Enzymatic Regulation Strategies (Full video)
The activity of proteins, including enzymes, often must be regulated so that they function at the proper time and place. The biological activity of proteins is regulated in four principal ways:
1.
Allosteric control. Allosteric proteins contain distinct regulatory sites and multiple functional sites. Regulation by small signal molecules is a significant means of controlling the activity of many proteins. The binding of these regulatory molecules at sites distinct from the active site triggers conformational changes that are transmitted to the active site. Moreover, allosteric proteins show the property of cooperativity: activity at one functional site affects the activity at others. As a consequence, a slight change in substrate concentration can produce substantial changes in activity. Proteins displaying allosteric control are thus information transducers: their activity can be modified in response to signal molecules or to information shared among active sites. This chapter examines two of the best-understood allosteric proteins: the enzyme aspartate transcarbamoylase (ATCase) and the oxygen-carrying protein hemoglobin. Catalysis by aspartate transcarbamoylase of the first step in pyrimidine biosynthesis is inhibited by cytidine triphosphate, the final product of that biosynthesis, in an example of feedback inhibition. The binding of O2 by hemoglobin is cooperative and is regulated by H+, CO2 and 2,3-bisphosphoglycerate (2,3-BPG).
2.
Multiple forms of enzymes. Isozymes, or isoenzymes, provide an avenue for varying regulation of the same reaction at distinct locations or times. Isozymes are homologous enzymes within a single organism that catalyze the same reaction but differ slightly in structure and more obviously in KM and Vmax values, as well as regulatory properties. Often, isozymes are expressed in a distinct tissue or organelle or at a distinct stage of development.
3.
Reversible covalent modification. The catalytic properties of many enzymes are markedly altered by the covalent attachment of a modifying group, most commonly a phosphoryl group. ATP serves as the phosphoryl donor in these reactions, which are catalyzed by protein kinases. The removal of phosphoryl groups by hydrolysis is catalyzed by protein phosphatases. This chapter considers the structure, specificity, and control of protein kinase A (PKA), a ubiquitous eukaryotic enzyme that regulates diverse target proteins.
4.
Proteolytic activation. The enzymes controlled by some of these mechanisms cycle between active and inactive states. A different regulatory motif is used to irreversibly convert an inactive enzyme into an active one. Many enzymes are activated by the hydrolysis of a few or even one peptide bond in inactive precursors called zymogens or proenzymes. This regulatory mechanism generates digestive enzymes such as chymotrypsin, trypsin, and pepsin. Caspases, which are proteolytic enzymes that are the executioners in programmed cell death, or apoptosis (Section 2.4.3), are proteolytically activated from the procaspase form. Blood clotting is due to a remarkable cascade of zymogen activations. Active digestive and clotting enzymes are switched off by the irreversible binding of specific inhibitory proteins that are irresistible lures to their molecular prey.
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