Enzyme Kinetics Flashcards
First order reaction
A -> P
V = k[A]
V = dP/dt = -dA/dt
Unit for k: s^-1
Second order reaction
2A -> P or A + B -> P
V = k[A}^2 or V = k[A][B]
Units for k: M^-1 s^-1
Reactions can also be pseudo first order, which means
[B]»_space;> [A] and A is present at low [ ] then rxn rate will appear to not depend on [B]
Reactions can be zero order, which means
They are not dependent on [ ] of reactants
What is velocity (rxn rate) determined by?
The rate constant and the concentrations of reactants and products
The Michaelis-Menten model assumes steady-state kinetics
- Pre-steady state: initial mixing of E + S, [ES] builds up
- Steady state: [ES] remains approx constant
- Steady-state kinetics: measurements of V0 while [ES] is relatively stable
The maximum velocity can only be obtained when
all the enzyme is bound to substrate so [ES] = [E]t
Vmax is directly dependent on [enzyme]
What happens when [S]»_space;> Km?
V0 = Vmax [S]/[S] = Vmax
The rate is zero order, independent of [S]
Km is equal to
the [S] at which the rxn rate is half its maximal value
Km reflects the affinity of an enzyme for
its substrate when k2 is rate limiting
low Km = high affinity for sub
high Km = low affinity for sub
What does Km depend on?
Substrate, pH, temperature, [ionic]
The cellular [ ] of a particular substrate is often close to the Km value
Lineweaver-Burk plot characteristics
Slope = Km/Vmax Y-int = 1/Vmax X-int = -1/Km Y axis: 1/V0 X axis: 1/[S]
What is the turnover number?
The number of substrate molecules that the enzyme can turn into product per unit time when fully saturated
kcat = k2 = Vmax/[E]t
When [S] «< Km
- V0 depends on [S], [E]t, and kcat/Km
- [E]t = ~[E]
- kcat/Km is the rate constant for ES formation
kcat/Km is
the specificity constant
- It is a measure of catalytic efficiency bc it takes into account rate of catalysis (kcat) and nature of the enzyme-substrate interaction (Km)
High specificity constant = high substrate preference
