Enzyme Regulation - Reversible Covalent Modification

Reversible Covalent ModificationReversible covalent modification is the making and breaking of a covalent bond between a non-protein group and an enzyme molecule. The most common reversible modification is the addition and removal of phosphate groups through the processes of phosphorylation and dephosphorylation.

Phosphorylation/Dephosphorylation Phosphorylation is catalysed by protein kinases using ATP as the phosphate donor. Dephosphorylation is catalysed by protein phosphatases. The removal of the phosphate group causes a change in the tertiary structure of the enzyme that alters it catalytic activity. A phosphorylated enzyme may be either more or less activated than its dephosphorylated form. The phosphorylation/dephosphorylation may be used as a rapid, reversible switch to turn a metabolic pathway on or off according to the needs of the cell.

Examples of Phosphorylation 1. Protein kinases transfer the phosphate group specifically onto the hydroxyl group of Ser or Tyr residue on their target enzymes.These are cAMP protein dependent kinases. A second class transfers the phosphate group onto the hydroxyl group of a tyrosine residue – Tyrosine Kinases. 2. Glycogen phosphorylase is an enzyme involved in glycogen breakdown. It is active in its phosphorylated form. The glycogen synthase involved in glycogen synthesis is active in its dephosphorylated form.

Advantages of Phosphorylation and DephosphorylationPhosphorylation is a highly effective means of controlling the activity of proteins because: 1.       A phosphoryl groups adds 2 negative charges to a modified protein. This disrupts electrostatic interactions in the unmodified protein and allows new ones to form. This structural change alters substrate binding and catalytic activity. 2.       A phosphoryl group can form 3 or more Hydrogen bonds. These bonds are highly directional allowing for specific interactions with hydrogen-bond donors. 3.       The free energy of phosphorylation is large. Half of it is consumed making phosphorylation irreversible and the other half is consumed in the phosphorylated protein. 4.       Both phosphorylation and dephosphorylation can occur very rapidly – the kinetics can be adjusted. 5.       It causes highly amplified effects. If the target protein is an enzyme, it may transform a large number of substrates. 6.       ATP usage as a phosphoryl donor group links the energy status of the cell with the regulation of metabolism.

Reversible Covalent Modification