Free Proteins MCQs with Answers

1. The bond that joins two amino acids in a polypeptide chain forms between

• A. two amino groups
• B. the carboxyl group of one amino acid and the amino group of the next
• C. two R groups
• D. the carboxyl group of one amino acid and the R group of the next

Explanation: A peptide bond is a condensation between the carboxyl group of one amino acid and the alpha amino group of the next, releasing water. The R groups stay free and determine how the chain folds and what the protein does. Interactions between R groups matter at the tertiary level, but they are not peptide bonds.

2. The alpha helix and beta pleated sheet of a protein are examples of its

• A. primary structure
• B. secondary structure
• C. tertiary structure
• D. quaternary structure

Explanation: Secondary structure is the regular local folding held together by hydrogen bonds between the backbone carbonyl and amide groups, giving the alpha helix and the beta pleated sheet. Primary structure is only the sequence of amino acids. Tertiary structure is the overall three-dimensional fold of one chain, and quaternary structure appears when two or more chains associate, as in haemoglobin.

3. Haemoglobin contains four polypeptide chains. This makes it an example of a protein with

• A. primary structure only
• B. secondary structure only
• C. tertiary structure only
• D. quaternary structure

Explanation: Quaternary structure exists whenever a functional protein is built from more than one polypeptide chain. Haemoglobin has two alpha and two beta chains, each holding a haem group, and their cooperative interaction is what produces the sigmoid oxygen dissociation curve. A single-chain protein such as myoglobin stops at tertiary structure.

4. When a protein is denatured by heat

• A. its peptide bonds are hydrolysed and amino acids are released
• B. its tertiary structure is destroyed but its primary structure remains intact
• C. its amino acid sequence is rearranged
• D. it becomes more soluble and more active

Explanation: Heat breaks the relatively weak hydrogen, ionic and hydrophobic interactions that hold the chain in its three dimensional shape, so the protein unfolds and loses function, but the strong covalent peptide bonds of the backbone survive. This is why boiling an egg changes the albumen irreversibly without breaking it into amino acids. Denatured proteins usually become less soluble, which is why the egg white turns opaque.

5. Essential amino acids are those that

• A. are found in every protein
• B. cannot be synthesised by the body and must be supplied in the diet
• C. are needed only during childhood
• D. make up the active site of every enzyme

Explanation: About nine of the twenty amino acids cannot be made by the human body at a useful rate, so they have to come from food, and a protein supplying all of them in good proportion is called a complete protein. Animal proteins are generally complete while most single plant proteins are not, which is why combinations such as rice with pulses matter in a vegetarian diet. All twenty are essential to the cell in the ordinary sense; the term refers only to dietary need.

6. The tertiary structure of a protein is held in shape by all of the following EXCEPT

• A. hydrogen bonds between R groups
• B. ionic bonds between charged R groups
• C. disulphide bridges between cysteine residues
• D. peptide bonds between R groups

Explanation: Peptide bonds join the amino group of one residue to the carboxyl group of the next along the backbone, so they build the primary structure and never form between R groups. The fold itself is maintained by the weaker hydrogen, ionic and hydrophobic interactions between side chains, reinforced by the covalent disulphide bridge where two cysteines meet. That is why gentle heating or a pH change can unfold a protein without breaking it apart.

7. Which test is used to detect proteins?

• A. Benedict's test
• B. Iodine test
• C. Biuret test
• D. Emulsion test

Explanation: Biuret test uses NaOH and CuSO4 to detect peptide bonds, giving a purple/violet color with proteins.

8. The secondary structure of protein includes

• A. α-helix and β-pleated sheet
• B. Quaternary folding
• C. Amino acid sequence
• D. Disulfide bridges

Explanation: Secondary structure involves local folding into α-helix or β-pleated sheet, stabilized by hydrogen bonds between backbone atoms.

9. The secondary structure of proteins is stabilized by:

• A. Ionic bonds
• B. Hydrogen bonds
• C. Disulfide bridges
• D. Hydrophobic exclusion

Explanation: Regular hydrogen bonding between backbone carbonyl and amide groups coils the chain into an alpha helix or folds it into a beta pleated sheet. Ionic bonds, disulphide bridges and hydrophobic exclusion all act between side chains and belong to the tertiary level instead. Being weak, these hydrogen bonds are the first to break on heating, which is why helices unwind during denaturation.

10. The following is an example of a globular protein:

• A. Keratin
• B. Collagen
• C. Hemoglobin
• D. Histone

Explanation: Haemoglobin folds into a compact soluble ball with hydrophilic residues outside, which is what lets it dissolve in the cytoplasm of a red cell and carry oxygen. Keratin and collagen are fibrous structural proteins, long and insoluble. Histones are globular too, which makes the last option arguable, but haemoglobin is the standard textbook example the paper is looking for.