Cyclic Protein Phosphorylation and Dephosphorylation Regulate Many Cellular Functions

As noted earlier, one of the most common mechanisms for regulating protein activity is phosphorylation, the addition and removal of phosphate groups from serine, threonine, or tyrosine residues. Protein kinases catalyze phosphorylation, and phosphatases catalyze dephosphorylation. Although both reactions are essentially irreversible, the counteracting activities of kinases and phosphatases provide cells with a "switch" that can turn on or turn off the function of various proteins (Figure 3-30). Phosphorylation changes a protein's charge and generally leads to a conformational change; these effects can significantly alter ligand binding by a protein, leading to an increase or decrease in its activity.

Active

Active

Inactive

▲ FIGURE 3-30 Regulation of protein activity by kinase/phosphatase switch. The cyclic phosphorylation and dephosphorylation of a protein is a common cellular mechanism for regulating protein activity. In this example, the target protein R is inactive (light orange) when phosphorylated and active (dark orange) when dephosphorylated; some proteins have the opposite pattern.

Nearly 3 percent of all yeast proteins are protein kinases or phosphatases, indicating the importance of phosphorylation and dephosphorylation reactions even in simple cells. All classes of proteins—including structural proteins, enzymes, membrane channels, and signaling molecules—are regulated by kinase/phosphatase switches. Different protein kinases and phosphatases are specific for different target proteins and can thus regulate a variety of cellular pathways, as discussed in later chapters. Some of these enzymes act on one or a few target proteins, whereas others have multiple targets. The latter are useful in integrating the activities of proteins that are co-ordinately controlled by a single kinase/phosphatase switch. Frequently, another kinase or phosphatase is a target, thus creating a web of interdependent controls.

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