IMAT 2017 Q26 [Enzyme Inhibitors]

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1

Which of the following statements is/are correct for all enzyme inhibitors?

  1. They alter the shape of the active site.
  2. They denature the enzyme.
  3. They increase the activation energy of the reaction.
  4. They reduce the rate of the enzyme catalysed reaction.

A. 4 only
B. 3 only
C. 1, 2 and 3 only
D. 1, 2 and 4 only
E. 2 and 4 only

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2

The correct answer is A.

First, an enzyme inhibitor (EI) is a compound that changes the catalytic action. Indeed, a catalyst increases the speed of the reaction, and the inhibitor does the opposite.

Second, we need to remind ourselves what collision theory is.

According to it, reactions occur when the molecules “collide effectively”. For that to happen, they need to :

  1. collide
  2. have a certain orientation in space
  3. do both of these things with a certain amount of energy

Knowing that, we can now start to review each item individually :

  • The EI binds to the enzyme’s active site, and thus prevents its binding by changing its shape OR by binding to the enzyme itself to prevent the reaction from being catalyzed.
    Therefore, the fact that all EI alter the shape of the active site is incorrect.

  • A denatured enzyme is an enzyme that can no longer proceed to catalyze a reaction because it has lost its 3D or tertiary structure. This happens when the enzyme is subjected to high temperatures, inadequate pH, or chemical solutions. Enzyme inhibitors do NOT cause denaturation.

  • Activation energy is the minimum energy required for a reaction to occur.

Temperature : the lower the temperature, the lesser the movements made by the reactant molecules. They are then less likely to collide and to make the chemical reaction happen.

  1. Reactant concentration : lowering the concentration of one or more reactants will lower the rate of reaction.

  2. Pressure : lower pressure means more space for the molecules to move in and therefore, they are less likely to collide.

    Therefore, the activation energy is independent of the presence or not of an EI.

  • Like we’ve said at the beginning, the EI’s purpose is to lower the speed of the reaction, thus making item 4 the only correct one.
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If enzymes are to decrease activation energy, then don’t inhibitors increase the energy compared to the enzyme-existant situation?