Protein Kinase A (PKA) is an enzyme that plays a crucial role in cellular signaling. It is activated by the molecule cyclic AMP (cAMP), which is produced in response to various hormones and signals. Once activated, PKA phosphorylates specific target proteins, leading to changes in their activity and function, which can affect processes like metabolism, gene expression, and cell growth. PKA is composed of two regulatory and two catalytic subunits. The binding of cAMP to the regulatory subunits causes a conformational change, releasing the catalytic subunits to perform their function. This mechanism allows PKA to act as a key mediator in various physiological responses, including the fight-or-flight response.