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.
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.
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 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.