Respiratory complex I, EC 7.1.1.2 (also known as NADH:ubiquinone oxidoreductase, Type I NADH dehydrogenase and mitochondrial complex I) is the first large protein complex of the respiratory chains of many organisms from bacteria to humans. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and translocates protons across the inner mitochondrial membrane in eukaryotes or the plasma membrane of bacteria.

This enzyme is essential for the normal functioning of cells, and mutations in its subunits lead to a wide range of inherited neuromuscular and metabolic disorders. Defects in this enzyme are responsible for the development of several pathological processes such as ischemia/reperfusion damage (stroke and cardiac infarction), Parkinson's disease and others.^{[citation needed]}

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), Coenzyme Q – cytochrome c reductase (complex III), and cytochrome c oxidase (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.

The reaction can be reversed – referred to as aerobic succinate-supported NAD⁺ reduction by ubiquinol – in the presence of a high membrane potential, but the exact catalytic mechanism remains unknown. Driving force of this reaction is a potential across the membrane which can be maintained either by ATP-hydrolysis or by complexes III and IV during succinate oxidation.

Complex I may have a role in triggering apoptosis. In fact, there has been shown to be a correlation between mitochondrial activities and programmed cell death (PCD) during somatic embryo development.

Complex I is not homologous to Na⁺-translocating NADH Dehydrogenase (NDH) Family (TC# 3.D.1), a member of the Na⁺ transporting Mrp superfamily.