Final answer:
A protein kinase adds phosphate groups to proteins, while a protein phosphatase removes them.
Step-by-step explanation:
A protein kinase and a protein phosphatase are both enzymes involved in the process of protein phosphorylation, a post-translational modification that regulates protein activity. A protein kinase adds a phosphate group to specific amino acids in a protein, usually serine, threonine, or tyrosine. This addition of phosphate groups can activate or deactivate the protein, depending on the signaling pathway involved.
On the other hand, a protein phosphatase removes the phosphate group from these phosphorylated amino acids, reversing the phosphorylation process and restoring the protein to its original state.
In summary, a protein kinase adds phosphate groups to proteins, while a protein phosphatase removes them.