Which enzyme indirectly adds a phosphate group to a substrate by transferring a phosphate from ATP?

• A. Phosphatase
• B. Kinase
• C. Phosphorylase
• D. Isomerase

Answer: (B)

Phosphorylation by kinases is the process of transferring a phosphate group from ATP to a substrate. The key idea is that the phosphate donor is ATP, and the enzyme just moves the gamma phosphate from ATP onto the target molecule, producing ADP and a phosphorylated substrate. This is how cells regulate many activities, from enzyme function to signaling pathways, because adding or removing a phosphate can change a molecule’s shape, charge, or interactions.

Think of it this way: a kinase binds ATP and the substrate, then catalyzes the transfer of the terminal phosphate from ATP to the substrate. An example is a protein kinase phosphorylating a serine, threonine, or tyrosine residue on a protein, which can switch the protein’s activity on or off. In contrast, phosphatases remove phosphate groups, phosphorylases use inorganic phosphate as the donor to add phosphate in specific catabolic steps, and isomerases rearrange atoms within a molecule without adding a phosphate. The scenario described—phosphate coming from ATP and being transferred to the substrate—fits kinase activity precisely.