Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. If you continue browsing the site, you agree to the use of cookies on this website. See our User Agreement and Privacy Policy.

Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. If you continue browsing the site, you agree to the use of cookies on this website. See our Privacy Policy and User Agreement for details.

Like this presentation? Why not share!

No Downloads

Total views

777

On SlideShare

0

From Embeds

0

Number of Embeds

5

Shares

0

Downloads

53

Comments

0

Likes

1

No embeds

No notes for slide

- 1. Chapter Six The Behavior of Proteins: Enzymes
- 2. Enzyme Catalysis <ul><li>Enzyme: a biological catalyst </li></ul><ul><ul><li>with the exception of some RNAs that catalyze their own splicing (Section 10.4), all enzymes are proteins </li></ul></ul><ul><ul><li>enzymes can increase the rate of a reaction by a factor of up to 10 20 over an uncatalyzed reaction </li></ul></ul><ul><ul><li>some enzymes are so specific that they catalyze the reaction of only one stereoisomer; others catalyze a family of similar reactions </li></ul></ul><ul><li>The rate of a reaction depends on its activation energy, G° ‡ </li></ul><ul><ul><li>an enzyme provides an alternative pathway with a lower activation energy </li></ul></ul>
- 3. Enzyme Catalysis (Cont’d) <ul><li>For a reaction taking place at constant temperature and pressure, e.g., in the body </li></ul><ul><li>• the change in free energy is </li></ul><ul><li>Difference in energies between initial state and final state </li></ul><ul><li>The change in free energy is related to the equilibrium constant, K eq , for the reaction by </li></ul>
- 4. Enzyme Catalysis (Cont’d) <ul><li>Consider the reaction </li></ul>
- 5. Temperature dependence of catalysis <ul><li>• Temperature can also catalyze reaction (increase rate) </li></ul><ul><li>• This is dangerous, why? </li></ul><ul><li>• Increasing temperature will eventually lead to protein denaturation </li></ul>
- 6. Enzyme Kinetics <ul><li>For the reaction </li></ul><ul><ul><li>The rate of reaction is given by rate equation </li></ul></ul><ul><ul><li>Where k is a proportionality constant called the specific rate constant </li></ul></ul><ul><ul><li>Order of reaction : the sum of the exponents in the rate equation </li></ul></ul>
- 7. Enzyme Kinetics (Cont’d) <ul><ul><li>Consider the reaction </li></ul></ul><ul><ul><li>Whose rate equation is given by the expression </li></ul></ul><ul><ul><ul><li>Determined experimentally, not always from balanced equations </li></ul></ul></ul><ul><ul><li>The reaction is said to be first order in A , first order in B , and second order overall </li></ul></ul><ul><ul><li>Consider this reaction of glycogen with phosphate </li></ul></ul>
- 8. How Enzymes bind to Substrate <ul><li>In an enzyme-catalyzed reaction </li></ul><ul><ul><li>Substrate , S: a reactant </li></ul></ul><ul><ul><li>Active site : the small portion of the enzyme surface where the substrate(s) becomes bound by noncovalent forces, e.g., hydrogen bonding, electrostatic attractions, van der Waals attractions </li></ul></ul>
- 9. Binding Models <ul><li>Two models have been developed to describe formation of the enzyme-substrate complex </li></ul><ul><ul><li>Lock-and-key model : substrate binds to that portion of the enzyme with a complementary shape </li></ul></ul><ul><ul><li>Induced fit model: binding of the substrate induces a change in the conformation of the enzyme that results in a complementary fit </li></ul></ul>
- 10. 2 Modes of E-S Complex Formation
- 11. Formation of Product
- 12. An Example of Enzyme Catalysis <ul><li>Chymotrypsin catalyzes </li></ul><ul><ul><li>The selective hydrolysis of peptide bonds where the carboxyl is contributed by Phe and Tyr </li></ul></ul><ul><ul><li>It also catalyzes hydrolysis of the ester bonds </li></ul></ul>
- 13. An Example of Enzyme Catalysis (Cont’d)
- 14. Non-Allosteric Enzyme Behavior <ul><li>• Point at which the rate of reaction does not change, enzyme is saturated, maximum rate of reaction is reached </li></ul>
- 15. ATCase: An Example of Allosteric Behavior <ul><li>• Sigmoidal shape- characteristic of allosterism </li></ul><ul><li>• Again Max. velocity reached, but different mechanism </li></ul>
- 16. Michaelis-Menten Kinetics <ul><li>Initial rate of an enzyme-catalyzed reaction versus substrate concentration </li></ul>
- 17. Michaelis-Menten Model <ul><li>For an enzyme-catalyzed reaction </li></ul><ul><li>The rates of formation and breakdown of ES are given by these equations </li></ul><ul><li>At the steady state </li></ul>
- 18. Michaelis-Menten Model (Cont’d) <ul><li>When the steady state is reached, the concentration of free enzyme is the total less that bound in ES </li></ul><ul><li>Substituting for the concentration of free enzyme and collecting all rate constants in one term gives </li></ul><ul><li>Where K M is called the Michaelis constant </li></ul>
- 19. Michaelis-Menten Model (Cont’d) <ul><li>It is now possible to solve for the concentration of the enzyme-substrate complex, [ES] </li></ul><ul><li>Or alternatively </li></ul>
- 20. Michaelis-Menten Model (Cont’d) <ul><ul><li>In the initial stages, formation of product depends only on the rate of breakdown of ES </li></ul></ul><ul><ul><li>If substrate concentration is so large that the enzyme is saturated with substrate [ES] = [E] T </li></ul></ul><ul><ul><li>Substituting k 2 [E] T = V max into the top equation gives </li></ul></ul>
- 21. Michaelis-Menten Model (Cont’d) <ul><ul><li>When [S]= K M , the equation reduces to </li></ul></ul>
- 22. Linearizing The Michaelis-Menten Equation <ul><ul><li>It is difficult to determine V max experimentally </li></ul></ul><ul><ul><li>The equation for a hyperbola </li></ul></ul><ul><ul><li>Can be transformed into the equation for a straight line by taking the reciprocal of each side </li></ul></ul>
- 23. Lineweaver-Burk Plot <ul><ul><li>The Lineweaver-Burke plot has the form y = mx + b, and is the formula for a straight line </li></ul></ul><ul><ul><li>a plot of 1/V versus 1/[S] will give a straight line with slope of K M /V max and y intercept of 1/V max </li></ul></ul><ul><ul><li>such a plot is known as a Lineweaver-Burk double reciprocal plot </li></ul></ul>
- 24. Lineweaver-Burk Plot (Cont’d) <ul><ul><li>K M is the dissociation constant for ES; the greater the value of K M , the less tightly S is bound to E </li></ul></ul><ul><ul><li>V max is the turnover number </li></ul></ul>
- 25. Turnover Numbers <ul><ul><li>• V max is related to the turnover number of enzyme:also called k cat </li></ul></ul><ul><ul><li>• Number of moles of substrate that react to form product per mole of enzyme per unit of time </li></ul></ul>
- 26. Enzyme Inhibition <ul><li>Reversible inhibitor : a substance that binds to an enzyme to inhibit it, but can be released </li></ul><ul><ul><li>competitive inhibitor: binds to the active (catalytic) site and blocks access to it by substrate </li></ul></ul><ul><ul><li>noncompetitive inhibitor: binds to a site other than the active site; inhibits the enzyme by changing its conformation </li></ul></ul><ul><li>Irreversible inhibitor : a substance that causes inhibition that cannot be reversed </li></ul><ul><ul><li>usually involves formation or breaking of covalent bonds to or on the enzyme </li></ul></ul>
- 27. Competitive Inhibition <ul><ul><li>Substrate must compete with inhibitor for the active site; more substrate is required to reach a given reaction velocity </li></ul></ul><ul><ul><li>We can write a dissociation constant, K I for EI </li></ul></ul>
- 28. Competitive Inhibition
- 29. Competitive Inhibition <ul><li>In a Lineweaver-Burk double reciprocal plot of 1/V versus 1/[S], the slope (and the x intercept) changes but the y intercept does not change </li></ul>
- 30. A Lineweaver-Burke Plot for Competitive Inhibition
- 31. Noncompetitive Inhibition (Cont’d) <ul><li>Several equilibria are involved </li></ul><ul><li>The maximum velocity V max has the form </li></ul>
- 32. Noncompetitive Inhibition (Cont’d)
- 33. A Lineweaver-Burke Plot for Noncompetitive Inhibition <ul><ul><li>Because the inhibitor does not interfere with binding of substrate to the active site, K M is unchanged </li></ul></ul><ul><ul><li>Increasing substrate concentration cannot overcome noncompetitive inhibition </li></ul></ul>
- 34. A Lineweaver-Burke Plot for Noncompetitive Inhibition (Cont’d)
- 35. Other Types of Inhibition <ul><li>Uncompetitive - inhibitor can bind to the ES complex but not to free E. V max decreases and KM decreases. </li></ul><ul><li>Mixed - Similar to noncompetitively, but binding of I affects binding of S and vice versa. </li></ul>

No public clipboards found for this slide

×
### Save the most important slides with Clipping

Clipping is a handy way to collect and organize the most important slides from a presentation. You can keep your great finds in clipboards organized around topics.

Be the first to comment