Enzyme Kinetics 2 Flashcards
What is First order reaction?
Where A disappears as B is produced
i.e. the rate is directly proportional to the concentration of A: [A]
A -> B
Does the Rate constant (k) of reaction change ?
It does not change
But with time, the velocity of the reaction reduces ?
As the reactant A is used up and B becomes a substrate for the reverse reaction
A < –> B (reversible)
The reaction reaches equilibrium when ?
Rates for forward and reverse reactions are EQUAL and the overall rate for the
reaction is zero. (i.e balanced)
How many reactants do Simple Kinetics I & II ?
Only one reactant
Explain Simple Kinetics IiI ?
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.
What is the Second order reaction based on ?
Based on concentration of two reactants
What does the equilibrium constant tells us ?
It tells us the extent of the reaction, NOT its speed
Explain the first step of Enzyme Catalysis ?
E+S ⇌ ES
The Enzyme molecule binds to Substrate to form intermediate ES in a reversible reaction
Explain the second step of Enzyme Catalysis ?
ES → P+E
In the intermediate ES decomposes irreversibly to yield product (P) and Enzyme. i.e. the enzyme is not changed (K2)
Overall the mechanism is ?
Made up of two consecutive reactions, with a reversible 1st step
If we assume [S] is greater than the [E]° ?
Then the enzyme will mostly be present as the intermediate ES and [ES] will remain constant
Enzyme concentration is LIMITING which is what prevents ?
The constant, infinite increase of P
What is Km ?
This is the concentration of S at half Vmax
Equation of the rate of ES dissociation ?
Steady state assumption - the rate of ES formation
When is the steady state reached ?
When the concentration of intermediate (ES) is constant
What is Km related to ?
The enzyme affinity for the substrate (higher the Km, lower the affinity)
Define Vmax ?
Maximum catalytic rate at full saturation
What is Ki ?
Dissociation constant for inhibitor
What does Pyruvate kinase (non-competitive inhibitor) catalyse ?
Catalyses the final step of Glycolysis
Phosphoenolpyruvate+ADP—–> Pyruvate+ATP
What is Pyruvate Kinase regulated and inhibited by?
By binding of Alanine. Alanine binding does not affect binding of the substrate
Explain the Kinetics of competitive inhibitors ?
- Increase [S] to overcome inhibition
- Vmax Remains the same
- Km is increased
Explain the kinetics of non-competitive inhibitor ?
- Increasing [S] cannot overcome inhibition
- Less E available,
- Vmax is lower
- Km remains the same for available E
Give equation Competitive inhibition and describe ?
E + S ⇌ ES → E + P
E ⇌ EI
[I] binds to free [E] only, and competes with [S]; increasing [S] overcomes Inhibition by [I]
Give equation Non-competitive inhibition and describe ?
E + S ⇌ ES → E + P
EI+S → EIS
[I] binds to free [E] or [ES] complex; Increasing [S] can not overcome [I] inhibition
What is Methotrexate ?
A competitive inhibitor of dihydrofolate reductase - Used to treat cancer
What is Tetrahydrofolate ?
Substrate for dihydrofolate reductase
role in purine & pyrimidine biosynthesis.
