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MCAT: Biochemistry > Chapter 2: Enzymes > Flashcards

Chapter 2: Enzymes Flashcards

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MCAT flashcards
1
Q

What is a catalyst?

A

Do not impact the thermodynamics of a biological reaction

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2
Q

Does an enzyme change the equilibrium position of a reaction?

A

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

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3
Q

Do enzymes affect the overall deltaG of a reaction?

A

No

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4
Q

How do enzymes increase the reaction rate?

A

By lowering the energy of activation

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5
Q

What are substrates?

A

The molecules upon which an enzyme acts

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6
Q

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

A 
- LI'L HOT
Ligase
Isomerase
Lyase
Hydrolase
Oxidoreductase
Transferase
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7
Q

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

A

donor: reductant
acceptor: oxidant

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8
Q

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

A

oxidoreductases

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9
Q

Enzymes with kinase are usually ____.

A

transferases

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10
Q

What do kinases catalyze?

A

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

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11
Q

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

A

Hydrolases

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12
Q

What do lyases catalyze?

A
  • The cleavage of a single molecule into two products (without water)
  • The synthesis of two molecules into a single molecule
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13
Q

What are synthases?

A

Lyases

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14
Q

What do ligases catalyze? What do they require?

A
  • Addition or synthesis reactions

- Often require ATP

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15
Q

Differentiate endergonic and exergonic reactions.

A

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

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16
Q

Do enzymes affect the kinetics of a reaction?

A

Yes, by lowering the energy of activation

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17
Q

How do enzymes act? (3)

A
  • By stabilizing the transition state
  • Providing a favorable micro-environment
  • Bonding with the substrate molecules
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18
Q

What is the active site of an enzyme?

A

The site of catalysis

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19
Q

What does the lock and key theory hypothesize?

A

That the enzyme and substrate are exactly complementary

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20
Q

What does the induced fit model hypothesize?

A

That the enzyme and substrate undergo conformational changes to interact fully

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21
Q

Differentiate cofactors and coenzymes.

A
  • Cofactors: metal cation (minerals)

- Coenzymes: small organic (vitamins)

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22
Q

Differentiate apoenzymes and holoenzymes.

A

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

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23
Q

What are prosthetic groups?

A

Tightly bound cofactors or coenzymes

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24
Q

When is an enzyme working at maximum velocity?

A

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

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25
Q

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

A

km = (S)

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26
Q

What is the Michaelis-Menton equation?

A

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

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27
Q

A low Km reflects a ____ affinity for the substrate?

A

high

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28
Q

What are the two definitions of Km?

A
  • (S) at which half of the enzyme’s active sites are full

- Measure of the affinity of the enzyme for its substrate

29
Q

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

A

No

30
Q

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

A
  • (1/Km)
31
Q

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

A

1/v

32
Q

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

A

Due to cooperativity among substrate binding sites

33
Q

What are cooperative enzymes?

A

They have multiple subunits and multiple active sites

34
Q

What is cooperativity?

A

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

35
Q

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

A
  • Low-affinity tense state (T)

- High-affinity relaxed state (R)

36
Q

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

A
  • Transition from T state to the R state

- Increases the likelihood of substrate binding by these other subunits

37
Q

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

A
  • Transition from the R state to the T state

- Promotes dissociation of substrate from the remaining subunits

38
Q

How can cooperative enzymes be subject to activation and inhibition?

A

Competitively and through allosteric sites

39
Q

Give examples of cooperative binding.

A
  • Hemoglobin (acts as a transport protein rather than an enzyme)
  • Regulatory enzymes in pathways (ex: PFK-1)
40
Q

What are the effects of increasing (E)?

A

Will always increase vmax, no matter the starting concentration

41
Q

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

A
  • v vs. (S)

- Hyperbolic curve

42
Q

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

A
  • 1/v vs. 1/(S)

- Straight line

43
Q

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

A

They are synonyms; do not differ

44
Q

For every 10oC increase, enzyme activity ____

A

doubles

45
Q

What is the optimal temperature for the human body?

A

37oC = 98.6oF = 310 K

46
Q

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

A
  • pH affects the ionization of the active site

- Changes in pH can lead to denaturation of the enzyme

47
Q

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

A

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

48
Q

Define feedback inhibition.

A

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
Q

What is reversible inhibition?

A

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

50
Q

What is irreversible inhibition?

A

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
Q

What are the four types of reversible inhibition?

A
  • Competitive inhibition
  • Noncompetitive inhibition
  • Mixed inhibition
  • Uncompetitive inhibition
52
Q

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

A
  • Inhibitor is similar to substrate and binds to the active site
  • Vmax unchanged
  • Km increases
53
Q

Which inhibition can be overcome by adding more substrate?

A

Competitive inhibition

54
Q

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

A
  • Inhibitor binds with equal affinity to the enzyme and the enzyme-substrate complex
  • Vmax decreases
  • Km unchanged
55
Q

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

A
  • 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
Q

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

A
  • Inhibitor binds only to the ES complex ONCE the substrate has already bound
  • Vmax decreases
  • Km decreases
57
Q

What are allosteric sites?

A

Non-catalytic regions of the enzyme that bind regulators

58
Q

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

A

Increases Km

Lowers affinity

59
Q

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

A

Decreases Km

Increases affinity

60
Q

Give a popular real-world example for irreversible inhibition.

A

Aspirin

61
Q

What do allosteric enzymes possess?

A

Multiple binding sites

62
Q

Differentiate allosteric activators and inhibitors.

A

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

63
Q

Give examples of transient modifications.

A

Allosteric activation or inhibition

64
Q

Give examples of covalent modifications.

A

Phosphorylation/Dephosphorylation and glycosylation

65
Q

What is a zymogen?

A

Precursors of an active enzyme

66
Q

What is the role of zymogens? Give an example.

A

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

67
Q

What are zymogens composed of?

A
  • Contain a catalytic (active) domain and a regulatory domain
  • Regulatory domain must be either removed or altered to expose the active site
68
Q

What is the suffix to most zymogens?

A

-ogen

MCAT: Biochemistry (12 decks)

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