of NADH that are saturating, NADH inhibits the peroxidase activity of the NADH peroxidase by converting the enzyme to an unstable intermediate. NAD+ behaves as an activator by reversing the equilibria that lead to the unstable intermediate, thus converting the enzyme to the kinetically active complex that reduces H2O2. Biological Function NADH eliminates potentially toxic hydrogen peroxide under aerobic growth conditions and represents an enzymatic defense available against H2O2-mediated oxidative stress. Second, the enzyme presents an additional mechanism for regeneration of the NAD+ essential to the strictly fermentative metabolism of this organism. The enzyme may also protect against exogenous H2O2 and contribute to bacterial virulence. The actual function of NADH peroxidases and oxidases in plants is still unclear, but they could act in early signaling of oxidative stress through producing H2O2. An alternative role may include regulation of H2O2 formation by NADH peroxidase and oxidase in cell wall loosening and reconstruction. References EC 1.11.1 NADPH-dependent enzymes NADH-dependent enzymes Flavoproteins Enzymes of known structure