Phosphofructokinase (PFK) is an enzyme that catalyzes a substrate level phosphorylation reaction.

Substrate-level phosphorylation

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Substrate-level phosphorylation exemplified with the conversion of ADP for ATP

Substrate-level phosphorylation is a metabolic reaction that results in the production of ATP or GTP by the transfer of a phosphate group of a substrate directly to ADP or GDP. Transfer of a higher energy product (whether or not a bound phosphate group) to a lower energy product. This process uses some of the released files chemical energyThe Gibbs free energyto transfer a phosphoryl (DUST3) to ADP or GDP from another phosphorylated compound. Occurs in glycolysis and the citric acid cycle.

Different oxidative phosphorylationoxidation and phosphorylation are not coupled in the phosphorylation process at the substrate level, and reactive intermediates are more often obtained in the course of phosphorylation. oxidation processes in catabolism. Most ATP is generated by oxidative phosphorylation in aerobic or anaerobic respiration while substrate-level phosphorylation provides a faster and less efficient source of ATP, independent of electron acceptors. It is the case of human erythrocytesthat does not have mitochondriaand in oxygen-depleted muscle.

Overview

Adenosine triphosphate is an important “energy currency” of the cell. The high-energy bonds between phosphate groups can be broken to power a variety of reactions used in all aspects of cell function.

Substrate-level phosphorylation occurs in the cytoplasm of cells during glycolysis and mitochondria during krebs cycle or by MTHFD1L (EC 6.3.4.3), an enzyme that interconverts ADP + phosphate + 10-formyltetrahydrofolate to ATP + formate + tetrahydrofolate (reversibly), under both aerobic and anaerobic conditions. At the Glycolysis compensation phasea network of 2 ATP is produced by phosphorylation at the substrate level.

glycolysis

Main article: glycolysis

The first substrate-level phosphorylation occurs after the conversion of 3-phosphoglyceraldehyde and Pi and NAD+ to 1,3-bisphosphoglycerate via glyceraldehyde 3-phosphate dehydrogenase. 1,3-bisphosphoglycerate is then dephosphorylated via phosphoglycerate kinaseproducing 3-phosphoglycerate and ATP through phosphorylation at the substrate level.

The second phosphorylation at the substrate level occurs by dephosphorylation phosphoenolpyruvatecatalyzed by pyruvate kinaseproducing pyruvate and ATP.

During the preparatory phase, each 6-carbon glucose molecule is broken into two 3-carbon molecules. Thus, in glycolysis, dephosphorylation results in the production of 4 ATP. However, the previous preparatory phase consumes 2 ATP, so the net yield in glycolysis is 2 ATP. 2 molecules of NADH are also produced and can be used in oxidative phosphorylation to generate more ATP.

mitochondria

ATP can be generated by substrate-level phosphorylation in mitochondria on a road independent of proton driving force. At the headquarters There are three reactions capable of substrate-level phosphorylation using phosphoenolpyruvate carboxykinase or succinate-CoA ligaseor Monofunctional C1-tetrahydrofolate synthase.

Phosphoenolpyruvate carboxykinase

It is believed that mitochondrial phosphoenolpyruvate carboxykinase participates in the transfer of phosphorylation potential from the matrix to the cytosol and vice versa. However, it is strongly favored for GTP hydrolysis, so it is not really considered an important source of intra-mitochondrial phosphorylation at the substrate level.

Succinate-CoA ligase

Succinate-CoA ligase is a heterodimer composed of an invariant α subunit and a substrate-specific ß subunit encoded by SUCLA2 or SUCLG2. This combination results in a ADP-forming succinate-CoA ligase (A-SUCL, EC 6.2.1.5) or a GDP-forming succinate-CoA ligase (G-SUCL, EC 6.2.1.4). ADP-forming succinate-CoA ligase is potentially the only matrix enzyme that generates ATP in the absence of a proton driving force, capable of maintaining matrix ATP levels under energy-limited conditions such as transients. hypoxia.

Monofunctional C1-tetrahydrofolate synthase

This enzyme is encoded by MTHFD1L and reversibly interconverts ADP + phosphate + 10-formyltetrahydrofolate to ATP + formate + tetrahydrofolate.

Other mechanisms

By working the skeletal muscles and the brain, Phosphocreatine is stored as a readily available supply of high-energy phosphate, and the enzyme creatine phosphokinase transfers a phosphate from phosphocreatine to ADP to produce ATP. Then ATP releases giving chemical energy. This is sometimes mistakenly thought of as substrate-level phosphorylation, although it is a transphosphorylation.

Importance of substrate-level phosphorylation in anoxia

During anoxiaThe provision of ATP by substrate-level phosphorylation in the matrix is ​​important not only as a mere means of energy, but also to prevent mitochondria from overtaxing glycolytic stores of ATP, maintaining the adenine nucleotide translocator in ‘direct mode’ transporting ATP into the cytosol.

oxidative phosphorylation

An alternative method used to create ATP is through oxidative phosphorylationthat occurs during cellular respiration. This process uses the oxidation of NADH for NAD+yielding 3 ATP, and from FADHtwo to FAD, yielding 2 ATP. The potential energy stored as a electrochemical gradient of protons (H+) across the inner mitochondrial membrane is required to generate ATP from ADP and PI (inorganic phosphate molecule), a fundamental difference from phosphorylation at the substrate level. This gradient is exploited by ATP synthase acting as a pore, allowing H+ from the mitochondrial intermembrane space to move down its electrochemical gradient in the matrix and couple free energy release to ATP synthesis. Electron transfer, on the other hand, provides the energy needed to actively pump H+ outside the matrix.

References


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