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BDS1 BAMS: Molecules and Cells > Enzymes > Flashcards

Enzymes Flashcards

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1
Q

What is an enzyme?

A
  • A biological catalyst/catalyst in living cells

- Speeds up chemical reactions

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2
Q

What effect do enzymes have on the activation energy required for the reaction to take place?

A

Enzymes lower the activation energy required for the reaction to take place

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3
Q

What effect do enzymes have on the equilibrium of the reaction?

A

Enzymes do not alter the equilibrium of the reaction; they accelerate the TIME taken to reach equilibrium

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4
Q

Why are enzymes important?

A
  • Vital for life: catalyse chemical reactions (our metabolism) in cells that keep us alive
  • 1000s of roles in living organisms including aiding respiration, digestion, muscle and nerve function, etc
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5
Q

What does amylase do?

A

Converts starch into sugars

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6
Q

Where is amylase found?

A

In our saliva

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7
Q

What is lysozyme?

A

An antimicrobial, breaks down peptidoglycan in cell wall of bacteria, found in secretions e.g. tears, human milk, mucous

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8
Q

What is maltase?

A

An enzyme in saliva that breaks down maltose into glucose

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9
Q

What is the ‘lock and key’ model?

A
  • The enzyme active site is complementary in shape to that of the substrate
  • Active site is precisely shaped to hold specific substrates
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10
Q

What is the ‘induced fit model’?

A
  • Active site of enzyme and substrate do not fit together exactly
  • Enzyme changes shape when substrate binds
  • Active site of enzyme has a shape complementary to the substrate only AFTER the substrate has bound
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11
Q

What are the only known enzymes that are not proteins?

A

Ribozymes - RNA molecules with enzymatic activity

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12
Q

What does sucrase break sucrose down into?

A

Glucose and fructose

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13
Q

Which 2 factors can affect enzyme activity?

A

pH and temperature

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14
Q

What can happen to an enzyme at extremes of pH or temperature?

A

The enzyme may become denatured (lose its 3D shape) and become totally inactive

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15
Q

What is the standard free energy change (△G)?

A

The difference between energies of the reactants and the products

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16
Q

What is the activation energy (EA)?

A

The energy input required to initiate the reaction

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17
Q

What does the reaction rate depend on?

A
  • The activation energy
  • Temperature
  • pH
  • Enzyme concentration
  • Substrate concentration
  • Inhibitors and activators
  • Covalent modification
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18
Q

What effect does an increase in enzyme concentration have on the reaction rate?

A

Increases the reaction rate - the equilibrium is reached more rapidly

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19
Q

What is Vmax?

A

The maximum rate of reaction (enzyme is saturated with substrate)

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20
Q

What does the relationship between the reaction rate (V) and substrate concentration ([S]) depend on?

A

Affinity of the enzyme for its substrate

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21
Q

What is the Michaelis constant?

A

Km - an inverse measurement of affinity

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22
Q

What does Km tell us?

A

Km is the Michaelis constant - an inverse measurement of affinity
Km is the concentration of substrate at which the rate of reaction is half its maximum value

23
Q

What does a low Km mean?

A

The enzyme is normally saturated with substrate and acts at a fairly constant rate regardless of variations in substrate concentration.

24
Q

What does a high Km mean?

A

The enzyme is not normally saturated with substate, its activity will vary with as substrate concentration varies. Rate of product formation depends on substrate availability.

25
Q

Why is measuring the initial rate (Vi) of an enzyme-catalysed reaction important?

A

Because substrate concentration changes with time

26
Q

What is the optimal temperature for a typical human body enzyme?

A

37.5 degrees C

27
Q

What is the Michaelis-Menten equation?

A

Represents a graph of V against [S]

V = Vmax[S]/(Km + [S])

28
Q

What is Km and how is it calculated?

A

Km is the substrate concentration at which V = 0.5 x Vmax

Km is a measure of affinity of the enzyme for its substate

29
Q

What does a low Km mean?

A

Enzyme has a high affinity for its substate

30
Q

What is dextran?

A

A polymer of glucose - one of the main components of dental plaque

31
Q

Which enzyme makes dextran and what is the substrate?

A

Dextran sucrase

Substrate = sucrose

32
Q

What is the Lineweaver-Burk plot?

A
  • A graph plotting 1/v against 1/[S]

- Give a straight line for most enzymes

33
Q

What do the intercepts at the x-axis and y-axis on a Lineweaver-Burk plot tell us, respectively?

A

Intercept on x-axis: -1/Km

Intercept on y-axis: 1/Vmax

34
Q

What effect do reversible inhibitors have on enzymes?

A
  • Full enzyme activity is regained when the inhibitor is removed
  • Most natural enzyme inhibitors fall into this category
35
Q

What effect do irreversible inhibitors have on enzymes?

A
  • Damage the enzyme beyond repair

- Generally cause covalent modification of the enzyme

36
Q

Are most enzyme inhibitors reversible of irreversible?

A

Reversible

37
Q

What is feedback inhibition?

A
  • When the product of a metabolic pathway builds up in the cell, inhibition of the pathway often occurs
  • Product of the pathway acts as an inhibitor of an earlier step in the pathway
38
Q

Describe feedback inhibition of threonine deaminase by isoleucine

A
  • If isoleucine is not metabolised by the cell, it accumulates and switches off further synthesis.
  • Isoleucine acts as an allosteric inhibitor of threonine deaminase (first enzyme in the pathway)
39
Q

On a Lineweaver-Burk plot, what does the steepness of the slope tell us about the speed of the reaction?

A

The less steep the slope, the faster the reaction is overall

40
Q

What is a competitive inhibitor?

A
  • Substrate and inhibitor compete for the same binding site on the enzyme. They are structural analogues
  • Competitive inhibitors prevent the substrate from binding
  • Inhibition can be overcome by adding large amounts of substrate - substrate will ultimately occupy all binding sites
  • This is an example of reversible inhibition
41
Q

What is a non-competitive inhibitor?

A
  • Structurally unrelated to the substrate
    Binds to an allosteric site on the enzyme i.e. one other than the active site
  • Alters the conformation of the enzyme so that the active site is no longer fully functional
  • Inhibition cannot be reversed by adding large quantities of substrate
42
Q

Describe the effects on Vmax in the presence of competitive inhibitors

A
  • Vmax is unaltered because a large amount of the substrate can outcompete the inhibitor
43
Q

Describe the effects on Km when competitive inhibitors are added

A
  • Km is increased - as the inhibitor competes with the substrate, the affinity for the the substrate will appear to decrease
  • (Km is an inverse measurement of affinity of enzyme for substrate)
44
Q

Describe the effects on Vmax when non-competitive inhibitors are added

A
  • Vmax is decreased: a proportion of enyzmes molecules will be inactive at any time due to binding by the inhibitor
45
Q

Describe the effect on Km when non-competitive inhibitors are added

A
  • Km is unchanged: binding of substrate can still occur
46
Q

What is an allosteric enzyme?

A
  • Allosteric enzymes possess multiple subunits
  • In addition to active sites, they also possess regulatory (allosteric) sites to which non-substrate modulators bind, creating conformational change in subunits
47
Q

What effect does a positive modulator have on an allosteric enzyme?

A

Increases affinity for substrate at active site

48
Q

What effect does a negative modulator have on an allosteric enzyme?

A

Decreases affinity for substrate at active site

49
Q

Which type of enzymes do NOT conform to Michaelis-Menten kinetics?

A
  • Allosteric enzymes

- Graph of V vs [S] gives a sigmoidal rather than hyperbolic curve; Lineweaver-Burk plot cannot be used

50
Q

Which amino acids is a phosphate group often adde to in covalent modification of enzymes by another enzyme?

A

A common modification is the addition of a phosphate group to serine, threonine or tyrosine hydroxyl group

51
Q

What does glycogen phosphorylase do?

A

An enzyme in liver and muscle which breaks down glycogen to provide glucose; it is activated by phosphorylation

52
Q

Which enzyme phosphorylates glycogen phosphorylase?

A

Phosphorylase kinase

53
Q

Which enzyme removes the phosphate group on glycogen phosphorylase?

A

Phosphorylase phosphatase

BDS1 BAMS: Molecules and Cells (13 decks)

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