Inactive (top) and active (bottom) forms of cAMP-dependent protein kinase. Flexible portions of the protein are shown schematically with dots.Download high quality TIFF image
Phosphate groups are perfect chemical groups for
modifying the function of proteins: they have a strong negative charge, they are
fairly bulky, and they can form multiple hydrogen bonds. When a phosphate group
is attached or removed to a protein, it may modify the shape and flexibility of the
protein chain, or provide a readily-visible handle for recognition by other proteins.
Cells take full advantage of these possibilities, and in a typical cell, phosphate groups
are used to regulate the function of about one third of their proteins.
Protein kinases are used to add phosphate groups to proteins. The one shown here
is cAMP-dependent protein kinase, also known as protein kinase A or PKA (PDB entries
It is composed of two types of subunits. The catalytic
subunit, shown here in pink, performs the phosphate-adding reaction. The
regulatory subunit, shown in blue, senses the level of cyclic AMP, and turns the
catalytic subunits on or off based on that level. When cAMP levels are low, a dimer of
the regulatory subunits binds to two copies of the catalytic subunit, forming an
inactive complex (shown at the top). When cAMP levels rise, it binds to the
regulatory subunit, releasing the catalytic subunit in an active form (shown at the
PKA is an essential link in a cascade of messages that control energy utilization in
cells. Receptors such as the beta-adrenergic receptor and the glucagon receptor
stimulate the production of cyclic AMP. This activates PKA, which then
phosphorylates many proteins involved in generation of cellular energy, regulating
proteins like glycogen synthase and pyruvate kinase directly and also changing the
synthesis of proteins by phosphorylating key transcription factors.
As you can imagine, these kinases must be carefully regulated to make sure they add
phosphates at the proper time and in the proper place. Several types of regulatory
subunits for PKA are built by our cells. By mixing and matching these subunits,
different types of cells can tune the function of PKA for their needs. In addition, the
regulatory subunits form complexes with other scaffolding proteins, known as
AKAP proteins. These scaffolds hold PKA close to its partners in the signaling
cascade, further tuning its function. A short peptide from an AKAP is shown in the
illustration in green.
Stopping the Signal
Cells also need a method to stop the signal carried by cAMP. Specialized
phosphodiesterases break the unusual cyclic phosphate bond in cAMP, forming AMP
and making it inactive as a signaling molecule. Cells make many types of
phosphodiesterases to tailor cAMP signaling for the needs of particular types of
cells. Caffeine (the stimulant in coffee) and theophylline (the stimulant in tea)
inhibit many forms of this enzyme, prolonging the energetic messages carried by
cAMP. Drugs like Levitra and Viagra, on the other hand, inhibit only one type of
phosphodiesterase (shown here from PDB entry
and have a more targeted effect.