Complex I (EC 1.6.5.3) (also referred to as NADH dehydrogenase or, especially in the context of the human protein, NADH dehydrogenase) is an enzyme of the respiratory chains of myriad organisms from bacteria to humans that falls under the H⁺ or Na⁺-translocating NADH Dehydrogenase (NDH) Family (TC# 3.D.1), a member of the Na⁺ transporting Mrp superfamily. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and, in eukaryotes, it is located in the inner membrane. NADH:ubiquinone oxidoreductases type I of bacteria and of eukaryotic mitochondria and chloroplasts couple electron transfer to the electrogenic transport of protons or Na⁺. It is one of the "entry enzymes" of cellular respiration or oxidative phosphorylation in the mitochondria.

Complex I is the first enzyme of the mitochondrial electron transport chain. There are three energy-transducing enzymes in the electron transport chain - NADH:ubiquinone oxidoreductase (complex I), (complex III), and (complex IV). Complex I is the largest and most complicated enzyme of the electron transport chain.

The reaction catalyzed by complex I is:

In this process, the complex translocates four protons across the inner membrane per molecule of oxidized NADH, helping to build the electrochemical potential difference used to produce ATP. Escherichia coli complex I (NADH dehydrogenase) is capable of proton translocation in the same direction to the established Δψ, showing that in the tested conditions, the coupling ion is H⁺. Na⁺ transport in the opposite direction was observed, and although Na⁺ was not necessary for the catalytic or proton transport activities, its presence increased the latter. H⁺ was translocated by the Paracoccus denitrificans complex I, but in this case, H⁺ transport was not influenced by Na⁺, and Na⁺ transport was not observed. Possibly, the E. coli complex I has two energy coupling sites (one Na⁺ independent and the other Na⁺dependent), as observed for the Rhodothermus marinus complex I, whereas the coupling mechanism of the P. denitrificans enzyme is completely Na⁺ independent. It is also possible that another transporter catalyzes the uptake of Na⁺. Complex I energy transduction by proton pumping may not be exclusive to the R. marinus enzyme. The Na⁺/H⁺ antiport activity seems not to be a general property of complex I. However, the existence of Na⁺-translocating activity of the complex I is still in question.

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