Protein kinases are enzymes that play a crucial role in cellular signaling by adding phosphate groups to specific proteins. This process, known as phosphorylation, can activate or deactivate the target proteins, thereby influencing various cellular functions such as growth, metabolism, and apoptosis. These enzymes are vital for regulating many biological processes, including the cell cycle and immune responses. There are two main types of protein kinases: serine/threonine kinases, which phosphorylate serine or threonine residues, and tyrosine kinases, which target tyrosine residues. Their activity is tightly regulated to maintain cellular homeostasis.