In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor.
This equation can be written in several ways that are nearly equivalent that describe the behaviors of various protonated states of ATP, ADP, and the phosphorylated product.
As is clear from the equation, a phosphate group per se is not transferred, but a phosphoryl group (PO
3-).
Phosphoryl is an
electrophile.
This process and its inverse, dephosphorylation, are common in
biology.
[ Text was copied from this source, which is available under a .] Protein phosphorylation often activates (or deactivates) many
.
During respiration
Phosphorylation is essential to the processes of both anaerobic and 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
of plant cells.
Phosphorylation of glucose
Glucose metabolism
Phosphorylation of
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
Glycolysis is an essential process of glucose degrading into two molecules of
Pyruvic acid, 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
(first half of glycolysis), and initiates step 6 of payoff phase (second phase of glycolysis).
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
While phosphorylation is performed by ATPs during preparatory steps, phosphorylation during payoff phase is maintained by inorganic phosphate. Each molecule of glyceraldehyde 3-phosphate is phosphorylated to form 1,3-bisphosphoglycerate. This reaction is catalyzed by 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.
Glycogen synthesis
Glycogen is a long-term store of glucose produced by the cells of the
liver. In the
liver, the synthesis of
glycogen is directly correlated with blood glucose concentration. High blood glucose concentration causes an increase in intracellular levels of glucose 6-phosphate in the liver,
skeletal muscle, and fat (
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).
The liver's crucial role in controlling blood sugar concentrations by breaking down glucose into carbon dioxide and glycogen is characterized by the negative Gibbs free energy (ΔG) value, which indicates that this is a point of regulation with. The hexokinase enzyme has a low Michaelis constant (K), 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,
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.
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.
Protein phosphorylation
Protein phosphorylation is the most abundant post-translational modification in eukaryotes. Phosphorylation can occur on
serine,
threonine and
tyrosine side chains (in other words, on their residues) through phosphoester bond formation, on
histidine,
lysine and
arginine through
Phosphoramidate, 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.
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.
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
and special procedures and separation techniques are required for their preservation alongside classical Ser, Thr and Tyr phosphorylation.
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).
Further reading
See also
-
Moiety conservation
-
Phosida
-
Phosphoamino acid analysis
-
Phospho3D
External links
