Protein kinase A (PKA) is an enzyme that plays a crucial role in cellular signaling. It is activated by cyclic AMP (cAMP), a molecule that transmits signals within cells. Once activated, PKA can modify various target proteins through a process called phosphorylation, which involves adding a phosphate group to the protein. This modification can change the activity, location, or function of the target proteins, influencing many cellular processes. PKA is involved in regulating several important functions, including metabolism, gene expression, and cell growth. It is found in many tissues throughout the body and is essential for responding to hormones like epinephrine and glucagon. By modulating these pathways, PKA helps maintain homeostasis and supports various physiological responses.