Free Macromolecules MCQs with Answers

11. The high specificity of an enzyme for its substrate is due to

• A. the shape and chemistry of its active site
• B. its molar mass
• C. the temperature of the reaction
• D. the concentration of the substrate

Explanation: Only a substrate whose shape and charge distribution complement the active site can bind, which is why sucrase acts on sucrose but not on the closely related maltose. The induced fit model refines this picture by allowing the site to mould itself around the substrate as binding occurs. Specificity is a structural property and is unaffected by temperature or concentration.

12. Enzyme activity is lost above about 45 degrees Celsius because

• A. the substrate evaporates
• B. the enzyme is denatured as the weak bonds holding its tertiary structure break
• C. the enzyme is used up faster
• D. the activation energy increases

Explanation: Heat disrupts the hydrogen and ionic interactions that hold the chain in shape, so the active site loses its precise geometry and can no longer bind the substrate, and the loss is usually permanent. Below the optimum, raising the temperature increases the rate in the normal way, which is why the curve rises to a peak and then falls sharply. The peak for human enzymes lies near 37 degrees.

13. A competitive inhibitor reduces the rate of an enzyme catalysed reaction by

• A. denaturing the enzyme permanently
• B. binding an allosteric site
• C. occupying the active site because it resembles the substrate
• D. lowering the temperature

Explanation: Substrate and inhibitor compete for the same site, so the inhibition can be overcome by increasing the substrate concentration and the maximum rate is eventually still reached. A non competitive inhibitor binds elsewhere, changes the shape of the enzyme and cannot be outcompeted, so it genuinely lowers the maximum rate. Many drugs are designed as competitive inhibitors of a specific enzyme.

14. Which pair correctly matches a macromolecule with its monomer?

• A. Protein and glucose
• B. Starch and amino acid
• C. Nucleic acid and fatty acid
• D. Protein and amino acid

Explanation: Proteins are polymers of amino acids, starch and cellulose of glucose, and nucleic acids of nucleotides, while fats are not true polymers at all but esters of glycerol and fatty acids. Matching each macromolecule to its monomer and its linkage is the core of this topic. All these polymers are built by condensation and broken by hydrolysis.

15. The quaternary structure of a protein exists only when the molecule

• A. contains a disulphide bridge
• B. is fibrous
• C. consists of two or more polypeptide chains associated together
• D. contains more than one hundred amino acids

Explanation: Quaternary structure describes how separate polypeptide subunits fit together, as in haemoglobin with its two alpha and two beta chains, and a single chain protein such as myoglobin therefore stops at the tertiary level. The subunits are held by the same weak interactions that stabilise tertiary structure. Chain length alone has nothing to do with it.

16. Conjugated proteins differ from simple proteins in that they contain

• A. only amino acids
• B. a non protein prosthetic group in addition to the polypeptide
• C. no peptide bonds
• D. two identical chains

Explanation: A conjugated protein is bound to a non protein group, so haemoglobin carries a haem group, glycoproteins carry carbohydrate and lipoproteins carry lipid, and in each case the prosthetic group is essential to the function. Simple proteins yield only amino acids on hydrolysis. Removing the prosthetic group leaves the protein inactive.

17. Heavy metal ions such as mercury and lead are poisonous partly because they

• A. increase enzyme activity uncontrollably
• B. are converted into proteins by the body
• C. supply too much energy to the cell
• D. bind to sulphur containing groups in enzymes and denature them irreversibly

Explanation: These ions attack the sulphydryl side chains of cysteine, breaking disulphide bridges and destroying the tertiary structure, so the affected enzymes lose all activity permanently. Because the damage cannot be reversed by removing the metal, the effect is poisoning rather than simple inhibition. This is also why drinking milk or egg white gives some first aid benefit, as those proteins bind the metal first.

18. Each enzyme works fastest at a particular pH because a change in pH

• A. alters the charges on the side chains, changing the shape of the active site
• B. changes the molar mass of the enzyme
• C. removes the substrate from the solution
• D. converts the enzyme into a carbohydrate

Explanation: Excess hydrogen or hydroxide ions interfere with the ionic and hydrogen bonds between side chains, distorting the active site so the substrate no longer fits, and a large shift denatures the enzyme entirely. This is why pepsin works at about pH 2 in the stomach while trypsin needs about pH 8 in the small intestine. Small deviations are reversible, large ones are not.

19. Proteins are essential in the diet mainly because they supply

• A. the fastest source of energy
• B. amino acids for the growth and repair of tissues
• C. essential fatty acids
• D. dietary fibre

Explanation: Digested protein provides the amino acids the body reassembles into its own enzymes, hormones, antibodies and structural tissue, and this cannot be done from carbohydrate or fat. Growing children, pregnant women and patients recovering from injury therefore need more of it. Severe protein deficiency in children causes kwashiorkor even when energy intake is adequate.

20. The primary structure of a protein refers to

• A. the sequence of amino acids joined by peptide bonds
• B. the coiling of the chain into a helix
• C. the folding of the chain into a globular shape
• D. the association of several chains

Explanation: Primary structure is simply the order of the residues, and it determines every higher level of structure, because the chain folds according to the interactions between its own side chains. A single change in that sequence can be catastrophic, as one substitution in haemoglobin causes sickle cell anaemia. The peptide bonds of the primary structure survive denaturation.