SFP6: Theory Of Enzyme Catalysis

Question 1

What is an enzyme?

Answer 1

A protein which catalyses biological reactions

Question 2

What is a catalyst?

Answer 2

A substance that increases the rate of a reaction without being consumed in the reaction

Question 3

What is the reaction rate equation?

Answer 3

Reaction rate = k[A]

Rate is dependent on concentration of reactants

Question 4

What does [A]o stand for?

Answer 4

Initial concentration of A

Question 5

What is reaction coordinate?

Answer 5

Depicts progress of reaction

Question 6

What is the path called that the reactants approach one another along?

Answer 6

Path of minimum, free energy

Question 7

At what value of deltaG is the reaction thermodynamically favourable?

Answer 7

< 0

Question 8

What is the peak of a free energy diagram called?

Answer 8

The peak is called the transition state (TS)

Question 9

Why do we need an enzyme if a reaction is thermodynamically favourable/spontaneous?

Answer 9

Spontaneous doesn’t necessarily mean fast

Question 10

What is the difference in free energy between reactants and transition state known as?

Answer 10

The activation energy for the reaction.

Question 11

Does a greater activation energy mean slower or faster reaction?

Answer 11

Slower reaction

Question 12

K is directly proportional to deltaG (activation energy)… true or false?

Answer 12

False

K is directly proportional to 1/deltaG

Question 13

K is directly proportional to [TS]… true or false

Answer 13

True

Question 14

[TS] is directly proportional to delta G… true or false?

Answer 14

False, [TS] is directly proportional to 1/deltaG

Question 15

What is eyring equation and what does it show?

Answer 15

K= ((Boltzmann constant X T) / h) exp (-delta G/ RT)

H=Plancks constant
R=gas constant

Shows that increased k decreases activation energy in a log effect (small decrease means big increase)

Question 16

How does an enzyme reduce the activation energy?

Answer 16

By stabilising the transition state

Question 17

What physically is a transition state?

Answer 17

A-B + C -> A + B-C

There must be a point at which A-B is being broken as B-C is being formed

Question 18

Is A- - - B- - -C high or low energy?

Answer 18

High energy, it is very unstable

Question 19

What is an example where transition state occurs?

Answer 19

Esters

They are broken down by lysozyme and must move through a very unstable state

Question 20

What can activation energy be separated into?

Answer 20

Enthalpic and entropic terms (H and S)

DeltaG= DeltaH- TdeltaS

Question 21

What are two catalytic mechanisms used by enzymes for changing deltaH?

Answer 21

1) general acid-base catalysis

2) electrostatic catalysis

Question 22

What are two catalytic mechanisms used by enzymes for changing deltaS?

Answer 22

1) proximity and orientation effects

2) covalent catalysis

Question 23

What is general acid-base catalysis (changing deltaH)?

Answer 23

Neutralise-stabilise energetically unfavourable charge formation

General acid catalysis- stabilises formation of developing -ve charge by H+ transfer from acid
General base catalysis - stabilises formation of developing +ve charge by H+ transfer to base

Question 24

What is electrostatic catalysis (changing deltaH)?

Answer 24

Changes stabilised by electric field from charged side chains or metal ions from the enzymes

Question 25

What is an example of electrostatic catalysis?

Answer 25

Chymotrypsin: peptide backbone stabilises -ve charge on tetrahedral oxyanion

Question 26

What is entropy?

Answer 26

Measure of the degree of ‘randomness’ or ‘disorder’ of a system
More disorder= higher entropy= more energetically favourable

Question 27

What is proximity and orientation (changing DeltaS)?

Answer 27

Enzyme brings the substrate into proximity and correct orientation
Increases effective concentration of substrate by changing a bimolecular reaction into a unimolecular reaction

Question 28

What is covalent catalysis (changing deltaS)?

Answer 28

The ultimate extension of proximity and orientation effects is when the substrate is held covalently at the active site
Enzyme contains a reactive group that becomes temporarily covalently attached to part of the substrate