Free Characteristics of Enzymes MCQs with Answers

1. A non-protein organic molecule that an enzyme needs in order to function, such as NAD, is called a

• A. cofactor metal ion
• B. coenzyme
• C. prosthetic group of a haemoglobin type
• D. substrate

Explanation: Coenzymes are organic helper molecules, often derived from vitamins, that bind loosely and carry chemical groups or electrons between reactions, and NAD is the classic hydrogen carrier of respiration. Inorganic helpers such as zinc or magnesium ions are cofactors. A prosthetic group is a helper bound tightly and permanently, such as the haem in catalase.

2. Which statement about enzymes is correct?

• A. They are consumed in the reactions they catalyse
• B. They remain chemically unchanged at the end of the reaction
• C. They can catalyse reactions that would otherwise be thermodynamically impossible
• D. They work equally well at every pH

Explanation: An enzyme releases the product and returns to its original state, ready to bind the next substrate molecule, which is why a small amount catalyses the conversion of a great deal of substrate. Enzymes only speed up reactions that were already energetically favourable; they cannot drive an unfavourable one. Each has a narrow pH optimum outside which activity falls off.

3. Chemically, almost all enzymes are

• A. globular proteins
• B. lipids
• C. polysaccharides
• D. nucleotides

Explanation: An enzyme is a globular protein whose precisely folded tertiary structure creates the active site, which is why anything that unfolds the protein destroys the catalytic power. The globular rather than fibrous form matters because it makes the molecule soluble and compact. The one exception is the small group of catalytic RNA molecules called ribozymes.

4. Enzymes such as pepsin and trypsin are secreted in an inactive form known as a

• A. coenzyme
• B. cofactor
• C. prosthetic group
• D. zymogen

Explanation: A zymogen, or proenzyme, carries an extra length of polypeptide that blocks the active site, and removing it activates the enzyme once it has reached the place where it is needed. This prevents a protease from digesting the very cells that made it, and failure of the mechanism in the pancreas causes the self digestion seen in acute pancreatitis. Coenzymes and cofactors are helper molecules, not inactive precursors.

5. An enzyme that catalyses the removal of hydrogen from a substrate is classified as a

• A. dehydrogenase
• B. hydrolase
• C. isomerase
• D. ligase

Explanation: Enzymes are usually named from the substrate or the reaction plus the suffix ase, so a dehydrogenase removes hydrogen, as succinate dehydrogenase does in the Krebs cycle, passing it to a coenzyme such as NAD. Hydrolases split bonds by adding water, isomerases rearrange a molecule into its isomer and ligases join two molecules together. Knowing the suffix system lets an unfamiliar enzyme name be interpreted on sight.

6. Catalase, one of the fastest known enzymes, catalyses the breakdown of

• A. starch into maltose
• B. protein into amino acids
• C. fat into fatty acids and glycerol
• D. hydrogen peroxide into water and oxygen

Explanation: A single catalase molecule can decompose millions of molecules of hydrogen peroxide per minute, which is necessary because the peroxide produced during metabolism would otherwise damage the cell within seconds. This is why fresh liver put into hydrogen peroxide froths violently, a standard classroom demonstration. Amylase, protease and lipase are the enzymes for the other three substrates listed.

7. A metal ion such as zinc or magnesium that an enzyme needs in order to work is called

• A. a cofactor
• B. a coenzyme
• C. a prosthetic protein
• D. a substrate

Explanation: Cofactor is the term for an inorganic helper, and zinc in carbonic anhydrase and magnesium in the kinases of respiration are standard examples. Coenzymes are organic helpers such as NAD and coenzyme A, usually derived from vitamins, so the distinction is simply inorganic against organic. This is why trace elements are needed in the diet in tiny but non negotiable amounts.

8. Many coenzymes are derived from

• A. minerals absorbed from the soil
• B. vitamins of the B group
• C. fatty acids stored in adipose tissue
• D. the amino acids of the enzyme itself

Explanation: NAD is made from niacin, FAD from riboflavin and coenzyme A from pantothenic acid, so a deficiency of these vitamins cripples respiration and produces widespread symptoms rather than one local effect. This is the biochemical reason a vitamin is needed in very small quantities but cannot be omitted. Minerals supply cofactors instead, which is the parallel case.

9. Enzymes are used in industry in an immobilised form, fixed to an inert support, mainly because

• A. immobilised enzymes work at a much lower temperature
• B. immobilisation changes their specificity
• C. the enzyme can be recovered and reused and does not contaminate the product
• D. immobilised enzymes need no substrate

Explanation: Fixing the enzyme to beads or a membrane lets the product flow past and be collected pure, while the expensive enzyme stays behind for the next batch, and immobilised enzymes also tolerate heat and pH changes better. Lactose free milk is produced this way, by passing milk over immobilised lactase. The specificity of the enzyme is unchanged by immobilisation.

10. An enzyme that is secreted and works outside the cell that made it is called an extracellular enzyme. An example is

• A. trypsin
• B. DNA polymerase
• C. catalase
• D. RNA polymerase

Explanation: The pancreas exports trypsin into the duodenum, where it digests food outside any cell. DNA and RNA polymerases and catalase all work inside the cells that make them, which is what makes them intracellular enzymes.