Lecture 8: Enzyme Kinetics

Question 1

Equilibrium constant

Answer 1

Keq = [products] / [reactants]

Question 2

Michaelis-Menten equation

Answer 2

v_o = Vm * [S] / (Km + [S])

Question 3

Assumptions of Michaelis-Menten equation

Answer 3

1. Enzyme and substrate form an ES complex
2. No back reaction occurs from product buildup
3. We use initial velocity for analysis
4. [ES] is in steady state
5. There is negligible substrate depletion ([S]&raquo_space; [E])

Question 4

Features of a Michaelis-Menten reaction

Answer 4

1. [Product] increases with time and [S]
2. Product formation eventually levels off
3. Product formation is initially linear
4. Initial velocity vs [S] forms a hyperbolic curve

Question 5

Michaelis-Menten reaction speed at different [S]

Answer 5

[S]&raquo_space; Km: v_o = Vmax = k_cat[E]_tot (all active sites saturated)
[S] = Km: v_o = 0.5Vmax (half active sites filled)
[S] &laquo_space;Km: v_o = Vmax*[S] / Km (Vm / Km is first order rate constant, v proportional to [S])

Question 6

Km and dissociation constant Kd

Answer 6

A small Km means binding is strong and Kd is low; a large Km means binding is weak and Kd is high

Question 7

Turnover number k_cat

Answer 7

k_cat describes the catalytic ability of an enzyme and is the number of S molecules converted to product by 1 E molecule per unit time at saturation. Usually 1-10^4 per sec, larger is faster

Question 8

Catalytic efficiency

Answer 8

k_cat / Km for [S] &laquo_space;Km (physiological conditions). Shows how well an enzyme reacts to dilute substrate

Question 9

Perfect enzyme catalytic efficiency

Answer 9

A perfect enzyme has a k_cat / Km around 10^8-10^9, so catalysis is limited only by the rate of diffusion of S to the enzyme.

Question 10

Fraction of active sites filled

Answer 10

= [ES] / [E]_tot = v_o / Vm = [S] / (Km + [S])

Question 11

Lineweaver-Burk plot features

Answer 11

x-intercept = -1 / Km
y-intercept = 1 / Vm
Slope = Km / Vm

Question 12

Michaelis constant Km

Answer 12

Substrate concentration at which 50% of active sites are filled, aka v_o is half of Vmax

Question 13

Perfect enzyme

Answer 13

Perfect enzyme has catalytic efficiency around 10^8 - 10^9. Best substrate gives highest catalytic efficiency; if tied then higher k_cat is better

Question 14

Bi-substrate reaction

Answer 14

A reaction with 2 substrates yielding 2 products: A + B <-> P + Q. Many biochemical reactions are like this.

Question 15

Types of bi-substrate reactions

Answer 15

1. Sequential (ternary)
2. Ping-pong (double displacement)

Question 16

Sequential bi-substrate reaction

Answer 16

All substrates bind enzyme to form ternary complex EAB before product release

Question 17

Types of sequential bi-substrate reactions

Answer 17

1. Ordered sequential
2. Random sequential

Question 18

Ordered sequential reaction

Answer 18

Specific order occurs for substrate binding then product leaving

Question 19

Random sequential reaction

Answer 19

Random order occurs for substrate binding then product leaving

Question 20

Ping-pong (double displacement) reaction

Answer 20

1 substrate binds enzyme and forms a product, then the second substrate binds the now-modified enzyme and releases the second product. Characterized by the temporarily modified enzyme intermediate.