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

• A. An enzyme that removes phosphate groups from proteins by hydrolysis
• B. An enzyme that catalyzes transfer of a phosphate group, typically from ATP, to a specific substrate protein or other molecule
• C. A small molecule that donates phosphate groups to ATP during oxidative phosphorylation
• D. A structural protein that stores phosphate groups for later use in signaling pathways

Answer: (B)