Covalent Modification

Many enzymes are regulated by covalent modification, most often by the addition or removal of phosphate groups from specific serine, threonine, or tyrosine residues of the enzyme. Protein phosphorylation is recognized as one of the primary ways in which cellular processes are regulated.
1. Phosphorylation and dephosphorylation: Phosphorylation reactions are catalyzed by a family of enzymes called protein kinases that use ATP as the phosphate donor. Phosphate groups are cleaved from phosphorylated enzymes by the action of phosphoprotein phosphatases (Fig. 1).

Figure 1:  Covalent modification by the addition and removal of phosphate groups. [Note: HPO4²⁻ may be represented as Pi and PO3²⁻ as P .] ADP =adenosine diphosphate.
2. Enzyme response to phosphorylation: Depending on the specific enzyme, the phosphorylated form may be more or less active than the unphosphorylated enzyme. For example, hormone-mediated phosphorylation of glycogen phosphorylase (an enzyme that degrades glycogen) increases activity, whereas phosphorylation of glycogen synthase (an enzyme that synthesizes glycogen) decreases activity .

مواضيع ذات صلة

Blood clotting (coagulation)

Platelet Plug Formation: Aggregation

Platelet Plug Formation: Activation

Platelet Plug Formation: Adhesion

Limiting Clotting : Fibrinolysis

Limiting Clotting : Inactivating proteins

Fibrin Meshwork Formation : Pathways

Fibrin Meshwork Formation : Formation of γ-carboxyglutamate residues

Fibrin Meshwork Formation :Role of phosphatidylserine and calcium

Fibrin Meshwork Formation: Proteolytic cascade

Blood Clotting : Overview

Biotechnology and Human Disease