Common Mechanisms for Regulating Protein Function
■ In allostery, the binding of one ligand molecule (a substrate, activator, or inhibitor) induces a conformational change, or allosteric transition, that alters a protein's activity or affinity for other ligands.
■ In multimeric proteins, such as hemoglobin, that bind multiple ligand molecules, the binding of one ligand molecule may modulate the binding affinity for subsequent lig-and molecules. Enzymes that cooperatively bind substrates exhibit sigmoidal kinetics similar to the oxygen-binding curve of hemoglobin (see Figure 3-26).
■ Several allosteric mechanisms act as switches, turning protein activity on and off in a reversible fashion.
■ The binding of allosteric ligand molecules may lead to the conversion of a protein from one conformational/
activity state into another or to the release of active sub-units (see Figure 3-27).
■ Two classes of intracellular switch proteins regulate a variety of cellular processes: (1) calmodulin and related Ca2+-binding proteins in the EF hand family and (2) members of the GTPase superfamily (e.g., Ras and Ga), which cycle between active GTP-bound and inactive GDP-bound forms (see Figure 3-29).
■ The phosphorylation and dephosphorylation of amino acid side chains by protein kinases and phosphatases provide reversible on/off regulation of numerous proteins.
■ Nonallosteric mechanisms for regulating protein activity include proteolytic cleavage, which irreversibly converts inactive zymogens into active enzymes, compartmentation of proteins, and signal-induced modulation of protein synthesis and degradation.
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