Many enzymes involved in signal transduction are regulated via reversible phosphorylation (see Figure 7.14A). An enzyme can be activated by a protein kinase, which adds a phosphate from ATP to one or more specific amino acids. This results in a change in the shape of the enzyme, making it active. Such activation is reversible because another enzyme called a protein phosphatase can catalyze the hydrolysis and removal of phosphate groups, so that the enzyme becomes inactive again. In addition to the enzymes involved in signal transduction, many other enzymes and proteins in the cell (such as ion channels) are regulated via reversible phosphorylation. Reflecting the important role of protein phosphorylation in cell functions, the human genome contains more than 500 protein kinase genes: about 2 percent of all the protein-