1.2 - Enzyme Kinetics II: Kinetics, Inhibition and Control Flashcards
A
substrate
P
product
reaction velocity in simple reaction (A -> P) (unimolecular reaction) (2)
- v = - Δ[A]
Δt
or - v = Δ[P]
Δt
t
time
v
reaction velocity
k
rate constant
first order rate equation where k is rate constant (unimolecular reaction)
v = k[A]
what is the rate directly proportional to (unimolecular reaction)
concentration of A ([A])
velocity and reaction constant units (unimolecular reaction) (2)
- velocity = Ms^-1
- reaction constant = s^-1
does k change in unimolecular reaction?
no
when does velocity reduce in unimolecular reaction?
as A is used up and B becomes substrate for reverse reaction
equilibrium
if rates of forward and reverse reactions are equal, overall rate is 0
reaction velocity in bimolecular reaction (A + B ⇌ P) (2)
- v = -Δ[A]
Δt
or - v = -Δ[B]
Δt
second order rate equation where k is the constant bimolecular reaction)
v = k[A][B]
what order is rate in [A] and [B] (bimolecular reaction)
first order
velocity and reaction constant units in bimolecular reaction? (2)
- velocity = Ms^-1
- reaction constant = M^-1s^-1
how would you think of a reaction in terms of enzyme kinetics?
S + E ⇌ P + E
S
substrate
why does rate of reaction stop increasing linearly as [S] increases
enzyme saturation
what do you plot for a saturated enzymatic reaction?
initial rate (reverse reaction can be ignored)
Vmax
maximal velocity of a reaction
Km
Michaelis constant
Michaelis constant (Km)
measure of an enzymes efficiency = substrate conc that has a reaction rate half the Vmax
k1, k-1, k2 (3)
- k1 = 1st forwards reaction
- k-1 = 1st reverse reaction
- k2 = 2nd forwards reaction
Michaelis-Menton equation (rate of enzymatic reaction when the substrate is in excess)
E + S ⇌ ES -> E + P
k1/k-1 k2
ES
intermediate (k1) - reversible and most populous form
how can the rate of product formation in Michaelis-Menton equation be expressed (2)
v = Δ[P] = k2[ES]
Δt
what is the rate of ES production equivalent to (Michaelis-menton)
difference between rate of k1 and of k-1 and k2
rate of ES equation (Michaelis-Menton)
Δ[ES]=k1[E][S] - k-1[ES] - k2[ES]
Δt
2 assumptions of Michaelis-Menten equation
- equilibrium: k-1»_space; k2
Ks = k-1 = [E][S]
k1 [ES]
(Ks = dissociation constant of 1st step in reaction) - steady state: rate of ES formation = rate of ES dissociation
Δ[ES] = 0
Δt
michaelis constant (Km)
concentration of substrate in which the rate is half the maximal rate:
Km = Vmax = k-1 + k2
2 k1
what is michaelis constant (Km) related to
the affinity of an enzyme for a substrate
what does michaelis constant affected by? (2)
- enzyme and substrate
- reaction conditions, such as temp and pH
how does michaelis-menton calculate reaction rate of product formation?
v = Δ[P] = k2[ES]
Δt
how can michaelis-menton be used to calculate [ES]
[ES] = [Et][S]
Km + [s]
Vmax calculation (when [S]»_space; Km)
Vmax = k2[Et]
michaelis-menton equation
v = -Vmax[S]
Km+[S]
Km is also dissociation of ES complex (equation):
Km = k-1 + k2
k1
Kcat
specificity constant (catalytic efficiency)
Kcat equation (number pf catalysed reactions per active site per unit time)
Kcat = Vmax
[Et]
- rearranging Kcate is equivalent to k2 in simple reactions
what is catalytic efficiency limited by?
diffusion
lineweaver-burk plot
gives common graph type that allows easy calculations of enzyme kinetic parameters (y = mx + c)
lineweaver-burk plot (x intercept)
equivalent to -1/Km
lineweaver- burk plot (y intercept)
equivalent to 1/Vmax
lineweaver-burk plot (gradient)
equivalent to Km/Vmax
disadvantage of lineweaver-burk plot
small [S] dominate plot, may result in less accurate measurements
classes of enzyme inhibitors (3)
- competitive inhibitors
- non-competitive inhibitors
- uncompetitive inhibitors
competitive inhibition
involves inhibitor binding to an enzymes active site preventing binding of substrate
what can overcome competitive inhibition?
increasing [S]
kinetic effect of competitive inhibition (on Km and Vmax) (2)
- Km increases
- Vmax doesn’t change
Ki
dissociation constant for inhibition
Ki equation (competitive inhibition)
Ki = [E][I]
[EI]
uncompetitive inhibition
involves inhibitor binding to enzyme-substrate complex
(not to free enzyme)
kinetic effect of uncompetitive inhibition (on Km and Vmax)
both decrease
Ki equation (uncompetitive inhibition)
K’i = [ES][I]
[ESI]
mixed/non-competitive inhibition
effects both substrate binding and catalytic activity
(both enzymes and enzyme-substrate complex can be bound)
kinetic effect of mixed/non-competitive inhibition (Km and Vmax) (2)
- Km does not change
- Vmax decreases
Ki equation (mixed/non-competitive inhibition)
K’i = [ES][I]
[ESI]
