In the context of phosphorylation, what is a protein kinase?

• A. A protein that binds phosphate groups to store energy without catalyzing any chemical reaction
• B. An enzyme that synthesizes $ATP$ by phosphorylating $ADP$ using a proton gradient
• C. An enzyme that transfers a phosphate group from $ATP$ to a target protein (often on Ser, Thr, or Tyr residues)
• D. An enzyme that removes phosphate groups from phosphorylated proteins, typically releasing inorganic phosphate

Answer: (C)