NEET Biology Enzymes MCQs Set 03

Answer: (b) a coenzyme

Explanation:
1. Many water-soluble vitamins are converted into coenzymes, which are small organic helpers of enzymes.
2. They are not enzymes themselves (which are proteins), nor complete holoenzymes, nor hormones.

Teacher's Note:
1. Think of vitamin B derivatives such as NAD and FAD.
2. Vitamins are needed in small amounts because coenzymes are reused.

Answer: (d) All of the above

Explanation:
1. NAD, NADP and FAD are all loosely attached organic molecules that carry hydrogen or electrons for enzymes.
2. So each of the three is a coenzyme.

Teacher's Note:
1. Coenzymes are organic and loosely bound to the apoenzyme.
2. Many are derived from vitamins.

Answer: (c) all the living cells of the plant body

Explanation:
1. Every living cell carries out metabolism, and metabolism needs enzymes.
2. So enzymes occur in all living cells of the plant.

Teacher's Note:
1. Enzymes are not tissue-specific in the sense of presence.
2. Only their kinds differ from cell to cell.

Answer: (b) Enzyme

Explanation:
1. An enzyme is regenerated unchanged at the end of the reaction.
2. Hormones, vitamins and nucleotides are used up or modified during their action.

Teacher's Note:
1. A catalyst is not used up, so a small amount of enzyme can convert a large amount of substrate.
2. This is why enzymes are effective in tiny quantities.

Answer: (b) It is decreased

Explanation:
1. An enzyme gives the reaction an easier path through a lower-energy transition state.
2. Therefore the activation energy falls, while the energy levels of substrate and product stay the same.

Teacher's Note:
1. Lowering the energy barrier speeds up the reaction.
2. The overall energy change of the reaction does not change.

Answer: (a) competitive inhibition

Explanation:
1. Malonate is similar in structure to succinate, the natural substrate.
2. It occupies the active site, so the inhibition is competitive.

Teacher's Note:
1. A substrate analogue competes for the active site.
2. More substrate can overcome this kind of inhibition.

Answer: (a) competitive

Explanation:
1. A substrate analogue resembles the substrate and competes with it for the active site.
2. This is the definition of competitive inhibition.

Teacher's Note:
1. Competitive inhibitors are often reversible.
2. Some antibacterial drugs work this way.

Answer: (a) denatured

Explanation:
1. High temperature breaks the weak bonds that hold the protein in its three-dimensional shape.
2. The active site is lost, so the enzyme is denatured.

Teacher's Note:
1. Low temperature only slows enzymes down.
2. High temperature damages them.

Answer: (c) pH

Explanation:
1. Enzyme activity is highest at a particular pH and falls sharply on either side.
2. Changes in pH alter the charges in the active site and the shape of the protein.

Teacher's Note:
1. Each enzyme has its own optimum pH.
2. Pepsin works in acid while trypsin works in slightly alkaline medium.

Answer: (b) dehydrogenases

Explanation:
1. Dehydrogenases belong to the oxido-reductases and remove hydrogen (electrons) from a substrate.
2. Hydrolases, transaminases and proteases do not mainly transfer electrons.

Teacher's Note:
1. Oxido-reductases carry out redox reactions.
2. Cytochrome oxidase is another example.

Answer: (d) both activation and inhibition

Explanation:
1. A molecule binding at the allosteric site, away from the active site, can either switch the enzyme on or switch it off.
2. So the allosteric site serves both activation and inhibition.

Teacher's Note:
1. Allosteric means other site.
2. Many metabolic pathways are regulated this way.

Answer: (b) the Km constant of the enzyme

Explanation:
1. This is the definition of the Michaelis-Menten constant Km.
2. It is read from the rate against substrate concentration curve at half of Vmax.

Teacher's Note:
1. A low Km means high affinity for the substrate.
2. Km is a constant for an enzyme under given conditions.

Answer: (c) Competitive inhibition occurs when the substrate and the inhibitor compete for the active site of the enzyme

Explanation:
1. Competitive inhibitors resemble the substrate and compete with it for the active site, so (c) is correct.
2. Adding more substrate overcomes competitive inhibition, not non-competitive, and non-competitive inhibitors are often reversible, so (a) and (d) are wrong.

Teacher's Note:
1. Non-competitive inhibitors bind elsewhere on the enzyme.
2. Raising substrate concentration cannot displace them.

Answer: (a) the Km value

Explanation:
1. Km shows how much substrate is needed to reach half of Vmax and so reflects the affinity and efficiency of the enzyme.
2. pH optimum, product formation in general and molecular size do not directly compare efficiency.

Teacher's Note:
1. A lower Km means the enzyme works well even at low substrate levels.
2. Compare Km values under the same conditions.