Lecture 4

Question 1

Define enzymes

Answer 1

• Enzymes - globular proteins of high molecular weight that act as catalysts (RNA is exception)
• Enzymes are specific - most cellular reactions catalysed by enzymes
Faster reaction rate

Question 2

What is the nomenclature for enzymes?

Answer 2

Named by adding -ase to end of substrate it catalyses , e.g. lactase, or to catalysed reaction e.g. polymerase

Question 3

What do some protein enzymes require for their activity?

Answer 3

• Some protein enzymes require non-protein group for their activity
• Haloenzymes - contains non-protein group = co-factor (non protein group), apoenzyme (protein)
Isoenzymes - occurs in different molecular form but catalyse same reaction

Question 4

Give an example of basic enzyme classification

Answer 4

e.g. oxidoreductases = oxidation-reduction reactions,

isomerases - isomerization reactions

Question 5

How do enzymes work?

Answer 5

• Lowers EA by forming enzyme-substrate complex

- Free energy change and equilibrium constant unaffected by enzymes

Question 6

What are the four steps in a catalytic reaction?

Answer 6

1. enzyme and substrate are available, 2. substrate binds to enzyme,
2. substrate converted to products,
3. products are released

Question 7

Describe the problems with the lock and key model

Answer 7

• Assume enzymes are rigid - they are flexible molecules
• Over and undersized molecules are observed to bond but not react
• Some reactions occur in fixed sequence
• Mechanism for reaction rate enhancement not clear

Question 8

Describe the induced fit model

Answer 8

• Enzyme partially flexible, substrate influence enzyme final shape
• Some compounds bind to enzyme but not react as enzyme distorted too much
• Some molecules too small to induce proper alignment, therefore cant react
• Only right substrate capable of inducing proper alignment of active site

Question 9

What is the Michaelis-Mentin equation?

Answer 9

v=(v_max [S])/(K_M+[S])

Question 10

Describe the saturation kinetics

Answer 10

• reversible step: enzyme substrate(ES) complex formed

- Dissociation step: ES complex dissociates to give product and free enzyme

Question 11

What assumptions are made in the MM equation?

Answer 11

Assumptions - high substrate conc. so enzyme saturated, ES complex established rapidly, negligible reverse reaction (2nd step)

Question 12

Describe the Rapid equilibrium approach

Answer 12

Occurs in first step, product formation step slower than first step= ES complex always at equilibrium conc.

Question 13

Describe the Quasi-steady state approach -

Answer 13

Initial substrate conc. > enzyme conc.

Question 14

What is an enzyme inhibitor?

Answer 14

Compounds other than substrate that may bind to enzymes and reduce their activity
Can be either reversible or irreversible - irreversible form stable complex, reversible easily dissociate

Question 15

Describe the three major classes on inhibitor mechanisms

Answer 15

• Competitive - inhibitor usually substrate analog, compete with substrate for enzyme active site => Vmax same, Km increases
• Non-competitive - not substrate analogs, bind on sites other than active sites and reduce enzyme affinity => Vmax decreases, Km same
• Un-competitive - inhibitors bind only to ES complex, no affinity towards free enzyme => Vmax decrease, Km decrease