What is the function of a protein kinase?

• A. Adds phosphates to proteins.
• B. Removes phosphates from proteins.
• C. Transports proteins across membranes.
• D. Catalyzes the breakdown of ATP.

Answer: (A)

Protein kinases regulate cellular activities by transferring a phosphate group from ATP to specific amino acids on target proteins, usually serine, threonine, or tyrosine. This phosphorylation changes the protein’s charge and shape, which can turn its activity on or off, alter how it interacts with other molecules, or determine where it localizes in the cell. Because phosphorylation is reversible, phosphatases can remove the phosphate, allowing precise control of signaling pathways. The other options don’t describe this regulatory role: removing phosphates is done by phosphatases, transporting proteins across membranes is the job of transporter proteins, and catalyzing the breakdown of ATP is done by ATPases, not kinases.