Phosphorylation
Phosphorylation is the process of adding a phosphate group (PO4)3− to a molecule. In biology, phosphorylation and its counterpart, dephosphorylation, are critical for many cellular processes. A large fraction of proteins are at least temporarily phosphorylated, as are many sugars, lipids, and other molecules. Phosphorylation is especially important for protein function as this modification activates (or deactivates) many enzymes, thereby regulating their function. Protein phosphorylation is one type of post-translational modification.
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).
Phosphorylation of sugars is often the first stage of their catabolism. It 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 because many sugars are first converted to glucose before they are metabolized further.
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.
Research D. G. Walker of the University of Birmingham determined the presence of two specific enzymes in adult guinea pig liver, both of which catalyze the phosphorylation of glucose to glucose 6 phosphate. The two enzymes have been identified as a specific glucokinase (ATP-D-glucose 6-phosphotransferase) and non-specific hexokinase (ATP-D-hexose 6-phosphotransferase).
Hepatic cell is freely permeable to glucose, and the initial rate of phosphorylation of glucose is the rate-limiting step in glucose metabolism by the liver (ATP-D-glucose 6-phosphotransferase) and non-specific hexokinase (ATP-D-hexose 6-phosphotransferase).
The role of glucose 6-phosphate in glycogen synthase: High blood glucose concentration causes an increase in intracellular levels of glucose 6 phosphate in liver, skeletal muscle and fat (adipose) tissue. (ATP-D-glucose 6-phosphotransferase) and non-specific hexokinase (ATP-D-hexose 6-phosphotransferase). In liver, synthesis of glycogen is directly correlated by blood glucose concentration and in skeletal muscle and adipocytes, glucose has a minor effect on glycogen synthase. High blood glucose releases insulin, stimulating the trans location of specific glucose transporters to the cell membrane.
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