Phosphorylation

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Serine in an amino acid chain, before and after phosphorylation.

In biochemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion.[1] This process and its inverse, dephosphorylation, are common in biology.[2] Protein phosphorylation often activates (or deactivates) many enzymes.[3][4]

During respiration and photosynthesis

Phosphorylation is essential to the processes of both

aerobic respiration, which involve the production of adenosine triphosphate (ATP), the "high-energy" exchange medium in the cell. During aerobic respiration, ATP is synthesized in the mitochondrion by addition of a third phosphate group to adenosine diphosphate (ADP) in a process referred to as oxidative phosphorylation. ATP is also synthesized by substrate-level phosphorylation during glycolysis. ATP is synthesized at the expense of solar energy by photophosphorylation in the chloroplasts
of plant cells.

Phosphorylation of glucose

Glucose metabolism

Phosphorylation of sugars is often the first stage in their catabolism. Phosphorylation allows cells to accumulate sugars because the phosphate group prevents the molecules from diffusing back across their transporter. Phosphorylation of glucose is a key reaction in sugar metabolism. The chemical equation for the conversion of D-glucose to D-glucose-6-phosphate in the first step of glycolysis is given by:

D-glucose + ATP → D-glucose 6-phosphate
+ ADP
ΔG° = −16.7 kJ/mol (° indicates measurement at standard condition)

Glycolysis

Main article: Glycolysis

Glycolysis is an essential process of glucose degrading into two molecules of pyruvate, through various steps, with the help of different enzymes. It occurs in ten steps and proves that phosphorylation is a much required and necessary step to attain the end products. Phosphorylation initiates the reaction in step 1 of the preparatory step[5] (first half of glycolysis), and initiates step 6 of payoff phase (second phase of glycolysis).[6]

Glucose, by nature, is a small molecule with the ability to diffuse in and out of the cell. By phosphorylating glucose (adding a phosphoryl group in order to create a negatively charged phosphate group[7]), glucose is converted to glucose-6-phosphate, which is trapped within the cell as the cell membrane is negatively charged. This reaction occurs due to the enzyme hexokinase, an enzyme that helps phosphorylate many six-membered ring structures. Phosphorylation takes place in step 3, where fructose-6-phosphate is converted to fructose 1,6-bisphosphate. This reaction is catalyzed by phosphofructokinase.

While phosphorylation is performed by ATPs during preparatory steps, phosphorylation during payoff phase is maintained by inorganic phosphate. Each molecule of

glyceraldehyde-3-phosphate dehydrogenase
(GAPDH). The cascade effect of phosphorylation eventually causes instability and allows enzymes to open the carbon bonds in glucose.

Phosphorylation functions is an extremely vital component of glycolysis, as it helps in transport, control, and efficiency.[8]

Glycogen synthesis

adipose) tissue. Glucose 6-phosphate has role in regulating glycogen synthase
.

High blood glucose releases insulin, stimulating the translocation of specific glucose transporters to the cell membrane; glucose is phosphorylated to glucose 6-phosphate during transport across the membrane by ATP-D-glucose 6-phosphotransferase and non-specific hexokinase (ATP-D-hexose 6-phosphotransferase).[9][10] Liver cells are freely permeable to glucose, and the initial rate of phosphorylation of glucose is the rate-limiting step in glucose metabolism by the liver.[9]

The liver's crucial role in controlling blood sugar concentrations by breaking down glucose into carbon dioxide and glycogen is characterized by the negative

Michaelis constant
(Km), indicating a high affinity for glucose, so this initial phosphorylation can proceed even when glucose levels at nanoscopic scale within the blood.

The phosphorylation of glucose can be enhanced by the binding of fructose 6-phosphate (F6P), and lessened by the binding fructose 1-phosphate (F1P). Fructose consumed in the diet is converted to F1P in the liver. This negates the action of F6P on glucokinase,[11] which ultimately favors the forward reaction. The capacity of liver cells to phosphorylate fructose exceeds capacity to metabolize fructose-1-phosphate. Consuming excess fructose ultimately results in an imbalance in liver metabolism, which indirectly exhausts the liver cell's supply of ATP.[12]

Allosteric activation by glucose 6-phosphate, which acts as an effector, stimulates glycogen synthase, and glucose 6 phosphate may inhibit the phosphorylation of glycogen synthase by cyclic AMP-stimulated protein kinase.[10]

Other processes

Phosphorylation of glucose is imperative in processes within the body. For example, phosphorylating glucose is necessary for insulin-dependent

mechanistic target of rapamycin pathway activity within the heart. This further suggests a link between intermediary metabolism and cardiac growth.[13]

Protein phosphorylation

Main article: Protein phosphorylation

post-translational modification in eukaryotes. Phosphorylation can occur on serine, threonine and tyrosine side chains (often called 'residues') through phosphoester bond formation, on histidine, lysine and arginine through phosphoramidate bonds, and on aspartic acid and glutamic acid through mixed anhydride linkages. Recent evidence confirms widespread histidine phosphorylation at both the 1 and 3 N-atoms of the imidazole ring.[14][15] Recent work demonstrates widespread human protein phosphorylation on multiple non-canonical amino acids, including motifs containing phosphorylated histidine, aspartate, glutamate, cysteine, arginine and lysine in HeLa cell extracts.[16] However, due to the chemical lability of these phosphorylated residues, and in marked contrast to Ser, Thr and Tyr phosphorylation, the analysis of phosphorylated histidine (and other non-canonical amino acids) using standard biochemical and mass spectrometric approaches is much more challenging[16][17][18] and special procedures and separation techniques are required for their preservation alongside classical Ser, Thr and Tyr phosphorylation.[19]

The prominent role of protein phosphorylation in biochemistry is illustrated by the huge body of studies published on the subject (as of March 2015, the MEDLINE database returns over 240,000 articles, mostly on protein phosphorylation).

See also

References

  1. ISBN 978-1-947172-04-3. Archived
    from the original on 2023-03-31. Retrieved 16 April 2023.
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  5. ^ Chapter 14: Glycolysis and the Catabolism of Hexoses. Archived from the original on 2021-10-17. Retrieved 2016-05-14.
  6. ^ Garrett R (1995). Biochemistry. Saunders College.
  7. ^ "Hexokinase - Reaction". www.chem.uwec.edu. Archived from the original on 2020-12-02. Retrieved 2020-07-29.
  8. ^ Maber J. "Introduction to Glycolysis". Archived from the original on 6 April 2017. Retrieved 18 November 2017.
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  11. PMID 5834248
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  12. ^ "Regulation of Glycolysis". cmgm.stanford.edu. Archived from the original on 2009-03-03. Retrieved 2017-11-18.
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  19. bioRxiv 10.1101/202820
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External links
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