Lecture 3: Regulation and Inhibition of Enzymes

Question 1

___ bind substrate(s), increase rate of reaction, and are NOT consumed during reaction

Answer 1

Enzymes

Question 2

____: Special pocket formed by protein folding that contains amino acids that participate in substrate binding and catalysis, which chemically facilitates a reaction

Answer 2

Active Site

Question 3

_____forms upon binding of substrate to the enzyme and leads to conformation change

Answer 3

Enzyme-substrate (ES) complex

Question 4

The ___ changes shape slightly as substrate binds

Answer 4

Enzyme

Question 5

Kcat (_____) = molecules substrate converted to
product(s) per molecule enzyme per second

Answer 5

Turnover number

Question 6

True or False: Enzymes have
catalytic efficiency and specificity

Answer 6

True

(enzymes usually interact with one or very few substrates and catalyze only
one type of reaction = specificity)

Question 7

_____:non-protein molecules that enhance
enzyme activity

Answer 7

Co-factor/Co-enzyme

Question 8

Zinc or iron is an example of a ____while small organic molecule derived from vitamins (NAD+) are examples of ____

Answer 8

cofactor; coenzyme

Question 9

___: an active enzyme associated with its non-protein
component

____: inactive enzyme without its non-protein component

Answer 9

Holoenzyme

Apoenzyme

Question 10

Enzymes provide a more energetically favorable path from reactants to
products by lowering the ______ barrier

Answer 10

activation energy

Question 11

True or False; Each reaction has an activation energy barrier separating the substrates (reactants) and the products

Answer 11

True

Question 12

____: energy difference
between reactants and transition state

Answer 12

Free energy of activation

Question 13

_____: high-energy intermediate formed
prior to product formation

Answer 13

Transition state (T*)

Question 14

Enzymes ↓ free energy of activation required to reach ___

Answer 14

Transition state (T*)

Question 15

True or False: Free energy of overall reaction (ΔG) is unchanged when enzyme acts on a reaction

Answer 15

True

Question 16

True or False: There is a difference in the free energy of the overall reaction (energy of reactants - energy of products) between the catalyzed and uncatalyzed reaction

Answer 16

False - there is NO difference in the free energy

Question 17

A ____ ⇆ T* (high-energy intermediate) ⇆ B (product)

Answer 17

A (substrate)

Question 18

A reaction occurs if molecules contain enough ___ to overcome the energy
barrier to achieve T*

Answer 18

energy

Question 19

Reaction _____ is determined by # of molecules that have sufficient
energy to overcome the barrier

Answer 19

rate or velocity

Question 20

True or False: If free energy of activation is low: less molecules have enough energy to reach T* and reaction proceeds more slowly

Answer 20

False - If free energy of activation is low: MORE molecules have enough energy to reach T* and reaction proceeds more quickly

Question 21

Why do enzymatic reactions have a more rapid reaction rate?

Answer 21

1) Enzyme provide an alternate reaction pathway
2) lower free energy of activation
3) more molecules possess enough energy to overcome barrier

Question 22

An enzyme’s ____ is a complex molecular machine that facilitates
conversion of substrate into product

Answer 22

active site

Question 23

Substrate binds and initiates conversion into ___

Answer 23

Substrate binds and initiates conversion into T*

Question 24

Question 25

What do amino acid residues at the active site do?

Answer 25

1. Donate or accept protons 2. Hydrogen bonding to promote T state formation

Question 26

True or False: Enzymes are usually least responsive to changes in [S]

Answer 26

False - Enzymes are usually most responsive to changes in [S]

Question 27

_____: # of substrate molecules converted to product per unit of time when enzyme levels are constant

Answer 27

Rate or velocity (V) of enzymatic reactions

Question 28

What three factors influence enzymatic reaction velocity?

Answer 28

* substrate concentration [S] * Temperature * pH

Question 29

With constant [E], reaction rate increases with ↑[S] until reaching ___

Answer 29

Vmax (max velocity)

Question 30

High [S] eventually leads to ____ of all binding sites on enzymes, which causes ______ to level off

Answer 30

saturation; reaction rate

Question 31

True or False: Once Vmax is reached, reaction rate increases with addition of more substrate

Answer 31

Once Vmax is reached, reaction rate does NOT increase with addition of more substrate

Question 32

True or False: Increasing temperature generally increases the rate of a reaction

Answer 32

True

Question 33

What happens to enzymes at extremely high or low pH?

Answer 33

Enzyme denatured and no longer functional

Question 34

An abnormal pH affects functionality of amino acids located in the active site and lowers what variable?

Answer 34

Vmax

Question 35

Answer 35

Michaelis-Menten kinetics

Question 36

Enzymes following Michaelis-Menten Kinetics show a ____ curve. Allosteric enzymes show a ___ curve

Answer 36

hyperbolic; sigmoid

Question 37

___: describes how reaction velocity varies with [S]

Answer 37

Michaelis-Menten Kinetics

Question 38

True or False: Enzyme (E) reversible combines with substrate (S) to form an enzyme- substrate (ES) complex, which yields a product. Free energy is generated.

Answer 38

Question 39

What assumption does MM-Kinetics make?

Answer 39

1) [S] >> [E] 2) rate of ES formation = rate of ES breakdown

Question 40

MM Equation?

Answer 40

Question 41

Answer 41

Question 42

Km =

Answer 42

[S] at ½ Vmax

Question 43

Km demonstrates the affinity of enzyme for _____

Answer 43

substrate

Question 44

Small Km means ___ affinity of enzyme for substrate and ___ [S] needed to reach Vmax

Answer 44

high; low

Question 45

Large Km means ___ affinity of enzyme for substrate and __ [S] needed to reach Vmax

Answer 45

low; high

Question 46

Answer 46

Question 47

Answer 47

Question 48

For Lineweaver burk plot:

Answer 48

Question 49

If an inhibitor is covalently bound to enzyme, preventing it from interacting with S and resulting in “loss” of enzyme activity (i.e., lead poisoning), it is likely to be what type of inhibitor?

Answer 49

Irreversible Inhibitor

Question 50

True or False: Reversible inhibitors use non-covalent binding with enzyme, which allows for recovery of enzyme

Answer 50

True

Question 51

What are the two types of reversible inhibitors?

Answer 51

-Competitive -Noncompetitive

Question 52

Why are competitive inhibitors called "competitive"

Answer 52

They bind to SAME site as substrate – in that way, then, they compete with S to bind enzyme’s active site

Question 53

Answer 53

Question 54

How does a competitive inhibitor affect... -Km apparent? -Vmax?

Answer 54

Vmax = unchanged Km apparent = increased

Question 55

True or False: Statin drugs (like: Pravastin) are examples of non-competitive inhibitors?

Answer 55

False - Statin drugs (like: Pravastin) are examples of competitive inhibitors

Question 56

Statin drugs inhibit rate-limiting step in ______ (HMG CoA => Mevalonic Acid)

Answer 56

cholesterol biosynthesis

Question 57

Statin drugs compete with substrate HMG-CoA for binding to _____

Answer 57

HMG CoA-reductase

Question 58

What type of inhibitor bind to different sites than the substrate and prevents an enzymatic reaction from occurring?

Answer 58

Non-competitive inhibitor

Question 59

How is it possible that non-competitive inhibitors can bind to either: a free enzyme or the ES complex?

Answer 59

The inhibitor binds to a different place than the substrate

Question 60

Answer 60

Question 61

How do non-competitive inhibitors affect.... -Km? -Apparent Vmax?

Answer 61

Km = unchanged Apparent Vmax = decreased

Question 62

True or False: For non-competitive inhibitors, large quantities of substrate CAN overcome inhibition

Answer 62

False - for non-competitive inhibitors, even large quantities of substrate cannot overcome inhibition

Question 63

Answer 63

Question 64

What type of inhibitors are Penicillin and Amoxicillin? Mechanism of action?

Answer 64

Irreversible Inhibitors Inhibition of bacterial cell wall synthesis

Question 65

What type of inhibitors are ACE inhibitors (captopril, enalapril, and lisinopril)?

Answer 65

Reversible Competitive Inhibitors

Question 66

NSAIDs are all ____inhibitors. However: acetylsalicylic acid (aspirin) is ____while ibuprofen is _____

Answer 66

competitive irreversible reversible

Question 67

True or False: Increase [S] increases reaction rate/velocity

Answer 67

True

Question 68

Other mechanisms (3) involved in regulating enzyme activity?

Answer 68

1) Allosteric regulation 2) Covalent modification 3) Induction/repression of enzyme synthesis

Question 69

True or False: Allosteric enzymes usually contain only one subunit and catalyze the last step in a pathway

Answer 69

False - allosteric enzymes often contain multiple subunits and frequently catalyze the committed, rate-limiting step in a pathway

Question 70

True or False: Allosteric enzymes do not follow M-M kinetics

Answer 70

True (makes a sigmoidal curve)

Question 71

What type of molecules regulate allosteric enzymes?

Answer 71

Effector molecules

Question 72

_____: bind noncovalently at a site other than active site

Answer 72

Effector molecules

Question 73

What type of effector reduces or inhibits enzyme activity? What type of effector stimulates or increases enzyme activity?

Answer 73

Negative effector Positive effector

Question 74

What can effectors modify?

Answer 74

1) Vmax 2) Affinity of enzyme for S (K0.5) 3) or both

Question 75

Homotropic effector are usually ___ effectors

Answer 75

Positive

Question 76

Binding of substrate at allosteric site enhances activity at substrate- binding site. This is known as ____

Answer 76

cooperativity

Question 77

For a homotropic effector, what type of curve is visible? Why?

Answer 77

Sigmoidal curve 1) higher [S] initially needed for reaction to proceed 2) substrate must bind at two different sites

Question 78

True or False: In homotropic effectors, substrate = effector

Answer 78

True

Question 79

True or False: In a heterotropic effector, substrate does not equal effector

Answer 79

True

Question 80

____: downstream product in pathway serves as effector for an enzyme required in an upstream reaction

Answer 80

Feedback Inhibition

Question 81

An example of covalent modifications is: phosphorylation of specific ___, ___, or ____ residues:

Answer 81

Ser, Thr or Tyr

Question 82

True or False: Phosphorylation only inhibits its target

Answer 82

False - phosphorylation may activate or inhibit its target enzyme

Question 83

What are two ways to induce/repress enzyme synthesis?

Answer 83

1) Change rate of synthesis or degradation of enzyme 2) Slow mode of regulation (hours to days)

Question 84

True or False: Allosteric and covalent modifications are slower

Answer 84

False - they are more rapid!(seconds to minutes)