Succinate dehydrogenase

The enzyme succinate dehydrogenase (SDH), more succinate: ubiquinone oxidoreductase ( systematic name ), also known as complex II, is an enzyme complex that is composed of four subunits. It is the only membrane-bound protein of the citric acid cycle and integrated as complex II of the respiratory chain directly into the electron transport chain of the inner mitochondrial membrane. The enzyme catalyzes

The oxidation of succinate to fumarate with FAD as the oxidant:

Simultaneous transfer of two electrons across the membrane boundary:

Using this electron reduction of ubiquinone (coenzyme Q) to ubiquinol ( EC ):

Mutations in one of four genes coding can humans responsible for inherited metabolic diseases (see table).


The protein consists of four human protein subunits (see Table ).

An evolutionary standpoint, the flavoprotein subunit is the oldest and homologous proteins are therefore already be found in many bacteria ( EC ). With the evolution of eukaryotes, the iron - sulfur protein was added, and the vertebrates, and the then -added cytochrome complex in the membrane was anchored.


The enzyme catalyzes the oxidation of succinate to fumarate and the reduction of ubiquinone (coenzyme Q) to ubiquinol. The resulting as a hydrogen carrier in this oxidation in the citric acid cycle FADH2 occurs - in contrast to NADH - not free to, but is bound as a prosthetic group to the SDHA subunit of the enzyme complex. His re-oxidation takes place by a chain of Einelektronenübertragungen starting with the three iron-sulfur clusters Fe2S2, Fe4S4, Fe3S4 the SDHB subunit toward the cytochrome B560 ( in mitochondria ) and cytochrome b556 ( in bacteria) of the subunits SDHC and SDHD. This eventually reduces the quinoid - thus oxidized - form of Coenzyme Q ( ubiquinone ), the phenolic form, ubiquinol.

Since the iron complexes involved are only capable Einelektronenübertragungen, carried the reoxidation of FADH2 to FAD or the reduction of ubiquinone, which provide or require two electrons in two stages via radical, but stable, semiquinoid intermediates. The binding site for succinate is located in the subunit SDHA; for ubiquinone is formed by the three subunits SDHB, SDHC, and SDHD. Ubiquinone / ubiquinol is, due to its lipophilic polyisoprenyl chain membranlöslich. Therefore, it serves as a mobile electron beam from the complex II in the complex III of the respiratory chain.

The redox chain of succinate dehydrogenase:

The accounting for this step of the citric acid cycle thus results in a transfer of two electrons from succinate to ubiquinone. A proton transfer does not take place, however, by this enzyme.


  • Malonate