Enzyme Regulation - Reversible Covalent Modification

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Reversible Covalent Modification

Reversible 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 Dephosphorylation

Phosphorylation 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