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BIOC192 > Lecture 12: HOW FAST IS THAT ENZYME? > Flashcards

Lecture 12: HOW FAST IS THAT ENZYME? Flashcards

1
Q

What is shown on the lineweaver-burk plot?

A

The double reciprocal of the michaelis-menten equation: 1/v=km/vmax x 1/[S] +1/vmax

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2
Q

What is on the x axis of the lineweaver burk plot?

A

1/[S]

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3
Q

What is on the y axis of the lineweaver burk plot?

A

1/vo

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4
Q

What is the slope of the lineweaver-burk plot?

A

km/Vmax

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5
Q

What is the x intercept of the lineweaver-burk plot?

A

-1/Km

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6
Q

What is the y-intercept of the lineweaver-burk plot?

A

1/vmax

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7
Q

What does km characterise?

A

One enzyme-substrate pair (if an enzyme can act on different substrates, it will have different Km values for each

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8
Q

What is the Km?

A

The [S] needed to reach half of vmax

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9
Q

What are the units of Km?

A

The units of concentration (mmol/L)

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10
Q

What is Km formally?

A

k-1 +k2 /k1

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11
Q

How do k2 and k-1 compare in many enzymes?

A

k2«

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12
Q

What is the more useful equation for Km?

A

k-1/k1 (the ES dissociation constant)

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13
Q

What does a low km mean?

A

High affinity between E and S

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14
Q

What does a high Km mean?

A

A low affinity between E and S

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15
Q

How is the [S] in a cell for a particular enzyme-substrate interaction?

A

Often below the Km

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16
Q

What does [S] being below Km in a cell mean?

A

Rate will rise to accommodate more substrate, tending to maintain steady state

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17
Q

What does hexokinase do?

A

Generates energy in muscle and brain (wants to use it straight away)

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18
Q

What is hexokinase Km for ATP?

A

0.04mM

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19
Q

What is hexokinase Km for glucose?

A

0.05mM

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20
Q

What is an isozyme of hexokinase?

A

Glucokinase

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21
Q

What does glucokinase do?

A

Store energy as glycogen in the liver (not used straight away)

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22
Q

What is the km of glucokinase for glucose?

A

6mM

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23
Q

What is the Km for each isozyme and substrate?

A

Different

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24
Q

What are the classes of inhibitor?

A

Irreversible and reversible (competitive or noncompetitive- pure or mixed)

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25
Q

How do irreversible inhibitors bind?

A

Covalently to the enzyme

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26
Q

How do reversible inhibitors bind?

A

Not covalently bound to the enzyme

27
Q

What do irreversible inhibitors do?

A

Bind to the enzyme and permanently inactivate it

28
Q

What do inhibitors react with?

A

A specific amino acid side chain, usually in the active site, and forms a covalent bond

29
Q

What do covalent inhibitors often react with?

A

Catalytic residues

30
Q

What happens with chymotrypsin (inhibitors)?

A

Addition of the bulky toysl-L-phenylalanine methylketone to the histidine disables the catalytic triad and fills the active site, blocking substrate binding

31
Q

What does a reversible inhibitor do?

A

Binds to the enzyme but can subsequently be released, leaving the enzyme in its original condition

32
Q

What happens in competitive inhibitor?

A

Inhibitor competes with the substrate for binding the active site (steric hinderance)

33
Q

What happens to the vmax in competitive inhibition?

A

No change (high [S] outcompetes the inhibitor) - same y intercept

34
Q

What happens to the Km in competitive inhibitor?

A

Increases (more substrate is needed to get v=vmax/2) - different x intercept

35
Q

What are often targets for drugs?

A

Enzymes

36
Q

What can transition state analogues do?

A

Make ideal enzyme inhibitors (competitive)

37
Q

What do enalapril and aliskiren do?

A

Lower blood pressure

38
Q

What do statins do?

A

Lower serum cholesterol

39
Q

What are protease inhibitors?

A

AIDS drugs

40
Q

What is juvenile hormone esterase?

A

A pesticide target

41
Q

What is tamiflu?

A

An inhibitor of influenza heuraminidase

42
Q

What is kcat?

A

The number of substrate molecules converter to product, per enzyme, per unit of time, when E is saturated with substrate

43
Q

What does kcat help to define?

A

The activity of one enzyme molecule - a measure of catalytic activity

44
Q

What is kcat if the Michaelis mention equation fits?

A

K2 - the rate determining step

45
Q

What should the kcat of effective enzymes be?

A

high (ability to turnover a lot of substrate into product per second)

46
Q

What should the km of effective enzymes be?

A

Low (low substrate concentration required to achieve near vmax, high affinity for the substrate under the Michaelis-menten assumptions)

47
Q

What is kcat/km?

A

An overall measure of enzyme efficiency, the higher the better

48
Q

What equation has kcat in it?

A

vmax=kcat[E]t

49
Q

What is the upper limit for kcat/km?

A

The diffusion controlled limit, the rate at which enzyme and substrate diffuse together

50
Q

What sets an absolute upper limit for kcat/km?

A

The viscosity of water (~10^9s-1M-1)

51
Q

What are referred to as perfect catalysts?

A

Enzymes with kcat/km above 10^8s-1M-1

52
Q

What is an inhibitor?

A

A compound that binds to an enzyme and reduces its activity

53
Q

Why are inhibitors important?

A
  • natural inhibitors regulate metabolism
  • many drugs, poisons and toxins are enzyme inhibitors
  • used to study enzyme mechanisms
  • used to study metabolic pathways
54
Q

What do transition state analogues make?

A

Tight binding inhibitors

55
Q

What does adenosine deaminase use?

A

A tetrahedral intermediate

56
Q

What actively inhibits adenosine deaminase?

A

A non-reactive analog, 1,6-dihydroinosine

57
Q

What can an alternate substrate do?

A

Compete for the active site of an enzyme. Typically, related molecules fail to react, but some do

58
Q

What happens in non-competitive inhibitor?

A

The inhibitor binds at a different site than the substrate, it is an allosteric inhibitor

59
Q

What can the enzyme bind in non-competitive inhibitor?

A

Substrate, or inhibitor or both

60
Q

What happens in pure non-competitive inhibition?

A

The inhibitor has no effect on the binding of substrate - the substrate binds to E and EI with the same affinity

61
Q

What does binding a non-competitive inhibitor do?

A

Changes the structure of the active site such that S still binds, but transition state stabilisation is no longer optimal

62
Q

What happens to the vmax in non-competitive inhibition?

A

Decreases (different y intercept)

63
Q

What happens to the Km in non-competitive inhibition?

A

Stays the same (same x intercept)

BIOC192 (38 decks)

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