Chapter 2: Enzymes Flashcards Preview

MCAT: Biochemistry > Chapter 2: Enzymes > Flashcards

Flashcards in Chapter 2: Enzymes Deck (68):
1

What is a catalyst?

Do not impact the thermodynamics of a biological reaction

2

Does an enzyme change the equilibrium position of a reaction?

No, by definition, catalyst do not change the equilibrium position

3

Do enzymes affect the overall deltaG of a reaction?

No

4

How do enzymes increase the reaction rate?

By lowering the energy of activation

5

What are substrates?

The molecules upon which an enzyme acts

6

What are the major classifications of enzymes? What is the mnemonic?

- LI'L HOT
Ligase
Isomerase
Lyase
Hydrolase
Oxidoreductase
Transferase

7

In reactions catalyzed by oxidoreductases, the electron donor is known as the _____, and the electron acceptor is known as the ____.

donor: reductant
acceptor: oxidant

8

Enzymes with dehydrogenase, reductase, or oxidase are usually ______.

oxidoreductases

9

Enzymes with kinase are usually ____.

transferases

10

What do kinases catalyze?

The transfer of a phosphate group, generally from ATP, to another molecule

11

Phosphatase, peptidase, nuclease, and lipase are examples of which enzyme class?

Hydrolases

12

What do lyases catalyze?

- The cleavage of a single molecule into two products (without water)
- The synthesis of two molecules into a single molecule

13

What are synthases?

Lyases

14

What do ligases catalyze? What do they require?

- Addition or synthesis reactions
- Often require ATP

15

Differentiate endergonic and exergonic reactions.

Endergonic: one that requires energy input (deltaG > 0)
Exergonic: one in which energy is given off (deltaG <0)

16

Do enzymes affect the kinetics of a reaction?

Yes, by lowering the energy of activation

17

How do enzymes act? (3)

- By stabilizing the transition state
- Providing a favorable micro-environment
- Bonding with the substrate molecules

18

What is the active site of an enzyme?

The site of catalysis

19

What does the lock and key theory hypothesize?

That the enzyme and substrate are exactly complementary

20

What does the induced fit model hypothesize?

That the enzyme and substrate undergo conformational changes to interact fully

21

Differentiate cofactors and coenzymes.

- Cofactors: metal cation (minerals)
- Coenzymes: small organic (vitamins)

22

Differentiate apoenzymes and holoenzymes.

Apoenzymes: enzymes without their cofactors
Holoenzymes: enzymes with their cofactors

23

What are prosthetic groups?

Tightly bound cofactors or coenzymes

24

When is an enzyme working at maximum velocity?

When it has become fully saturated, adding more substrate will not increase the rate of reaction

25

When the reaction rate is equal to half of vmax, km = ?

km = (S)

26

What is the Michaelis-Menton equation?

v = (vmax (S)) / (Km + (S))

27

A low Km reflects a ____ affinity for the substrate?

high

28

What are the two definitions of Km?

- (S) at which half of the enzyme's active sites are full
- Measure of the affinity of the enzyme for its substrate

29

Can you alter Km by changing the concentration of substrate or enzyme?

No

30

What is the intercept of the line with the x-axis in the Lineweaver-Burk plot?

- (1/Km)

31

What is the intercept of the line with the y-axis in the Lineweaver-Burk plot?

1/v

32

Why do certain enzymes show sigmoidal (S-shaped) kinetics and not the normal hyperbola in the v vs (S) plot?

Due to cooperativity among substrate binding sites

33

What are cooperative enzymes?

They have multiple subunits and multiple active sites

34

What is cooperativity?

Refers to the interactions between subunits in a multisubunit enzyme or protein

35

Subunits and enzymes may exist in one of two states. What are they?

- Low-affinity tense state (T)
- High-affinity relaxed state (R)

36

In a cooperative enzyme, what does the binding of the substrate encourage?

- Transition from T state to the R state
- Increases the likelihood of substrate binding by these other subunits

37

In a cooperative enzyme, what does the loss of the substrate encourage?

- Transition from the R state to the T state
- Promotes dissociation of substrate from the remaining subunits

38

How can cooperative enzymes be subject to activation and inhibition?

Competitively and through allosteric sites

39

Give examples of cooperative binding.

- Hemoglobin (acts as a transport protein rather than an enzyme)
- Regulatory enzymes in pathways (ex: PFK-1)

40

What are the effects of increasing (E)?

Will always increase vmax, no matter the starting concentration

41

What are the axes in the M&M graph? What kind of curve does that create for monomeric enzymes?

- v vs. (S)
- Hyperbolic curve

42

What are the axes in the Lineweaver-Burk graph? What kind of curve does that create for monomeric enzymes?

- 1/v vs. 1/(S)
- Straight line

43

How do the terms enzyme activity, enzyme velocity, and enzyme rate differ?

They are synonyms; do not differ

44

For every 10oC increase, enzyme activity ____

doubles

45

What is the optimal temperature for the human body?

37oC = 98.6oF = 310 K

46

What are the two reasons that explain why enzymes depend on pH to function properly?

- pH affects the ionization of the active site
- Changes in pH can lead to denaturation of the enzyme

47

How can altering the concentration of salt change enzyme activity in vitro?

Changes in salinity can disrupt bonds within an enzyme, causing disruption of tertiary and quaternary structures, which leads to loss of enzyme function

48

Define feedback inhibition.

A regulatory mechanism whereby the catalytic activity of an enzyme is inhibited by the presence of high levels of a product later in the same pathway

49

What is reversible inhibition?

The ability to replace the inhibitor with a compound of greater affinity or to remove it using mild laboratory treatment

50

What is irreversible inhibition?

Alters the enzyme in such a way that the active site is unavailable for a prolonged duration or permanently; new enzyme molecules must be synthesized for the reaction to occur again

51

What are the four types of reversible inhibition?

- Competitive inhibition
- Noncompetitive inhibition
- Mixed inhibition
- Uncompetitive inhibition

52

What is competitive inhibition? Where does the inhibitor bind? How do vmax and Km change?

- Inhibitor is similar to substrate and binds to the active site
- Vmax unchanged
- Km increases

53

Which inhibition can be overcome by adding more substrate?

Competitive inhibition

54

What is noncompetitive inhibition? Where does the inhibitor bind? How do vmax and Km change?

- Inhibitor binds with equal affinity to the enzyme and the enzyme-substrate complex
- Vmax decreases
- Km unchanged

55

What is mixed inhibition? Where does the inhibitor bind? How do vmax and Km change?

- Inhibitor binds with unequal affinity to the enzyme and the enzyme-substrate complex
- Vmax decreases
- Km increased or decreased depending on if the inhibitor has higher affinity for the E or the ES complex

56

What is uncompetitive inhibition? Where does the inhibitor bind? How do vmax and Km change?

- Inhibitor binds only to the ES complex ONCE the substrate has already bound
- Vmax decreases
- Km decreases

57

What are allosteric sites?

Non-catalytic regions of the enzyme that bind regulators

58

If a mixed inhibitor preferentially binds to the enzyme, what happens to the Km value?

Increases Km
Lowers affinity

59

If a mixed inhibitor preferentially binds to the ES complex, what happens to the Km value?

Decreases Km
Increases affinity

60

Give a popular real-world example for irreversible inhibition.

Aspirin

61

What do allosteric enzymes possess?

Multiple binding sites

62

Differentiate allosteric activators and inhibitors.

Activators: shift that makes the active site more available for binding
Inhibitors: shift that makes the active site less available for binding

63

Give examples of transient modifications.

Allosteric activation or inhibition

64

Give examples of covalent modifications.

Phosphorylation/Dephosphorylation and glycosylation

65

What is a zymogen?

Precursors of an active enzyme

66

What is the role of zymogens? Give an example.

Critical that certain enzymes (digestive enzymes of the pancreas) remain inactive until arriving at their target site

67

What are zymogens composed of?

- Contain a catalytic (active) domain and a regulatory domain
- Regulatory domain must be either removed or altered to expose the active site

68

What is the suffix to most zymogens?

-ogen