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Bio chem > Lecture 12 - Enzyme rate (Inhibition & Activation) > Flashcards

Lecture 12 - Enzyme rate (Inhibition & Activation) Flashcards

1
Q

The Michaelis-Menten equation

A

describes The V vs. [S] curve

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

based on model reaction

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2
Q

model reaction

A

E + S ⟷ ES ⟶ E + P

⟷

  • K1
  • K-1

⟶
- K2

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3
Q

Vmax

A

how fast it can go if all enzyme is in ES complex

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4
Q

The more ES complex

A

faster it will go

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5
Q

Lineweaver - Burk Plot

X intercept

Y intercept

A

Double reciprocal
1 / S concentration vs 1 / V (kinetic rate)
Linear

-1 / Km

1 / Vmax

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6
Q

Significance of KM

A

substrate concentration needed to reach half Vmax

units mmol/ L

Characterises one enzyme-substrate pair (if an enzyme can act on different substrates, it will have different KM values for each).

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7
Q

For many enzymes k 2 &laquo_space;k -1 , so approximation neglects k 2 :

A

Km = K-1 / K1

the ES dissociation constant

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8
Q

Low KM

A

high affinity between E and S;

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9
Q

high KM

A

low affinity between E and S;

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10
Q

Physiological significance of KM

in enzyme-substrate interaction,

A

[S] is below the KM.

rate will rise to accommodate more substrate, tending to maintain steady state.

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11
Q

KM: substrate preference and response

KM for each isozyme and substrate is same or different?

A

different

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12
Q

Isozyme glucokinase

A

stores energy as glycogen in the liver.

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13
Q

Turnover number, kcat

A

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

define the activity of one enzyme molecule – a measure of catalytic activity.

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14
Q

If the Michaelis-Menten model fits , kcat = k2

kcat describes

A

the ‘rate limiting’ step.

Vmax = kcat [E]T

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15
Q

most effective enzymes should have…

A

A high kcat

A low KM

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16
Q

kcat / KM measure

A

enzyme efficiency;

the higher the better.

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17
Q

A high kcat

A

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

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18
Q

A low KM

A

(low substrate concentration required to achieve near Vmax; high affinity for the substrate under the Michaelis-Menten assumptions).

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19
Q

kcat, KM and “catalytic perfection”

The upper limit for kcat / KM is the

A

diffusion-controlled limit; i.e. the rate at which enzyme and substrate diffuse together.

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20
Q

Viscosity of water sets an absolute upper limit at

A

~10^9 s-1 M-1.

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21
Q

Enzymes with kcat / KM above 10^8 s-1 M-1 are referred

to as

A

‘perfect’ catalysts.

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22
Q

Enzymes are optimized for

A

specific roles

23
Q

Inhibitor:

A

compound that binds to an enzyme and reduces its activity.

24
Q

why are Enzyme inhibitors Important?

A

o Natural inhibitors regulate metabolism.

o Many drugs, poisons & toxins are enzyme inhibitors.

o Used to study enzyme mechanisms.

o Used to study metabolic pathways.

25
Q

Two classes of inhibitor

A

Irreversible inhibitor – binds covalently to the enzyme.

Reversible inhibitor – not covalently bound to the enzyme.

26
Q

Irreversible inhibitor

A

binds covalently to the enzyme.

reacts with a specific amino acid side chain in the active site, and forms a covalent bond.

27
Q

Reversible inhibitor

A

binds to the enzyme but can be released, leaving the enzyme in its original condition.

not covalently bound to the enzyme.

Competitive
or
Non-competitive (pure or mixed).

28
Q

Covalent inhibitors often react with catalytic residues.

eg in chymotrypsin

A

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

29
Q

Competitive inhibition definition

A

Inhibitor competes with the substrate for binding in

the active site.

30
Q

Competitive inhibition

michaelis-menten plot

A

No change in Vmax:

high Km

High [S] than the
inhibitor.

31
Q

Competitive inhibition

Lineweaver-burk plot

A

Increases KM:

Vmax unchange

More substrate is needed to get to V = Vmax / 2.

32
Q

Transition state analogues as drugs

A

comp inhibitor

33
Q

Enzymes are often targets for

A

drugs

34
Q

Transition state analogues can make ideal

A

enzyme inhibitors.

35
Q

Enalapril and Aliskiren lower

A

blood pressure.

36
Q

Statins lower

A

serum cholesterol.

37
Q

Protease inhibitors are

A

AIDS drugs.

38
Q

Juvenile hormone esterase is a

A

pesticide target.

39
Q

Tamiflu is an inhibitor of

A

influenza neuraminidase.

40
Q

Transition state analogs make tight binding inhibitors

eg in Adenosine deaminase

A

use tetrahedral
intermediate.

• A non-reactive analog, 1,6-dihydroinosine, effectively inhibits the enzyme.

41
Q

Substrate analogue inhibitors of HIV protease

A

•inhibitor fills the active
site.

• Two catalytic aspartic
acid residues.

42
Q

Non-competitive inhibition

A

Inhibitor binds at a different site than the substrate.

Enzyme can bind substrate, or inhibitor (I), or both.

43
Q

In pure non-competitive inhibition,

A

binding of I has no effect on the binding of S

i.e. the substrate binds to E and EI with the same affinity.

44
Q

Competitive vs. pure non-competitive inhibition

michaelis menten plor

A

same Vmax / 2

different Km (comp more than pure)

45
Q

Competitive vs. pure non-competitive inhibition

lineweaver burk plot

A

non

  • km unchanged
  • vmax reduce

comp

  • km increase
  • vmax unchange
46
Q

Pure non-competitive inhibition

lineweaver burk plot

A

Vmax decreases;
KM stays the same.

Binding I changes the structure of the active site such that S still binds, but transition state stabilisation is no longer optimal.

47
Q

Mixed non-competitive inhibition

A

Vmax decreases;
KM increases.

binding of the inhibitor does affect binding
of the substrate à mixed non-competitive inhibition.

48
Q

Competitive inhibitors may react

A

alternate substrate can compete for the active site
of an enzyme,

as for alcohol dehydrogenase, on the right. Typically, related molecules fail to react., but some do.

49
Q

Michaelis-Menten equation is based on

A

binding theory and simple chemical reaction rates.

50
Q

Inhibition can be

A

reversible or irreversible.

51
Q

Inhibition can change either

A

binding (KM) or

catalysis (kcat) or both.

52
Q

Competitive inhibition gives a change in apparent

A

KM.

53
Q

Competitive inhibitors often resemble

A

substrates or transition states.

Bio chem (36 decks)

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