Controlling Enzymes

1. Controlling Enzymes

2. Reaction Rate • Without enzyme reaction rate is dependent on concentration. • With enzyme, rate still depends on concentration but. . . • there is also a saturation phenomenon

3. Each line represents a metabolic pathway. • Each dot represents a waypoint and a reaction (and an enzyme) in the pathway.

4. Regulation • Enzymes organize and regulate metabolic pathways.

5. Inhibitors can regulate enzyme activity • Inhibitors can bind to enzymes, slowing down the rates of enzyme-catalyzed reactions. • IRREVERSIBLE INHIBITION -- Inhibitor covalently bond to certain side chains at the active site of an enzyme, thereby permanently inactivating the enzyme by destroying its capacity to interact with its normal substrate.

6. Irreversible Inhibition • DIPF (diisopropylphosphorofluoridate) reacts with serine. • DIPF is an irreversible inhibitor of acetylcholinesterase, essential to the normal function of the nervous system. • DIPF and other similar compounds are classified as nerve gases.

7. REVERSIBLE INHIBITION • Inhibitor is similar enough to a particular enzyme’s natural substrate to bind noncovalently. • Different enough that the enzyme catalyzes no chemical reaction. • When bound to enzyme, the natural substrate cannot enter the active site; wastes the enzyme’s time by preventing its catalytic action. Called competitive inhibitor because competes with the natural substrate for the active site. • Inhibition is reversible because when the concentration of the competitive inhibitor is reduced, it detaches from the active site, and the enzyme is active again.

8. Non Competitive Inhibitor • Noncompetitive inhibitors bind to the enzyme at a site different from the active site. • Binding can cause a change in the shape of the enzyme that alters the active site. • Active site may still bind substrate molecules, but the rate of product formation may be reduced. • Noncompetitive inhibitors, like competitive inhibitors, can become unbound, so their effects are reversible.

9. Allosteric Inhibitor • The change in enzyme shape due to noncompetitive inhibitor binding is an example of allostery (allo, “different”; stery, “shape”). • Binding of the inhibitor induces the protein to change its shape. • More common are enzymes that already exist in the cell in more than one possible shape.

10. Allostery • Active form of the enzyme has the proper shape for substrate binding. • The inactive form of the enzyme has a shape that cannot bind the substrate but can bind an inhibitor. • Binding of an inhibitor to a site separate from the active site (i.e., the site where substrate binds) stabilizes the inactive form, and it becomes less likely to convert to the active form. • The active form can be stabilized by binding of an activator to a third site on the enzyme.

11. Allosterism • Binding of inhibitors and activators highly specific. • Most enzymes that are allosterically regulated are proteins with quaternary structure. • Active site is present on one subunit, called the catalytic subunit, • Regulatory site(s) for activators and/or inhibitors are on different polypeptide sequences, called regulatory subunits.

12. Reaction Rate • Allosteric enzymes and nonallosteric enzymes differ greatly in their reaction rates when the substrate concentration is low. • For a nonallosteric enzyme reaction rate first increases very sharply with increasing substrate concentration, then tapers off to a constant maximum rate as the supply of enzyme becomes saturated with substrate. Concentration of Substrate

13. Reaction Rate • Plot for many allosteric enzymes is radically different, having a sigmoid (S-shaped) appearance. • Increase in reaction rate with increasing substrate concentration is slight at low substrate concentrations, but within a certain range, the reaction rate is extremely sensitive to relatively small changes in substrate concentration. Because of this sensitivity, allosteric enzymes are important in regulating entire metabolic pathways. Concentration of Substrate

14. Metabolic Pathways • Usually involves a starting material, intermediate products, and end product needed by the cell. • In pathway, there are a number of reactions, each forming an intermediate product and each catalyzed by a different enzyme. • First step in a pathway is called commitment -- once this enzyme-catalyzed reaction occurs, the other reactions happen in sequence, leading to end product.

15. Why Regulation? • If the cell has no need for that product—eg. that product is available from its environment. • Cell should not waste energy to make something it does not need.

16. Feedback Inhibition • One way to solve this problem is to shut down the metabolic pathway by having the final product allosterically inhibit the enzyme that catalyzes the commitment step. • This mechanism is known as feedback inhibition or end-product inhibition. When the end product is present at a high concentration, some of it binds to an allosteric site on the commitment step enzyme, thereby causing it to become inactive.

17. Feedback Inhibition

18. Thyroid Regulation • If your thyroid is producing enough thyroxine (T4), pituitary secretion of thyroid stimulating hormone (TSH) is turned down. • If your thyroid is not producing enough T4, such as if there is iodine deficiency, the production of TSH by pituitary increases and so thyroid gland is stimulated to increase production.

19. Goiter

20. Question • Which statement about enzyme inhibitors is not true? • a. A competitive inhibitor binds the active site of the enzyme. • b. An allosteric inhibitor binds a site on the active form of the enzyme. • c. A noncompetitive inhibitor binds a site other than the active site. • d. Noncompetitive inhibition cannot be completely overcome by the addition of more substrate. • e. Competitive inhibition can be completely overcome by the addition of more substrate.

21. Question • Which statement about enzyme inhibitors is not true? • a. A competitive inhibitor binds the active site of the enzyme. • b. An allosteric inhibitor binds a site on the active form of the enzyme. • c. A noncompetitive inhibitor binds a site other than the active site. • d. Noncompetitive inhibition cannot be completely overcome by the addition of more substrate. • e. Competitive inhibition can be completely overcome by the addition of more substrate.

23. Another Question • Which statement about feedback inhibition of enzymes is not true? • a. It is exerted through allosteric effects. • b. It is directed at the enzyme that catalyzes the first committed step in a metabolic pathway. • c. It affects the rate of reaction, not the concentration of enzyme. • d. It acts very slowly. • e. It is an example of irreversible inhibition.

24. Another Question • Which statement about feedback inhibition of enzymes is not true? • a. It is exerted through allosteric effects. • b. It is directed at the enzyme that catalyzes the first committed step in a metabolic pathway. • c. It affects the rate of reaction, not the concentration of enzyme. • d. It acts very slowly. • e. It is an example of irreversible inhibition.