NADH dehydrogenase (EC 184.108.40.206) (also referred to as "NADH:quinone reductase" or "Complex I") is an enzyme located in the inner mitochondrial membrane that catalyzes the transfer of electrons from NADH to coenzyme Q (CoQ). It is the "entry enzyme" of oxidative phosphorylation in the mitochondria. 
NADH Dehydrogenase is the first enzyme (Complex I) of the mitochondrial electron transport chain. There are three energy-transducing enzymes in the electron transport chain - NADH dehydrogenase (Complex I), Coenzyme Q – cytochrome c reductase (Complex III), and cytochrome c oxidase (Complex IV).  NADH dehydrogenase is the largest and most complicated enzyme of the electron transport chain..
The reaction of NADH dehydrogenase is:
In this process, the complex translocates four protons across the inner membrane per molecule of oxidized NADH, helping to build the electrochemical potential used to produce ATP.
The reaction can be reversed - referred to as aerobic succinate-supported NAD+ reduction - in the presence of a high membrane potential, but the exact catalytic mechanism remains unknown. 
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. 
All redox reactions take place in the extramembranous portion of NADH dehydrogenase. NADH initially binds to NADH dehydrogenase, and transfers two electrons to the flavin mononucleotide (FMN) prosthetic group of complex I, creating FMNH2. The electron acceptor - the isoalloxazine ring - of FMN is identical to that of FAD. The electrons are then transferred through the second prosthetic group of NADH dehydrogenase via a series of iron-sulfur (Fe-S) clusters, and finally to coenzyme Q (ubiquinone). This electron flow causes four hydrogen ions to be pumped out of the mitochondrial matrix. Ubiquinone (CoQ) accepts two electrons to be reduced to ubiquionol (CoQH2). 
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