The NADH dehydrogenase (EC 1.6.5.3)) complex, also called complex I, or NADH coenzyme Q oxidoreductase, is the first complex in the electron transfer chain of mitochondria and catalyzes the transfer of electrons from NADH to coenzyme Q (CoQ):

NADH + CoQ + H⁺ → NAD⁺ + CoQH₂

In the process of reducing coenzyme Q, this complex also translocates protons, helping to provide the electrochemical potential used to produce ATP. It is a large complex, the mammalian enzyme containing 46 separate protein subunits, as well as a flavin prosthetic group and eight iron-sulfur clusters.

Seven of the 46 mammalian protein subunits are encoded in the mitochondrial DNA. Defects in the subunits of Complex I are associated with various mitochondrial diseases.

More information can be found on the Complex I home page.