Oxidative phosphorylation is the process by which ATP is formed as electrons are transferred from NADH or FADH2 to molecular oxygen (O2) by a series of electron carriers. The energy released form the oxidation of glucose, fatty acids, and amino acids is stored as the reduced coenzymes NADH or FADH2. There is a step by step transfer of electrons from NADH or FADH2 to specific protein complexes, which are part of the electron transport chain.

The ultimate acceptor of these electrons is O2. The electron-transport chain, also known as respiratory chain, is a series of linked electron carriers that transfer electrons from NADH and FADH2 to molecular oxygen (O2). The respiratory chain is found in the inner mitochondrial membrane and is composed of 3 enzyme complexes and 2 mobile carriers, also known as respiratory assemblies. Respiratory assemblies are enzyme complexes of acceptor proteins, coenzymes, and metal ions, are located in the inner mitochondrial membrane. The respiratory assemblies are made up of 3 enzyme complexes, which are the sites of the proton pumps.

Enzyme Complexes Mobile carriers
NADH-Q reductase Q (ubiquinone)
Cytochrome reductase Cytochrome c
Cytochrome oxidase  

In addition to the 3 enzyme complexes above (NADH-Q reductase, cytochrome reductase, and cytochrome oxidase), all of which span the inner mitochondrial membrane and are proton pumps, a fourth complex succinate-Q reductase is positioned between NADH-Q reductase and cytochrome reductase. This complex is found on the matrix side of the inner mitochondrial membrane does not span the membrane.   Succinate-Q reductase is the point of entry into the respiratory chain for electrons from FADH2. As indicated in the following table, all of these enzymes are large, multisubunit complexes containing several prosthetic groups. The prosthetic groups are the molecules that actually carry the electrons within the complex.

Complex number

Enzyme complex

Molecular weight (x 10-3)

# of subunits

I NADH-Q reductase 880,000 >34
II Succinate-Q reductase 140,000 4
III Cytochrome reductase 250,000 10
IV Cytochrome oxidase 160,000 10

The prosthetic groups are the units involved in the actual transfer of electrons within each complex and from one complex to another and are classified as 1) iron sulfur proteins (Fe-S),   2) hemes,   3) copper ions,    and  4) flavins.

All of them serve to carry electrons but each enzyme complex is associated with specific prosthetic groups. 

Complex number

Enzyme complex

Prosthetic groups

I NADH-Q reductase FMN, Fe-S
II Succinate-Q reductase FAD, Fe-S
III Cytochrome reductase Heme b-562, Heme b-566, Heme c1, Fe-S
IV Cytochrome oxidase Heme a, Heme a3, CuA, CuB

The following diagram illustrates the linear sequence of the 4 enzyme complexes in the inner mitochondrial membrane and shows the flow of electrons within this respiratory chain and their blockage by specific poisons and/or antibiotics.

An important feature to remember is that the processes of electron transport and ATP synthesis are distinct from each other and occur in different parts of the mitochondrial membranes; but they are coupled processes.

The structure of the mitochondria is a very important feature of the oxidative phosphorylation process. See the diagrams in your textbook to review the important properties of the mitochondria with respect to oxidative phosphorylation. It is very important to remember that while the outer mitochondrial membrane is permable to most molecules of 5,000 daltons MW, the inner mitochondrial membrane is impermeable to ions and small molecules.

This structure is very important because the process of electron transport establishes a proton gradient across the mitochondrial membrane, which is then used to synthesize ATP. If the inner mitochondrial membrane was not impermeable to protons, no gradient could be established.


  1. The transport of electrons from NADH and FADH2 provide "energy" for synthesis of ATP
  2. Electrons from NADH & FADH2 are passed thru a series of electron transport complexes
  3. Oxidation/reduction reactions are coupled
  4. These electrons are passed to molecular oxygen as the ultimate acceptor
  5. The integrity of the mitochondrial structure is essential for oxidative phosphorylation
  6. Four enzyme complexes and 2 mobile carriers comprise the respiratory chain
  7. NADH & FADH2 used for electron transport result from oxidation of foods, mainly glucose
  8. As the electrons are transferred, protons are pumped into the inter-membrane space of the mitochondria resulting in the formation of a proton gradient.
  9. The chemiosmotic hypothesis of Peter Mitchell states that a proton-motive force was responsible for driving the synthesis of ATP
  10. Much experimental evidence supports the pumping of protons by the respiratory chain complexes as the primary energy conserving event of oxidative phosphorylation


Note:    a new value for ATP per NADH is 2.5   &   ATP per FADH2 is 1.5.
You may see the numbers ATP per NADH = 3   and   ATP per FADH 2 = 2 in some texts. For more information on the re-evaluation of the number of ATP's/nucleotide coenzyme see Hinkle, et al.  Mechanistic stoichiometry of mitochondrial oxidative phosphorylation. Biochemistry 30:3576-82, 1991.

The different values of 30 or 32 ATP/glucose via NADH/FADH  depend on the method used to transport cytoplasmic NADH, formed by glycolysis, into the mitochondria, i.e. the shuttles.

    cm 2002