Final answer:
The inactive PKA being a tetramer means that it consists of four subunits, specifically two regulatory and two catalytic subunits. The regulatory subunits inhibit the catalytic ones until cyclic AMP induces a change and activates the catalytic subunits for the phosphorylation process.
Step-by-step explanation:
When we say that the inactive PKA is a tetramer, it means that it is a protein complex composed of four subunits. Protein Kinase A (PKA) is an enzyme that plays a vital role in cellular processes by transferring phosphate groups from ATP to specific substrates (phosphorylation). This process is essential for the regulation of many cellular pathways. The inactive form of PKA typically consists of two regulatory and two catalytic subunits. The regulatory subunits bind to the catalytic subunits to inhibit their activity. Once cyclic AMP (cAMP) binds to the regulatory subunits, it induces a conformational change that releases the active catalytic subunits. These activated catalytic subunits can then phosphorylate target proteins, leading to downstream cellular effects.