Nadh‑ubiquinone Oxidoreductase Complex I

NADH‑ubiquinone oxidoreductase, commonly called Complex I, is a massive protein assembly that sits in the inner membrane of mitochondria and in the equivalent membranes of many bacteria. Its primary job is to take electrons from NADH, pass them through a series of iron‑sulfur clusters, and hand them off to ubiquinone, a small lipid‑soluble carrier. As this electron flow occurs, Complex I changes shape in a way that pushes protons across the membrane, building up an electrochemical gradient that later powers the synthesis of ATP, the cell’s main energy currency.

The importance of Complex I lies in its role as the first and largest entry point for reducing power into the respiratory chain. By linking the oxidation of nutrients to a proton motive force, it determines how efficiently cells can harvest energy from oxygen. Mutations or damage to any of its many subunits can cripple cellular respiration and are linked to a range of mitochondrial diseases and neurodegenerative conditions.

You will find Complex I wherever living systems depend on oxidative phosphorylation: in the mitochondria of animal, plant and fungal cells, as well as in the membranes of aerobic bacteria that use similar respiratory chains. Its conserved architecture makes it a central object of study for biochemists, structural biologists, and anyone interested in how organisms turn chemical fuel into usable energy.