Type II NADH–quinone oxidoreductase (NDH-2)

Type II NADH–quinone oxidoreductase is a membrane-bound enzyme that catalyzes the transfer of electrons from NADH to quinones within the respiratory chain. Unlike complex I (Type I), it does not pump protons across the membrane but instead provides an alternative route for electron flow, contributing to cellular energy production by regenerating NAD+, which is essential for various metabolic pathways. This enzyme is found widely among bacteria, fungi, plants, and some protozoa but is absent from mammalian mitochondria. NDH‑2 enzymes are structurally distinct from other quinone reductases; they typically contain FAD as a cofactor and have separate binding sites for their two substrates—NAD(P)H and quinones. The catalytic mechanism involves oxidation of NAD(P)H at one site with concurrent reduction of a quinone at another site. In pathogenic bacteria such as Staphylococcus aureus, NDH‑2 plays a critical role in respiration and survival under stress conditions. Because mammals lack this enzyme class in their mitochondria, NDH‑2 has emerged as an attractive antimicrobial drug target—selective inhibitors can disrupt pathogen energy metabolism without affecting host cells. HQNO is one such inhibitor studied for its ability to block this pathway. No major safety concerns or established biomarkers are currently associated with targeting this molecule; however, further research may be needed before clinical application.

Other names

NDH-2Type II NADH:quinone oxidoreductaseAlternative NADH dehydrogenaseNon-proton-pumping NADH dehydrogenaseNdh (gene/protein symbol in some species)

Inhibition of electron transfer from NADH to quinone, disrupting energy metabolism and respiratory function in bacteria