Krebs cycle step 5

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Step 5 of the Krebs cycle: Succinyl-CoA Synthetase

Image:Succinyloca_2.jpg

Figure: Formation of Succinate

The reaction of succinyl-CoA to succinate (see figure) proceeds via the intermediate succinyl phosphate. In the first step, the coenzyme A (green), which is located at the succinyl group, is substituted by a hydrogen phosphate ion (yellow). Succinyl phosphate remains bound in the enzyme an dis therefore not released. In the second step, succinyl phosphate transfers its phosphoric acid residue on GDP (guanosine diphosphate), so that the energy-rich compound GTP (guanosin triphosphate) and succinate produced.

GTP and ATP can be converted into each other by the enzyme nucleosiddiphosphatkinase: GTP + ADP → GDP + ATP. This enzyme transfers the phosphate group from GTP to ADP.

This reaction, which is called the substrate chain, is the only one in the Krebs cycle where direct energy is obtained as GTP.

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Verena Pietzner, Jaime Prilusky

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