Substrate-level phosphorylation
Mentioned in the substrate chain, also Substratstufenphosphorylierung or substrate phosphorylation, which in all organisms, functioning as an indispensable energy carrier of the cell metabolism adenosine triphosphate ( ATP) or guanosine triphosphate (GTP ) is obtained, outside the Elektronentransportphosphorylierung. This is done in the oxidative degradation of organic compounds of living cells, phosphorylated in consequence of the phosphate radical intermediates to adenosine diphosphate ( ADP) or guanosine diphosphate (GDP ) is transmitted. It is thereby no activation energy is required.
In this process, the phosphorylated intermediate has a higher group transfer potential than ATP own.
Process
In the substrate chain phosphorylated intermediate transfers its phosphoryl group (inorganic phosphate) to ADP. Since the group transfer potential of ATP is less than that of the intermediate, the reaction takes place in one direction.
In the substrate chain an oxidation step is often installed upstream. If an aldehyde is oxidized to the carboxylic acid, which is released during this process, energy is used by a free phosphate group is esterified with the carboxylic acid. The result is a phosphoric anhydride, a compound with high group transfer potential. Alternatively, a molecule with a ketone, such as pyruvate, is oxidatively decarboxylated. Here, the energy of oxidation is conserved by a thioester bond with coenzyme A is formed. After transesterification with a phosphate group, in turn, results in a phosphorylated compound with a sufficiently high group transfer potential for substrate chain.
Importance
The substrate chain is used for rapid recovery of ATP regardless of whether external electron acceptors for respiration processes are available. In humans, this is for example the case in specialized cells such as erythrocytes, which can not operate an aerobic respiration and their energy solely from substrate chain phosphorylation in glycolysis. The muscles can win at a lower supply of oxygen their energy only in this way. However, in general, more ATP is produced in aerobic and anaerobic respiration. Plants can produce during photosynthesis ATP.
Without substrate chain part of the energy released in the reaction or oxidation of energy -rich compounds would simply fizzle into heat and thus lost.
Occurrence
The following reactions belong to the substrate chain:
- In the Amortisierungsphase of glycolysis:
- In the citric acid cycle:
- In the course of fermentation, for example,