Enzyme catalysed reactions

Question 1

What are the two models of substrate-enzyme interaction?

Answer 1

Lock and key

Induced fit

Question 2

Briefly explain the lock and key model?

Answer 2

• Active site has rigid shape
• Only substrates with matching shape can fit
• Substrate is a key that fits the lock of the active site

Question 3

Briefly explain the induced fit model

Answer 3

• Substate enters the active sit of enzyme
• Enzyme slightly changes shape as the substrate binds
• Complex forms and products released

-Binding of substrate conduces modification of active site allowing the complex to form

Question 4

How does substrate (reactant) concentration affect rate of reaction in enzyme catalysed reactions?

Answer 4

-Increased substrate conc. increases reaction rate to a maximum velocity and no further as enzyme becomes saturated
(at constant enzyme conc.)

Question 5

What is the Michaelis-Menten equation and what is it used for?

Answer 5

-dCs/dt = -rs = (Vmax*Cs)/ (Km + Cs)

-rs: volumetric rate of reaction
Cs: conc. of substrate
Vmax: max. rate of reaction at infinite substrate conc.
Km: = 1/2 Vmax, Michaelis constant for the substrate

Used when enzyme conc. is the limiting factor to represent the kinetics of enzyme reactions

Question 6

What does a high Km show?

Answer 6

• Lots of substrate is needed to saturate the enzyme
• The enzyme has low affinity (attractive force) for the substrate

-The opposite is true for a low Km

Question 7

What is the main feature of zero-order reactions, what is the Michaelis-Menten equation used?

Answer 7

-High substrate conc. Cs»>Km

: -rs ≈ Vmax

Question 8

What is the main feature of first-order reactions, what is the Michaelis-Menten equation used?

Answer 8

-Low substrate conc. Km»>Cs

: -rs ≈ (Vmax*Cs)/Km

Question 9

What is Km when the substrate conc. is such that the reaction rate Is equal to one-half the maximum rate?

Answer 9

Km = Cs(1/2)

From: Vmax/2 = (Vmax*Cs(1/2))/ (Km + Cs(1/2))

Question 10

What units does Km have?

Answer 10

M (molar->moldm^-3)

Question 11

When do you need to integrate the Michaelis-Menten equation and what should the limits be?

Answer 11

-When substrate is being converted/its conc. is changing over a period of time as change isn’t constant

• On the left hand side, limits of new conc. (top) and original conc. (bottom)
• On the right hand side, limits of time