In the context of phosphorylation, what does a protein kinase do?

• A. Hydrolyzes $ATP$ to generate $cAMP$ as a second messenger
• B. Transfers a phosphate group, typically from $ATP$, to a target protein residue (e.g., Ser, Thr, or Tyr)
• C. Removes a phosphate group from a phosphorylated protein, releasing inorganic phosphate
• D. Binds DNA and directly increases transcription of the kinase target gene

Answer: (B)