such as oxygen (O), nitrogen (N), and sulfur (S). Proteins exert preferences for binding specific metal ions because of their atomic structure. However, when metal homeostasis is distorted, metal ions such as Cu²⁺ are able to compete with other essential metals, for instance, iron ions (Fe²⁺), and prevent proteins from binding with the correct cofactor and consequently inhibiting protein function. For instance, the protease of HIV-1 is inhibited by the binding of copper ions to the amino acid cysteine. Another example is the inactivation of protein tyrosine phosphatases through the oxidation of cysteine residues on the phosphatases by copper ions. Furthermore, the enzyme isopropylmalate dehydratase in E. coli is inactivated by displacing iron atoms by copper ions from the iron-sulfur clusters within the enzyme, a process called mismetallation.

（To be continued）

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