Substrate chain phosphorylation

In substrate chain phosphorylation, also known as substrate stage phosphorylation or substrate phosphorylation , adenosine triphosphate (ATP) or guanosine triphosphate (GTP), which functions as an indispensable energy carrier for cell metabolism in all organisms, is obtained outside of electron transport phosphorylation . This happens during the oxidative breakdown of organic compounds in living cells, as a result of which the phosphate residue of phosphorylated intermediate products is transferred to adenosine diphosphate (ADP) or guanosine diphosphate (GDP). The transfer is enzymatically catalyzed. During this process, the phosphorylated intermediate must have a higher group transfer potential than ATP.

process

Very general representation of the substrate chain phosphorylation in which ATP is formed.

In substrate chain phosphorylation, a phosphorylated intermediate transfers its phosphoryl group (inorganic phosphate ) to ADP. Since the group transfer potential of ATP is lower than that of the intermediate, the reaction takes place in one direction.

The substrate chain phosphorylation is often preceded by an oxidation step. When an aldehyde group is oxidized to a carboxylic acid, the energy released during this process is used by combining a free phosphate group with the carboxylic acid. A phosphorus anhydride is formed , a compound with a high group transfer potential. Alternatively, a molecule with a keto group , e.g. B. pyruvate , are oxidatively decarboxylated . The oxidation energy is conserved by forming a thioester bond with coenzyme A. After transesterification with a phosphate group, a phosphorylated compound with a sufficiently high group transfer potential for substrate chain phosphorylation is again formed.

meaning

The substrate chain phosphorylation is used for the rapid production of ATP regardless of whether external electron acceptors are available for respiratory processes . In humans, for example, this is the case in specialized cells such as erythrocytes , which are unable to perform aerobic respiration and derive their energy exclusively from substrate chain phosphorylation in glycolysis . The muscles can also only obtain their energy in this way if there is insufficient oxygen supply. Nevertheless, more ATP is usually obtained in aerobic or anaerobic respiration. Plants can produce ATP as part of photosynthesis .

Without substrate chain phosphorylation, part of the energy released during the conversion or oxidation of high-energy compounds would simply fizzle out into heat and thus be lost.

Occurrence

The following reactions are part of the substrate chain phosphorylation:

• in the amortization phase of glycolysis:

${\displaystyle \mathrm {{\text{1,3-Bisphosphoglycerat}}+ADP\longrightarrow {\text{3-Phosphoglycerat}}+ATP} }$

(involved enzyme : phosphoglycerate kinase )

${\displaystyle \mathrm {Phosphoenolpyruvat+ADP\longrightarrow Pyruvat+ATP} }$
(involved enzyme : pyruvate kinase )

• in the citric acid cycle :

${\displaystyle \mathrm {{\text{Succinylphosphat}}+GDP\longrightarrow Succinat+GTP} }$

(involved enzyme: succinyl-CoA synthetase )

In some organisms (e.g. plants) ATP can also be generated at this point.

• in the course of fermentations , for example

in propionic acid fermentation and heterofermentative lactic acid fermentation :

${\displaystyle \mathrm {Acetylphosphat+ADP\longrightarrow Acetat+ATP} }$

(involved enzyme: acetate kinase )

in butyric acid fermentation :

${\displaystyle \mathrm {Butyrylphosphat+ADP\longrightarrow Butyrat+ATP} }$

(involved enzyme: butyrate kinase)

Individual evidence

1. ↑ Georg Fuchs (Ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart, 8th edition 2007, ISBN 3-13-444608-1 , p. 200.
2. ↑ Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: Biochemistry . 6 edition. Spectrum Academic Publishing House, Heidelberg 2007; ISBN 978-3-8274-1800-5 ; P. 543.