Enzymes II : Thermodynamics and Kinetics

Question 1

What criterion can determine the spontaneity of a reaction and what information does it carry?

Answer 1

• Spontaneity of reaction can be determined by the Change in Gibbs Free Energy, ΔG
• ΔG < 0 (spontaneous) Favorable in the direction indicated
• likely to occur without an input of energy
• ΔG > 0 (non-spontaneous) Unfavorable in the direction indicated
• Requires input of energy to proceed.
• ΔG = 0 (equilibrium)

Question 2

Define activation energy

Answer 2

The energy needed to transform the substrate molecules into the transition states

Question 3

Define transition state

Answer 3

An unstable chemical forms part-way between the substrates and the product which has the highest free energy.

Question 4

Gibbs free energy of activation, ΔG

Answer 4

The energy difference between the transition

state and the substrate.

Question 5

Explain the influence of enzymes in a reaction and diagram

Answer 5

• Enzyme works by stabilizing the transition state of a chemical reaction and decreasing the Gibbs free energy of activation.
• Enzyme didn’t alter the energy level of substrate or product
• Thus it increases the rate of reaction without affecting the overall change of the reaction.

Question 6

How do we make a reaction more favorable?

Answer 6

• Using Coupled Reaction
• By coupling the energetically unfavorable reaction (Endergonic) with a more energetically favorable one (Exergonic)
• As long as the overall pathway of the reaction is exergonic, the reaction will operate in the desired (forward) direction.
• ΔG°: Standard change of free energy at constant pH of 7.0

Question 7

What is the influence of enzymes in the equilibrium position of a reaction?

Answer 7

• Enzyme didn’t alter the eqm. position of a reaction, but it accelerates the forward and reverse reaction to the same extent.
• Thus it will only accelerate the attainment of the eqm. position but will never shift its position.
• (decrease the time requires for a reaction to reach an equilibrium state)

Question 8

Define enzyme velocity

Answer 8

The rate of enzyme-catalyzed reaction

Question 9

When is the enzyme velocity at its fastest pont?

Answer 9

• The rate is fastest at the point where no product is yet present
• Because:
  a) The concentration of substrate is at its maximum
  b) No feedback inhibition by the formed product

Question 10

Define Vo

Answer 10

Initial rate of an enzyme-catalyzed reaction

• (measured before >10% of the substrate is converted to product)
• (obtained from the slope of the linear part of the product-time curve)

Question 11

Define enzyme kinetics

Answer 11

The study of the rates of enzyme-catalysed chemical reactions

Question 12

Define the state of dynamic equilibrium

Answer 12

At this point, although new molecules of substrate and product are continually being transformed and formed, the ratio of substrate to product remains at a constant value

Question 13

What is the most commonest way to express enzyme activity?

Answer 13

Vo, the initial rate of the reaction being catalyzed

Question 14

State the two standard units of enzyme activity that can be used

Answer 14

1) Enzyme unit (U)
- an enzyme unit is the amount of enzyme which will catalyze the transformation of 1miumol of substrate per minute at 25oC under optimal conditions (pH and temperature) for that enzyme
2) Katal (kat)
- defined as that catalytic activity which will raise the rate of a reaction by one mole per second in a specified system

Question 15

Define total activity

Answer 15

Refers to the total units of enzyme in the sample

Question 16

Define specific activity

Answer 16

The number of enzyme units per milligram of protein (units/mg)
- Measure the purity of an enzyme

Question 17

List out the factors that affect the enzyme activity

Answer 17

• substrate concentration
• enzyme concentration
• temperature
• pH

Question 18

Explain the effect of substrate concentration

Answer 18

a) Low substrate concentration
- Double [S] = double Vo
b) High substrate concentration
- The enzyme becomes saturated (all enzyme molecules have bound substrate)
- Increase in [S] lead to very small changes in Vo

Question 19

What happens at the saturation point during of substrate concentration?

Answer 19

The overall enzyme rate is now dependent on the rate at which the product can dissociate from the enzyme, and adding further substrate will not affect this

Question 20

Explain the effect on enzyme concentration

Answer 20

At high concentration of substrate
- Increase in [E] will also increase the Vo
(triple [E] = triple Vo)

Question 21

Explain the effect of temperature by 1st effect

Answer 21

1) 1st effect: Increase the thermal energy of the substrate molecules
- Increase the no. of substrate molecules with sufficient energy to overcome the Gibbs free energy of activation and hence increases the ROR

Question 22

Explain the effect of temperature by the 2nd effect

Answer 22

Higher temperature will lead to the denaturation (unfolding) of enzyme and thus decrease the catalytic activity
- will increase the chance of breaking multiple weak bonds that hold the enzyme together

Question 23

Explain the effect of pH on enzyme activity

Answer 23

Each enzyme has an optimum pH at which the rate of catalyzed reaction is at its maximum

• Small pH deviation: decrease catalytic activity (due to change in the ionization groups at the active sites)
• Large pH deviation: denaturation of the enzyme protein

Question 24

Explain the Michaelis-Menten Model

Answer 24

• It describes that ES (enzyme-substrate) complex can dissociate again to form E and S, or can proceed chemically to form E and P (product)
• ES will remain constant until all the substrates have been converted

Question 25

Define Allosteric Enzymes

Answer 25

Multiple-subunit enzymes consist of one or more active sites on each subunit

Question 26

Define allostery or cooperative binding

Answer 26

Binding of one substrate to the enzyme facilities binding of other substrate molecules

Question 27

Explain the two models of the allosteric enzyme’s action mechanism

Answer 27

• This occurs when the binding of a substrate at one active site induces a conformational change in the enzyme.
• This will affect the other active sites and alter their affinity for substrate molecules.
• Eg: Aspartate transcarbamoylase
• 2 models of mechanism: Concerted & Sequential