Activation and Inactivation
of Proteins




Often a specific protein has both an active and an inactive state. Indeed, many regulatory molecules work by causing the protein to switch between the two states. Three different types of protein activation or inactivation are described below.


Allosteric Regulation

In allosteric regulation, a regulatory molecule binds to a site separate from the catalytic site (active site) of an enzyme. This binding causes a change in the three dimension conformation of the protein, turning off (or turning on) the catalytic site.



Phosphorylation of the Protein

Many proteins in cells are regulated by phosphorylation. An enzyme, termed a protein kinase, transfers a phosphate group from ATP to the protein. Quite a few hormones and most growth factors work by activating a protein kinase. A different enzyme, termed a protein phosphatase, removes the phosphate group, thereby returning the protein to its original state.



GTP Binding Proteins

GTP binding proteins are important in the action of many hormones and other physiological regulatory molecules. These proteins are active if GTP is bound to the protein and inactive if GDP is bound. The protein is reactivated by GTP replacing GDP on the molecule. You will hear more about these important proteins later in the quarter.




Quick Quiz

Fill in Answer Correct False Correct Answer
Name the type of regulation in which a regulatory molecule changes the conformation of an enzyme by binding to a site other than the catalytic site.
What type of molecule is GTP?
What type of enzyme cleaves a phosphate from a molecule?
Which figure in this diagram shows the step in which a GTP binding protein is activated?

(Spelling must be correct)
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