Free Macromolecules MCQs with Answers

21. Which bond is responsible for holding the alpha helix of a protein in shape?

• A. peptide bonds along the backbone
• B. hydrogen bonds between backbone carbonyl and amide groups
• C. disulphide bridges between cysteines
• D. ionic bonds between side chains

Explanation: Regular hydrogen bonding between a carbonyl oxygen and an amide hydrogen four residues further along coils the chain into a helix. Peptide bonds create the chain itself rather than its coiling, and disulphide and ionic interactions belong to the tertiary level. Heat breaks the hydrogen bonds easily, which is why helices unwind on denaturation.

22. A disulphide bridge in a protein forms between the side chains of two

• A. cysteine residues
• B. glycine residues
• C. lysine residues
• D. serine residues

Explanation: Two sulphydryl groups are oxidised to form a covalent S-S link, the strongest of the interactions holding tertiary structure. This is why heavy metal ions, which attack sulphur, denature proteins irreversibly. Permanent waving of hair works by breaking and re-forming these bridges in keratin.

23. Which protein is responsible for the transport of oxygen in blood?

• A. collagen
• B. haemoglobin
• C. insulin
• D. keratin

Explanation: Haemoglobin is a globular conjugated protein of four chains, each holding an iron containing haem group that binds one oxygen molecule reversibly. Collagen and keratin are fibrous structural proteins and insulin is a hormone. The variety of these roles is why protein is described as the most versatile class of biological molecule.

24. Fibrous proteins differ from globular proteins in that fibrous proteins are

• A. soluble in water and act as enzymes
• B. insoluble in water and serve structural roles
• C. always conjugated
• D. made of a single amino acid

Explanation: Long parallel chains bundled together make fibrous proteins tough and insoluble, which suits keratin in hair, collagen in tendons and fibroin in silk. Globular proteins fold into compact soluble balls with hydrophilic residues outside, which suits enzymes, antibodies and transport proteins. Both classes are built from the same twenty amino acids.

25. The reaction that joins two amino acids releases

• A. carbon dioxide
• B. a molecule of water
• C. ammonia
• D. hydrogen

Explanation: Condensation between the carboxyl group of one residue and the amino group of the next expels water and creates the peptide link. The reverse process, hydrolysis, adds water back and is how proteins are digested. Every biological polymer is assembled by condensation and broken by hydrolysis.

26. An enzyme lowers the activation energy of a reaction by

• A. raising the temperature of the substrate
• B. binding the substrate at its active site and stabilising the transition state
• C. supplying energy to the reactants
• D. changing the equilibrium position

Explanation: Binding orients the substrate correctly and strains the bonds that are about to break, offering a lower energy route to the products. The enzyme is regenerated unchanged and does not shift the equilibrium, only the speed at which it is reached. This last point is what separates a catalyst from a reactant.

27. Compared with an inorganic catalyst such as manganese dioxide, an enzyme is

• A. far less efficient
• B. highly specific and effective at mild temperature and pH
• C. unaffected by temperature
• D. consumed during the reaction

Explanation: Enzymes achieve rate enhancements that industrial catalysts cannot approach and do so at body temperature and near neutral pH, but they work on one substrate or one class of substrate only. Their protein nature also makes them sensitive to heat and pH, which an inorganic catalyst is not. Like all catalysts, they emerge unchanged.

28. The turnover number of an enzyme measures

• A. how many substrate molecules one enzyme molecule converts per unit time
• B. the number of enzymes in a cell
• C. the temperature at which it denatures
• D. its molar mass

Explanation: Carbonic anhydrase converts around 600,000 molecules of carbon dioxide per second, which is why the reaction in red blood cells is essentially instantaneous. A high turnover number means a small quantity of enzyme suffices. It is a measure of catalytic efficiency rather than of abundance.

29. Which statement about the active site of an enzyme is correct?

• A. It occupies most of the enzyme's surface
• B. It is a small region whose shape and charges complement the substrate
• C. It binds any molecule of similar mass
• D. It is destroyed after each reaction

Explanation: Only a few residues, often far apart in the primary sequence but brought together by folding, form the active site, which is why the whole three dimensional structure matters. Complementarity of shape and charge is what produces specificity. The site is regenerated after each catalytic cycle.

30. Milk is curdled in cheese making by the enzyme

• A. amylase
• B. rennin
• C. lipase
• D. pepsin

Explanation: Rennin, also called chymosin, clots the milk protein casein, which is the first step in cheese production and is also present in the stomach of young mammals. Amylase acts on starch, lipase on fats and pepsin on proteins generally. Industrial enzymes of this kind are chosen for one specific substrate.