Enzyme Kinetics

Question 1

First order reaction

Answer 1

A -> P
V = k[A]
V = dP/dt = -dA/dt
Unit for k: s^-1

Question 2

Second order reaction

Answer 2

2A -> P or A + B -> P
V = k[A}^2 or V = k[A][B]
Units for k: M^-1 s^-1

Question 3

Reactions can also be pseudo first order, which means

Answer 3

[B]&raquo_space;> [A] and A is present at low [ ] then rxn rate will appear to not depend on [B]

Question 4

Reactions can be zero order, which means

Answer 4

They are not dependent on [ ] of reactants

Question 5

What is velocity (rxn rate) determined by?

Answer 5

The rate constant and the concentrations of reactants and products

Question 6

The Michaelis-Menten model assumes steady-state kinetics

Answer 6

• 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

Question 7

The maximum velocity can only be obtained when

Answer 7

all the enzyme is bound to substrate so [ES] = [E]t

Vmax is directly dependent on [enzyme]

Question 8

What happens when [S]&raquo_space;> Km?

Answer 8

V0 = Vmax [S]/[S] = Vmax

The rate is zero order, independent of [S]

Question 9

Km is equal to

Answer 9

the [S] at which the rxn rate is half its maximal value

Question 10

Km reflects the affinity of an enzyme for

Answer 10

its substrate when k2 is rate limiting
low Km = high affinity for sub
high Km = low affinity for sub

Question 11

What does Km depend on?

Answer 11

Substrate, pH, temperature, [ionic]

The cellular [ ] of a particular substrate is often close to the Km value

Question 12

Lineweaver-Burk plot characteristics

Answer 12

Slope = Km/Vmax
Y-int = 1/Vmax
X-int = -1/Km
Y axis: 1/V0
X axis: 1/[S]

Question 13

What is the turnover number?

Answer 13

The number of substrate molecules that the enzyme can turn into product per unit time when fully saturated
kcat = k2 = Vmax/[E]t

Question 14

When [S] «< Km

Answer 14

• V0 depends on [S], [E]t, and kcat/Km
• [E]t = ~[E]
• kcat/Km is the rate constant for ES formation

Question 15

kcat/Km is

Answer 15

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

Question 16

Temperature and Enzyme Catalysis

Answer 16

Enzyme act increases with temp until the enzyme is denatured

Question 17

pH and Enzyme Catalysis

Answer 17

Optimal pH for enzyme activity varies and reflects the environment
pH influences enzymes bc it changes the ionization state of the active site