What group do protein kinases add to their target proteins to alter activity?

Protein kinases are enzymes that play a crucial role in cellular signaling and regulation by modifying other proteins. Specifically, they add phosphate groups to target proteins through a process known as phosphorylation. This addition of phosphate groups typically occurs on specific amino acids, such as serine, threonine, or tyrosine residues, which can lead to a conformational change in the protein. The alteration in the protein's shape can affect its activity, interactions, localization, and stability.

Phosphorylation can either activate or deactivate the target protein, depending on the context and the specific protein being modified. This regulatory mechanism is vital in various biological processes, including cell division, metabolism, and response to external signals, making protein kinases essential components of signal transduction pathways. Other substances like nucleotides, amino acids, or sugars do not perform this specific function of adding phosphate groups to alter protein activity.