Enzyme Kinetics 2

Question 1

What is First order reaction?

Answer 1

Where A disappears as B is produced
i.e. the rate is directly proportional to the concentration of A: [A]
A -> B

Question 2

Does the Rate constant (k) of reaction change ?

Answer 2

It does not change

Question 3

But with time, the velocity of the reaction reduces ?

Answer 3

As the reactant A is used up and B becomes a substrate for the reverse reaction
A < –> B (reversible)

Question 4

The reaction reaches equilibrium when ?

Answer 4

Rates for forward and reverse reactions are EQUAL and the overall rate for the
reaction is zero. (i.e balanced)

Question 5

How many reactants do Simple Kinetics I & II ?

Answer 5

Only one reactant

Question 6

Explain Simple Kinetics IiI ?

Answer 6

There are two reactants
A + B ⇌ C
We assume that the rate of forward reaction is linearly proportional to the concentrations of A and B, and the reverse reaction is linearly proportional to the concentration of C.

Question 7

What is the Second order reaction based on ?

Answer 7

Based on concentration of two reactants

Question 8

What does the equilibrium constant tells us ?

Answer 8

It tells us the extent of the reaction, NOT its speed

Question 9

Explain the first step of Enzyme Catalysis ?

Answer 9

E+S ⇌ ES

The Enzyme molecule binds to Substrate to form intermediate ES in a reversible reaction

Question 10

Explain the second step of Enzyme Catalysis ?

Answer 10

ES → P+E

In the intermediate ES decomposes irreversibly to yield product (P) and Enzyme. i.e. the enzyme is not changed (K2)

Question 11

Overall the mechanism is ?

Answer 11

Made up of two consecutive reactions, with a reversible 1st step

Question 12

If we assume [S] is greater than the [E]° ?

Answer 12

Then the enzyme will mostly be present as the intermediate ES and [ES] will remain constant

Question 13

Enzyme concentration is LIMITING which is what prevents ?

Answer 13

The constant, infinite increase of P

Question 14

What is Km ?

Answer 14

This is the concentration of S at half Vmax

Question 15

Equation of the rate of ES dissociation ?

Answer 15

Steady state assumption - the rate of ES formation

Question 16

When is the steady state reached ?

Answer 16

When the concentration of intermediate (ES) is constant

Question 17

What is Km related to ?

Answer 17

The enzyme affinity for the substrate (higher the Km, lower the affinity)

Question 18

Define Vmax ?

Answer 18

Maximum catalytic rate at full saturation

Question 19

What is Ki ?

Answer 19

Dissociation constant for inhibitor

Question 20

What does Pyruvate kinase (non-competitive inhibitor) catalyse ?

Answer 20

Catalyses the final step of Glycolysis

Phosphoenolpyruvate+ADP—–> Pyruvate+ATP

Question 21

What is Pyruvate Kinase regulated and inhibited by?

Answer 21

By binding of Alanine. Alanine binding does not affect binding of the substrate

Question 22

Explain the Kinetics of competitive inhibitors ?

Answer 22

• Increase [S] to overcome inhibition
• Vmax Remains the same
• Km is increased

Question 23

Explain the kinetics of non-competitive inhibitor ?

Answer 23

• Increasing [S] cannot overcome inhibition
• Less E available,
• Vmax is lower
• Km remains the same for available E

Question 24

Give equation Competitive inhibition and describe ?

Answer 24

E + S ⇌ ES → E + P

E ⇌ EI

[I] binds to free [E] only, and competes with [S]; increasing [S] overcomes Inhibition by [I]

Question 25

Give equation Non-competitive inhibition and describe ?

Answer 25

E + S ⇌ ES → E + P

EI+S → EIS

[I] binds to free [E] or [ES] complex; Increasing [S] can not overcome [I] inhibition

Question 26

What is Methotrexate ?

Answer 26

A competitive inhibitor of dihydrofolate reductase - Used to treat cancer

Question 27

What is Tetrahydrofolate ?

Answer 27

Substrate for dihydrofolate reductase

role in purine & pyrimidine biosynthesis.