1. Chapter 6
CHM 341
Fall 2016
Suroviec

2. Increase likelihood that reactants can interact productively.
I. Introduction
Biological Catalysts
Accelerate biochemical reactions by physically interacting with reactants and products to provide a more favorable pathway for the transformation of one to another.
Increase likelihood that reactants can interact productively.
CANNOT promote reactions where G>0.

3. II. General Properties
Higher Reaction Rates
Milder reaction conditions
Greater reaction specificity
Capacity for regulation

4. Normally named by adding “-ase” to the name of enzyme’s substrate
A. Nomenclature
Normally named by adding “-ase” to the name of enzyme’s substrate
Currently most enzymes are named by using a more systematic method
International Union of Biochemistry and Molecular Biology

5. Uses non-covalent forces to bind the substrate into the active site
B. Specificity
Uses non-covalent forces to bind the substrate into the active site

6. II. Activation Energy and Reaction Coordinates
One understanding of how enzymes catalyzes come from transition state theory
Intermediate must be formed
These are high energy and unstable

7. II. Activation Energy and Reaction Coordinates
Reactants generally approach using path of LOWEST energy
Reaction Coordinate

8. II. Activation Energy and Reaction Coordinates
No longer symmetrical - why?
G‡ =

9. II. Activation Energy and Reaction Coordinates
From the Arrehenius equation
Reaction rates depend on energy and frequency of collisions

10. II. Activation Energy and Reaction Coordinates
Reactions usually occur in 2 steps
2 transition steps = 2 activation barriers

11. II. Activation Energy and Reaction Coordinates
Catalysts reduce G‡
Provide reaction pathway with transition state whose free energy is lower than that in uncatalyzed reaction

12. III. Catalytic Mechanisms
Cofactors and Coenzymes
In an enzyme, functional groups in the active site can perform the same catalytic faction as in chemical reactions such as acid/base reactions, transient covalent bonds and charge-charge interactions.
Functional groups cannot do redox reactions or group transfer reactions. For those you need a bound cofactor.

13. B. Acid-Base Catalysis
Acid catalysis is process in which a proton is transferred between the enzyme and the substrate

14. B. Covalent or Nucleophilic Catalysis
Accelerates reaction through transient formation of catalyst – substrate covalent bond
Need nucleophilic group on catalyst with electrophilic group on substrate

15. Mediate redox reactions
D. Metal Ion Catalysis
Metalloenzymes
Fe2+, Cu2+
Mediate redox reactions
Promote reactivity of other groups in active sites by electrostatic effects

16. E. Transition state stabilization
Bind the transition state of the reaction it catalyzes with greater affinity than its substrates or products
Binding transition state increases its concentration and increases reaction rate

17. F. Catalysis through Proximity/Orientation
Bring reactants close together
Binding causes conformational shift

18. IV. Structure and Function
The structure and mechanism can reveal quite a bit about an enzyme’s function

19. S  P Progress of this reaction can be expressed as a velocity
A. Reaction Kinetics
S  P Progress of this reaction can be expressed as a velocity

20. A. Reaction Kinetics
When enzyme concentration is help constant the reaction velocity will vary with [A]

21. V. Michaelis – Menton Eqn.
Rate equations describe chemical processes
Unimolecular reactions
Bimolecular reactions

22. B. Michaelis Menton Equation is rate equation
Simplest cases enzyme binds to substrate before converting to product
The reaction of E + P converting back to ES is a step that we assume does NOT happen.

23. B. Michaelis Menton Equation
We can measure n by choosing the experimental conditions
We then further choose experimental conditions to simplify the calculations: STEADY STATE

24. C. Michaelis Constant (Km) and Initial Velocity (n) and Maximal Velocity (Vmax)

25. KM is unique for each enzyme- substrate pair
C. Michaelis Constant (Km) and Initial Velocity (n) and Maximal Velocity (Vmax)
Occurs a high substrate concentration when enzyme is saturated
At substrate concentration at which [S] = KM
So if enzyme has small Km achieves max catalytic efficiency at low [S]
KM is unique for each enzyme- substrate pair

26. C. Michaelis Constant (Km) and Initial Velocity (n) and Maximal Velocity (Vmax)
Michaelis Constant is a combination of 3 rate constants that is experimentally determined.

27. D. kcat catalytic constant = turnover #
How fast an enzyme operates after it has selected and bound its substrate.
Number of catalytic cycles that each active site undergoes per unit time

28. E. kcat/KM = measure of efficiency
Enzyme effectiveness depends on ability to bind substrates and rapidly convert

30. F. Analysis: Find Vmax , KM
High values of [S] lead to vo asymptotically reaching Vmax
Use linear plot
Lineweaver-Burk

31. G. Exceptions to M-M Model
1. Multi-substrate reaction

32. G. Exceptions to M-M Model
2. Multi-step reactions
3. Non-hyperbolic reaction

33. Substance that reduces an enzyme’s activity by influencing
VI. Inhibition
Substance that reduces an enzyme’s activity by influencing
Binding of substrate
Turnover number
Variety of mechanisms
Irreversible enzyme inhibitors
Inactivators
Reversible
Diminish enzyme’s activity by interacting reversibly
Structurally resemble substrates
Affect catalytic activity without interfering with substrate binding
A. Competitive Inhibition
Compete directly with normal substrate for binding site
Resemble substrate
Specifically binds to active site
Product inhibition

34. A. Competitive Inhibition
Transition state analogs
Depends on inhibitor binding selectively with RAPID equilibrium

35. A. Competitive Inhibition
M-M equation for competitive inhibition reaction

36. 1. KI can be measured

37. B. Transition State Inhibitors
Inhibitor binds to ES complex
Not to free enzyme

38. Interact in a way with the enzyme that affects substrate binding
C. Mixed Inhibition
Interact in a way with the enzyme that affects substrate binding

39. III. Allosteric Regulation
Organism must be able to regulate catalytic activities
Metabolic processes
Respond to changes in environment
Control of enzyme availability
Amount of given enzyme in a cell depends on its rate of synthesis and its rate of degradation
Control of enzyme activity
Catalytic activity controlled through structural alteration
Can cause large changes in enzymatic activity

40. Chymotrypsin