Phosphorylation

 Phosphorylation

Introduction

The importance of organic phosphates in metabolic reactions was first indicated by the studies of Harden and Young, who found fermentation of glucose by cell-free yeast juice increased after inorganic phosphorus is added to the medium, which was found to be converted to organic phosphate. The work of Embden, Meyerhof, Cori and their associates show that organic phosphates concerned with the metabolism of glucose by yeast are also intermediates in the metabolism of glycogen and glucose in muscle and other tissues.

The presence of ATP, ADP, CP and other high-energy phosphate compounds in the tissues and their participation in different metabolic processes indicate the importance of organic phosphates in metabolism. The sugar phosphates, although not high-energy compounds, are obligatory intermediates. Their formation involves Phosphorylation by the energy compound ATP in presence of suitable enzymes.

Definition

A biochemical process that involves the addition of phosphate to an organic compound is called Phosphorylation.

Examples include the addition of phosphate to glucose to produce glucose monophosphate and the addition of phosphate to adenosine diphosphate (ADP) to form adenosine triphosphate (ATP). Phosphorylation is carried out through the action of enzymes known as phosphotransferases or kinases.

The term Phosphorylation includes all chemical reactions in the body which require association of a molecule with phosphoric acid. The reverse changes are called Dephosphorylation, where phosphoric acid is dissociated from the compound. Same enzyme system is used for both of these reversible reactions.

Physiological Functions of Phosphorylation- Phosphoric acid is part of the composition of cell protoplasm. Hence Phosphorylation is an essential chemical process for all cells. In addition to this it takes an essential part during absorption and metabolism of different molecules. The main functions of Phosphorylation- Dephosphorylation reactions are given below-

1.     Functions in relation to Carbohydrate

a.      Absorption of glucose through intestinal mucosa takes place by forming hexose phosphate which enters mucosal cell, then Dephosphorylation occurs and hexose is absorbed and phosphoric acid is left out.

b.      Reabsorption of glucose through renal tubules also takes place by the above mentioned process.

c.      Glycogenesis and Glycolysis are the processes dependent upon Phosphorylation Dephosphorylation reactions.

d.     Conversion of ATPs into ADPs to release the energy during muscular contraction is due to Dephosphorylation while the reverse process is due to Phosphorylation. This reaction takes place during oxidation process in the cell mostly in the cristae of Mitochondria.

2.     Functions in relation to Fats

a.      During absorption of fats, neutral fats and phospholipids are synthesized in the absorbing epithelium through Phosphorylation carried out by the enzyme, phosphorylase.

b.     Liver synthesizes phospholipids, especially lecithin. It is a very important step in the transport of fat. It also acts as a primary stage in the further oxidation of fatty acids. Fatty acid oxidation is a function of mitochondria.

c.      Phospholipids are part of the composition of element constant of the cell. Each cell can synthesize its own phospholipids locally by a process of Phosphorylation.

d.     Kephalin or cephalin, one of the phospholipids, is formed by Phosphorylation which initiates blood clotting.

3.     Functions in relation to Proteins

a.      All the phosphoproteins are synthesized by Phosphorylation, such as nucleoproteins, caseinogens, etc.

b.     Phosphorylation takes an important part in issue oxidation during which proteins, fats and carbohydrates are finally broken down.

4.     Functions in relation to Vitamins

Some members of the vitamin B group are phosphorylated compounds, e.g., thiamine pyrophosphate, riboflavin phosphate, etc. They act as coenzymes in oxidation and reduction processes in the cell.

5.     Functions in relation to Inorganic Salts

a.      Sodium plays an important part in Phosphorylation.

b.     The acid and alkaline phosphates, act as a very important buffer system in the body.

c.      The complex interrelation between calcium and phosphorus is primarily a process of Phosphorylation and Dephosphorylation as seen in bone’s dynamic physiological state.

Control of Phosphorylation

1.     Enzymes- The enzymes that take part in this process are phosphorylase, phosphatase, etc.

2.     Hormones-

a.      Glucocorticoids- Glucocorticoids secreted by adrenal cortex inhibit Phosphorylation,

b.     Adrenocorticotrophic hormone- secreted by anterior Pituitary gland exerts control on Phosphorylation, through adrenal cortex.

c.      Growth hormone- secreted by anterior Pituitary gland affects Phosphorylation in a similar way as Glucocorticoids.

3.     Inorganic Salts- Sodium and phosphates play a role in Phosphorylation through adrenal cortex.