protein kinase A
Protein kinase A is the foreman who acts on the alarm. Once cyclic AMP has rung the cell's internal bell, protein kinase A is the worker who reads the alarm and starts flipping switches on machines throughout the cell to get the job done.
A kinase is an enzyme that attaches a phosphate group to other proteins, and adding a phosphate often turns a target protein on or off. Protein kinase A is held inactive until cyclic AMP binds and releases its active part; the freed enzyme then phosphorylates many targets — enzymes in metabolism, ion channels, and even gene-regulating proteins in the nucleus — carrying out the hormone's instructions.
Because one active protein kinase A molecule can phosphorylate many substrates, this step adds another round of signal amplification on top of the cyclic AMP burst. Its effects are also reversible: enzymes called phosphatases remove the phosphates, so when the hormone and cyclic AMP fade, protein kinase A's marks are erased and the cell returns to baseline.
Adrenaline raises cyclic AMP in liver cells, activating protein kinase A, which phosphorylates and switches on the enzyme that breaks glycogen down to release glucose.
Protein kinase A converting a cyclic AMP signal into enzyme action.
Phosphorylation is reversible and quick, which is why hormone effects through protein kinase A can be switched on and off far faster than effects that require making new proteins.