CBI6 - Enzymes and Kinetics of Biocatalysis

Question 1

Define basic mechanism of an enzyme substrate reaction.

Answer 1

Enzyme and substrate bond to form enzyme-substrate reaction. Following this a product is released leaving the enzyme ready for re-use.

Question 2

Define notation for Gibbs activation energy.

Answer 2

Delta G with transition state symbol.

Question 3

Discuss how a catalyst works.

Answer 3

Lowers activation energy by providing alternative pathway. Not used up itself so can be reused.

Question 4

Which enzymes are not proteins.

Answer 4

Ribozymes - enzymes made up RNA not a protein.

Question 5

How can you define an irreversible reaction by Gibbs energy.

Answer 5

Largely negative change in Gibbs energy.

Question 6

How many numbers are used for enzyme classification.

Answer 6

4 numbers.

Question 7

Discuss kinetics of enzyme catalysed reaction with no inhibitor at low and high substrate concentration.

Answer 7

Low [S] - first order - directly proportional.

High [S] - zero order - no proportionality.

Question 8

Define Vmax.

Answer 8

Maximum velocity of enzyme reaction when all enzymes are saturated with substrate. Approached but never reached.

Question 9

Define kcat.

Answer 9

No. Of substrate molecules that are turned over by enzymes in a second.

Question 10

Define Michaelis Menten equation.

Answer 10

V = Vmax[S]/(Km + [S])

Question 11

Is Km the same for all enzymes and what is it.

Answer 11

Different for each enzyme but is independent of [E].

Km is [S] needed to reach half Vmax

Question 12

What does a low Km indicate.

Answer 12

High affinity of enzyme for the substrate.

Question 13

How can a Lineweaver Burk ploy be obtained from the Michaelis Menten equation.

Answer 13

Taking double reciprocal forming linear equation in y=Mx+c format.

Question 14

What formula can be used to describe catalytic efficiency.

Answer 14

Kcat/Km.

Question 15

What factors are taking into consideration with catalytic efficiency.

Answer 15

Combines catalytic potential with the ability to bind substrate at low [S].

Question 16

Name three types of inhibitor.

Answer 16

Competitive. Uncompetitive. Non-competitive.

Question 17

What is the difference between reversible and irreversible enzyme reactions.

Answer 17

Formation of temporary bonds vs permanent covalent bonds.

Question 18

Discuss effects of a competitive inhibitor.

Answer 18

Binds to active site of enzyme. Degree of inhibitor depends on [S]. Effect of inhibitor can be overcome by increasing [S].

Question 19

Discuss effects of uncompetitive inhibitors.

Answer 19

Binds to E-S complex. Prevents release of product. Cannot be overcome by increasing [S].

Question 20

Discuss effects of non-competitive inhibitor.

Answer 20

Binds at allosteric site of enzyme. Prevents enzyme from carrying out its catalytic function.

Question 21

What is an enzyme cofactor.

Answer 21

Small non protein chemicals or metals. Aid function of enzyme. Commonly drives from vitamins.

Question 22

How do the active sites of enzymes aid the function of enzymes as catalysts.

Answer 22

Active sites lower activation energy by stabilising the transition state.

Question 23

Give numbers and associated class names for first number of enzyme classification.

Answer 23

1. Oxidoreductases.
2. Transferases.
3. Hydrolases.
4. Lyases.
5. Isomerases.
6. Ligases.

Question 24

What is km.

Answer 24

(K-1 + k2)/k1 where:
K-1 = ES -> E + S
K2 = ES -> P
K1 = E + S -> ES

Question 25

What combination of kcat and km gives highest efficiency of enzyme.

Answer 25

Larger kcat and smaller km

Question 26

How do competitive inhibitors affect Km and Vmax.

Answer 26

Vmax remains same.

Km increases.

Question 27

How do uncompetitive inhibitors affect Vmax and Km.

Answer 27

Vmax decreases.

Km decreases.

Question 28

How do non competitive inhibitors affect Vmax and Km.

Answer 28

Vmax is decreased

Km remains same

Question 29

What are two conditions affecting enzymes.

Answer 29

Temperature and pH.

Question 30

How does pH affect enzymes.

Answer 30

Enzymes have own optimal pH. Too high or too low means it will be denatured.

Question 31

How does temperature affect enzyme.

Answer 31

Optimum temperature needed. Temperature too high breaks bonds in enzyme causing it to become denatured.

Question 32

Give types of reversible covalent modifications.

Answer 32

Phosphorylation, acetylation, methylation, carboxylation.

Question 33

Give name of enzyme that adds phosphate group to a molecule.

Answer 33

Kinase.

Question 34

Give type of enzyme that removes phosphate group from molecule.

Answer 34

Phosphatase.

Question 35

Define an allosteric inhibitor.

Answer 35

Inhibitors that act on enzymes by binding to the allosteric site.

Question 36

Which type of inhibitor is always an allosteric inhibitor.

Answer 36

Non competitive inhibitors are all allosteric inhibitors.

Question 37

What effect on enzyme activity can allosteric inhibitors have.

Answer 37

Either cause inhibition of enzyme activity or activation of enzyme activity.

Question 38

How do allosteric inhibitors work on enzyme activity.

Answer 38

Upon binding, cause conformational change which either gives correct active site (activating enzyme) or distorts active site (inhibiting enzyme).

Question 39

What graph shape is obtained for allosteric enzymes and discuss enzyme kinetics.

Answer 39

Sigmoidal. At low [S], very gradual increase in velocity. One substrate binds to enzyme causing conformational change allowing more substrate to bind. Eventually all sites become saturated. Graph plateaus.

Question 40

Give examples of enzyme following allosteric sigmoid shape.

Answer 40

Haemoglobin binding to oxygen.