Oxoglutarate dehydrogenase complex
The α -ketoglutarate dehydrogenase complex ( OGDC ) is a protein complex in eukaryotes and some bacteria, which catalyzes the oxidative decarboxylation of α -ketoglutarate, and the subsequent reaction with coenzyme A to succinyl -CoA. This reaction is part of the citric acid cycle. The overall reaction is:
Construction
The α -ketoglutarate dehydrogenase complex, three enzymes are involved: the α -ketoglutarate dehydrogenase E1 subunits (A, EC 1.2.4.2 ), the dihydrolipoamide succinyltransferase (B, EC 2.3.1.61 ), and dihydrolipoamide dehydrogenase, the (C -E, EC 1.8.1.4 ). Therefore, the complex has a similar structure as the pyruvate dehydrogenase complex (PDC).
The α -ketoglutarate dehydrogenase, together with the pyruvate dehydrogenase and branched -chain α - ketoacid dehydrogenase multienzyme complexes of a family of so-called α - ketoacid dehydrogenases.
In active multienzyme complex the succinyltransferase is too high symmetric multimers aggregated ( Tetracosamer the point group 432, Hexacontamer the point group 532 ) to which the various dehydrogenases are bound.
In eukaryotes, a single variant of the complex was identified in mitochondria.
Reaction
OGDC and PDC use the same coenzymes and also catalyze an analogous reaction, and the reaction mechanism is very similar in both complexes. α -ketoglutarate is oxidatively decarboxylated to succinyl -CoA and causes NADH. The citric acid cycle that is fed into the respiratory chain and is used for energy production.
Formation of succinate in prokaryotes
Changes in citric acid cycle pathways in which a part is missing step in bacteria are the norm ( 13 of 17 examined ). The missing step can be replaced by other reaction steps, or not. In fact, only three kinds of bacteria with enzymes ketoglutarate dehydrogenase activity ( KDH) known: Euglena gracilis, Bacillus japonicum and Escherichia coli - The bacterium Escherichia coli travels under aerobic conditions, the complete citrate as described.. Under anaerobic conditions, it is able to disable the KDH. The metabolic pathways that previously formed a circle are now connected tree -like structure. M. tuberculosis, however, can switch between two different Citratzyklen, both of which are different from the eukaryotic pathway.
Archaea, but also some bacteria, such as Helicobacter pylori, which grows under microaerophilic conditions, catalyze the conversion of α -ketoglutarate to succinyl -CoA by means of an oxidation-sensitive 2- oxoglutarate: ferredoxin oxidoreductase ( ogor, EC 1.2.7.3 ). In contrast to this OGDC containing iron -sulfur cluster; it lacks the flavin and the Liponsäureamid. Instead of ferredoxin NADH is used as a reducing agent. Mycobacterium tuberculosis also contains a CoA - dependent enzyme, which is, however, stable even under aerobic conditions.
With various mycobacteria ( including Mycobacterium tuberculosis ), the E1 subunit of ketoglutarate dehydrogenase is replaced by a ketoglutarate decarboxylase which initially produced independently of coenzyme A succinate semialdehyde, which is dehydrated by an NADP -dependent succinate semialdehyde dehydrogenase for succinate will.