Factors Affecting the Rate of Enzyme Reactions

Factors Affecting the Rate of Enzyme Reactions MDCAT MCQs with answers

26 past paper MCQs on Factors Affecting the Rate of Enzyme Reactions, from the Enzymes unit of MDCAT Biology. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

Read the notes first Practise the unit as a quiz

1.What is the best physiological pH for the working of most of the cellular enzymes of humans? (MDCAT 2022)

  1. 2-3
  2. 3-5
  3. 6-8
  4. 8-10
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Correct answer: (c) 6-8

Most enzymes inside the cell work near neutrality; pepsin in the stomach is the well-known exception.

2.Which enzyme can work at pH 7 of the medium? (MDCAT 2023)

  1. pepsin
  2. sucrase
  3. chymotrypsin
  4. pancreatic lipase
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Correct answer: (b) sucrase

Sucrase works near neutrality; pepsin needs a strong acid and the pancreatic enzymes an alkaline medium.

3.Which of the following statements is incorrect regarding the rate of enzymatic action? (MDCAT 2024)

  1. an increase in enzyme concentration increases the rate
  2. an increase in enzyme concentration reduces the rate
  3. all enzymes work at their maximum rate at their optimum temperature
  4. all enzymes work at their maximum rate at their optimum pH
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Correct answer: (b) an increase in enzyme concentration reduces the rate

More enzyme means more active sites, so with substrate to spare the rate rises rather than falls.

4.Enzymes activity decreases at very low or high pH because: (UHS MDCAT 2025)

  1. Substrate concentration increases
  2. Enzymes become denatured
  3. Product formation increases
  4. Temperature becomes constant
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Correct answer: (b) Enzymes become denatured

An extreme pH breaks the bonds holding the tertiary structure, so the active site loses its shape.

5.Effect of increased substrate concentration on enzyme activity: (UHS MDCAT 2025)

  1. Decrease the rate of reaction
  2. Increase the reaction rate until all active sites are saturated
  3. Have no effect on the reaction rate
  4. Increase the rate of reaction in a straight diagonal line
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Correct answer: (b) Increase the reaction rate until all active sites are saturated

The rate climbs with substrate until every active site is occupied, after which it levels off.

6.What is the required medium for the maximum activity of the given enzymes? (Lipase / Pepsin) (NUMS MDCAT 2025)

  1. Acidic / Basic
  2. Acidic / Acidic
  3. Basic / Acidic
  4. Basic / Basic
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Correct answer: (c) Basic / Acidic

Pancreatic lipase works in the alkaline intestine, pepsin in the acid of the stomach.

7.Which of the following statement is incorrect regarding rate of enzymatic action (UHS MDCAT 2024)

  1. Increase in enzyme concentration increases the rate
  2. Increase in enzyme concentration reduces the rate
  3. All enzymes work at their maximum rate at optimum temperature
  4. All enzymes work at their maximum rate at optimum pH
Show answer

Correct answer: (b) Increase in enzyme concentration reduces the rate

More enzyme means more active sites, so the rate rises, not falls.

8.Optimum pH for pancreatic lipase is: (KMU MDCAT 2024)

  1. 2
  2. 4
  3. 6
  4. 8
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Correct answer: (d) 8

Pancreatic lipase works in the alkaline intestine, at about pH 8.

9.Catalase can be activated at pH: (KMU MDCAT 2024)

  1. 1
  2. 3
  3. 5
  4. 7
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Correct answer: (d) 7

Catalase works best at about neutral pH.

10.What is the best physiological pH for optimum functioning for most of the cellular enzymes of human? (UHS MDCAT 2023)

  1. 2-3 pH
  2. 3-5 pH
  3. 6-8 pH
  4. 8-10 pH
Show answer

Correct answer: (c) 6-8 pH

Most enzymes of the human cell work best near neutrality, about pH 7.

11.The maximum enzymatic activity of trypsin is shown at: (SZABMU MDCAT 2023)

  1. pH 2
  2. pH 4
  3. pH 6
  4. pH 8
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Correct answer: (d) pH 8

Trypsin works in the alkaline intestine, at about pH 8.

12.Most enzyme have an optimum temperature of around: (SZABMU MDCAT 2023)

  1. 30 O C
  2. 40 O C
  3. 50 O C
  4. 20 O C
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Correct answer: (b) 40 O C

Most human enzymes are fastest near body temperature, about $40^{\circ}\mathrm{C}$.

13.In terms of enzyme action, maximum temperature refers to a temperature at which: (SZABMU MDCAT 2023)

  1. Enzyme start to denature
  2. Enzyme start to re-nature
  3. Enzyme work best
  4. Enzyme are reactivated
Show answer

Correct answer: (a) Enzyme start to denature

Above the maximum temperature the bonds holding the shape give way and the enzyme is denatured.

14.Most enzymes work the best at the following temperature: (NUMS MDCAT 2023)

  1. 30°C
  2. 40°C
  3. 50°C
  4. 20°C
Show answer

Correct answer: (b) 40°C

Most human enzymes are fastest near body temperature, about $40^{\circ}\mathrm{C}$.

15.In term of enzyme action, 'maximum temperature' refers to a temperature at which: (NUMS MDCAT 2023)

  1. Enzymes start to denature
  2. Enzymes start to re-nature
  3. Enzymes work best
  4. Enzymes are reactivated
Show answer

Correct answer: (a) Enzymes start to denature

Above that temperature the bonds holding the shape give way.

16.The rate of an enzyme catalyzed reaction: (PMC MDCAT 2022)

  1. Can be reduced by inhibitors
  2. Is constant under all conditions
  3. Cannot be measured
  4. Has no relation with substrate concentration
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Correct answer: (a) Can be reduced by inhibitors

An inhibitor slows it; temperature, pH and substrate all change it too.

17.What is meant by enzyme denaturation? (PMC MDCAT 2022)

  1. An enzyme begin to catalyze reverse reaction
  2. The enzyme loses its three dimensional structure
  3. Enzyme stops its activity temporarily
  4. Enzyme becomes apoenzyme
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Correct answer: (b) The enzyme loses its three dimensional structure

Denaturation is the loss of the folded shape, and with it of the active site.

18.What happens when an enzyme gets denatured by extreme pH? (PMC MDCAT 2022)

  1. Its amino acids are ionized
  2. Its number of amino acids Is changed
  3. Its Tertiary structure gets disrupted.
  4. It temporarily stops its activation
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Correct answer: (c) Its Tertiary structure gets disrupted.

The bonds holding the folded shape give way, so the active site loses its form.

19.Which of the following is the optimum temperature for all enzymes to work in body? (PMC MDCAT 2022)

  1. 0oC
  2. 15oC
  3. 25°C
  4. 370C
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Correct answer: (d) 370C

Human enzymes work best at body temperature, about $37^{\circ}\mathrm{C}$.

20.Optimum pH for salivary amylase Is: (PMC MDCAT 2022)

  1. 2
  2. 6.8
  3. 7.6
  4. 9
Show answer

Correct answer: (b) 6.8

Salivary amylase works best at about pH 6.8, near neutral.

21.Which enzyme works in alkaline pH? (NUMS MDCAT 2022)

  1. Pepsin
  2. Sucrose
  3. Enterokinase
  4. Pancreatic lipase
Show answer

Correct answer: (d) Pancreatic lipase

Pancreatic lipase works in the alkaline intestine; pepsin needs acid.

22.Considering enzyme action minimum temperature is the term used when: (NUMS MDCAT 2022)

  1. Enzymes start denaturing
  2. Enzyme becomes hyperactive
  3. Enzyme work at their best
  4. Inactive enzyme getting reactivated
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Correct answer: (d) Inactive enzyme getting reactivated

Below the minimum temperature the enzyme is inactive but unharmed, and warming brings it back.

23.If we add more substrate to an already occurring enzymatic reaction and it has no affect on the rate of reaction, the process is called? (PMC Practice 2021)

  1. Denaturation
  2. Saturation
  3. Composition
  4. Inhibition
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Correct answer: (b) Saturation

Once every active site is occupied, more substrate can make no difference.

24.Most enzymes have an optimum temperature of around: (PMC MDCAT 2020)

  1. 30 C
  2. 40 C
  3. 50 C
  4. 20 C
Show answer

Correct answer: (b) 40 C

Human enzymes work fastest near body temperature.

25.The temperature that promotes the maximum activity of enzymes is referred to as ______. (UHS MDCAT 2018)

  1. Fixed temperature
  2. Optimum temperature
  3. Controlled temperature
  4. Active temperature
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Correct answer: (b) Optimum temperature

The temperature at which an enzyme is fastest is its optimum temperature.

26.Pepsin, protein digesting enzymes, sets best pH: (UHS MDCAT 2015)

  1. 3.00
  2. 4.50
  3. 2.00
  4. 6.00
Show answer

Correct answer: (c) 2.00

Pepsin works in the acid of the stomach, at about pH 2.

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