Biology Question Bank – 14 MCQs on “Enzymes” – Answered!

14 Questions with Answers and Explanations on Enzymes for Biology Students.

1. Enzymes having slightly different molecular structure but performing identical activity are

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(a) holoenzymes

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(b) isoenzymes

(c) apoenzymes

(d) coenzymes.


Answer and Explanation:

1. (b): Enzymes having slightly different molecular structures but performing identical activities are called isoenzymes. Over 100 enzymes are known to have isoenzymes. Thus a-amylase of wheat endosperm has 16 isozymes, lactic dehydrogenase has 5 isoenzymes in man.

Holoenzyme is a complex comprising an enzyme molecule and its cofactor. Apoenzyme is an inactive enzyme that must associate with a specific cofactor molecule or ion in order to function. Coenzyme is an organic nonprotein molecule that associates with an enzyme molecule in catalysing biochemical reactions.

2. The nuclease enzyme, which begins its attack from free end of a polynucleotide, is


(a) polymerase

(b) endonuclease

(c) exonuclease

(d) kinase.

Answer and Explanation:

2. (c): Exonuclease activity means cleavage of nucleotides only at the end while endonuclease breaks DNA strand at an internal position.

DNA polymerase has 3?-5? exonuclease activity which removes any nucleotide which mispairs during elongation of growing strand. A small segment of DNA polymerase also shows 5?-3? exonuclease activity which removes DNA segment which comes as an obstruction in way of growing DNA strand. Polymerase catalyses the elongation of a polymeric molecule.

Endonucleases are very specific and cut DNA at very specific nucleotide sequences. These are called restriction enzymes. Kinase is an enzyme that can transfer a phosphate group.

3. The enzyme enterokinase helps in the conversion of

(a) caesinogen into caesin

(b) trypsinogen into trypsin

(c) pepsinogen into pepsin

(d) proteins into polypeptides.

Answer and Explanation:

3. (b): The enzyme enterokinase which is also known as enteropeptidase helps in the conversion of trypsinogen into trypsin. Trypsin acts on proteins and breaks them for digestion. It occurs in the brush borders of duodenum.

4. Which is a typical example of ‘feedback inhibition’?

(a) cyanide and cytochrome reaction

(b) sulpha drugs and folic acid synthesizer bacteria

(c) allosteric inhibition of hexokinase by glucose 6-phosphate

(d) reaction between succinic dehydrogenase and succinic acid.

5. Co-factor (prosthetic group) is a part of holoenzyme. It is

(a) loosely attached organic part

(b) loosely attached inorganic part

(c) accessory non-protein substance attached firmly

(d) none of these.

Answer and Explanation:

5. (c): Enzymes are simple if they are made of only proteins (e.g. pepsin, amylase etc.) while conjugate enzy.mes have an additional non protein cofactor which may be organic or inorganic. Loosely attached organic cofactor is coenzyme (e.g. NAD+, FAD) while firmly attached one is prosthetic group (e.g. heme, biotin). The intact enzyme-cofactor complex is called a holoenzyme when the cofactor is removed; the remaining protein which is inactive is called an apoenzyme.

6. Some of the enzymes, which are associated in converting fats into carbohydrates, are present in

(a) microsomes

(b) glyoxysomes

(c) liposomes

(d) golgi bodies.

Answer and Explanation:

6. (b): Glyoxysomes are microbodies found in fungi and germinating oil seeds that are involved in converting fat into carbohydrates. They contain enzymes for P oxidation of fatty acids and glyoxylate cycle. Glyoxylate cycle is a metabolic pathway in plants and microorganisms that is a modified form of the Krebs cycle.

It utilizes fats as a source of carbon and enables the synthesis of carbohydrate from fatty acids by avoiding the stages of the Krebs cycle in which carbon dioxide is released. It occurs in tissues rich in fats, such as those of germinating seeds; the enzymes involved in the cycle, which have not been found in mammals, are contained in organelles (microbodies) called glyoxysomes.

7. Enzymes are not found in

(a) fungi

(b) algae

(c) virus

(d) cyanobacteria.

Answer and Explanation:

7. (c): Viruses do not have enzymes so they cannot synthesize proteins. They multiply only inside the living host cell and for multiplication and metabolism they take over the machinery of the host cell. They lack their own cellular machinery and enzymes.

8. Which factor is responsible for inhibition of enzymatic process during feed back?

(a) substrate

(b) enzymes

(c) end product

(d) temperature.

9. Role of enzyme in reactions is to/as

(a) decrease activation energy

(b) increase activation energy

(c) inorganic catalyst

(d) none of the above.

10. Enzyme first used for nitrogen fixation

(a) nitrogenase

(b) nitroreductase

(c) transferase

(d) transaminase.

Answer and Explanation:

10. (a): Nitrogen fixation involves conversion of atmospheric nitrogen to ammonia. It is done with the help of nitrogenase enzyme which occurs inside thick walled heterocysts of the blue green algae. These provide suitable anaerobic environment for nitrogenase activity even in aerobic conditions.

11. In which one of the following enzymes, is copper necessarily associated as an activator?

(a) carbonic anhydrase

(b) tryptophanase

(c) lactic dehydrogenase

(d) tyrosinase.

Answer and Explanation:

11. (d): The additional nonprotein cofactor may be inorganic and are termed as activators. They are minerals of different types like Ca, Fe, Cu, Zn, Mn etc. Copper is associated as an activator with tyrosinase. It is widely distributed in plants, animals and man. It is also known as polyphenol oxidase or catecholase. It oxidizes tyrosine to melanin in mammals and causes the cut surfaces of many fruits land vegetable to darken.

12. Which one of the following statements regarding enzyme inhibition is correct?

(a) competitive inhibition is seen when a substrate competes with an enzyme for binding to an inhibitor protein

(b) competitive inhibition is seen when the substrate and the inhibitor compete Tor the active site on the enzyme

(c) non-competitive inhibition of an enzyme can be overcome by adding large amount of substrate

(d) non-competitive inhibitors often bind to the enzyme irreversibly.

Answer and Explanation:

12. (b): Competitive inhibition is a reversible inhibition where inhibitor competes with the normal substrate for the active site of enzyme. A competitive inhibitor is usually chemically similar to the normal substrate and therefore, fits into the active site of an enzyme and binds with it.

The inhibition is thus due to substrate analogue. The enzyme, now cannot act upon the substrate and reaction products are not formed. Hence, action of an enzyme may be reduced or inhibited.

Since a competitive inhibitor occupies the site only temporarily, the enzyme action is not permanently affected. The activity of succinate dehydrogenase is inhibited by malonate and is the most common example of competitive inhibition.

13. The catalytic efficiency of two different enzymes can be compared by the

(a) formation of the product

(b) pH of optimum value

(c) Km value

(d) molecular size of the enzyme,

Answer and Explanation:

13. (c): Km value or Michaelis constant is defined as the substrate concentration at which half of the enzyme molecules are forming (ES) complex or concentration of the substrate when the velocity of the enzyme reaction is half the maximal possible.

The Km varies from enzyme to enzyme and is used in characterizing the different enzymes. A smaller Km value indicates greater affinity of the enzyme for its substrate, hence, shows a quicker reaction. Km value is a constant characteristic of an enzyme for its conversion of a substrate.

14. An organic substance bound to an enzyme and essential for its activity is called

(a) isoenzyme

(b) coenzyme

(c) holoenzyme

(d) apoenzyme.

Answer and Explanation:

14. (b): Enzymes are simple if they are made of only proteins (eg. pepsin, amylase etc.) while conjugate enzymes have an additional non protein cofactor which may be organic or inorganic. Loosely attached organic cofactor is coenzyme.

It plays an accessory role in enzyme catalyzed processes often by acting as a donor or acceptor of a substance involved in the reaction. ATP and NAD are common coenzymes.

Isozymes are closely related variants of enzymes. It has similar function as another enzyme but has a different set of amino acids. Holoenzyme is the intact enzyme cofactor complex. Apoenzyme is an enzyme without its cofactor. It is the protein molecule to which a coenzyme will bind to produce the holoenzyme.


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