Final answer:
Protein Kinase A (PKA) catalytic subunits phosphorylate multiple substrate molecules, bringing about cellular changes through phosphorylation after cAMP-dependent activation.
Step-by-step explanation:
The kinase activity of the released catalytic subunits of Protein Kinase A (PKA) after cAMP-dependent activation phosphorylates multiple substrate molecules. This process leads to various changes in cellular processes by modifying the structural orientation and activity of these substrates. Upon activation by a signal, the G-protein interacts with adenylyl cyclase to convert ATP to cAMP. The cAMP then binds to PKA, causing a conformational change that separates the inactive tetramer into two active catalytic subunits and two regulatory subunits.
The free catalytic subunits can then phosphorylate phosphorylase kinase, which is only one example of the many different substrates that can be phosphorylated, each potentially triggering distinct metabolic pathways and cellular responses. After activation of adenylate cyclase, cAMP is synthesized and binds to the inactive PKA, causing a conformational change that dissociates the tetramer into active PKA subunits. These active PKA enzymes then catalyze the phosphorylation of target proteins, activating them in the process.