Free Classification of Proteins MCQs with Answers

1. The linkage that joins amino acid units in a protein is

• A. a glycosidic linkage
• B. a peptide linkage, formed by condensation between a carboxyl and an amino group
• C. an ester linkage
• D. a hydrogen bond

Explanation: The carboxyl group of one amino acid condenses with the amino group of the next, releasing water and leaving a CO-NH amide link known as the peptide bond. Glycosidic links join sugars and ester links join glycerol to fatty acids, so each class of macromolecule has its own characteristic linkage. Hydrogen bonds are important in proteins but hold the chain in shape rather than joining the units.

2. An amino acid is described as amphoteric because it

• A. dissolves only in organic solvents
• B. contains a benzene ring
• C. contains both an acidic carboxyl group and a basic amino group, so it reacts with both acids and bases
• D. has no charge under any conditions

Explanation: The carboxyl group can donate a proton and the amino group can accept one, so an amino acid neutralises both acids and alkalis. Within a single molecule the proton transfers from one group to the other, giving the doubly charged zwitterion that exists at intermediate pH. This ionic character is why amino acids are crystalline solids with high melting points and dissolve readily in water.

3. The pH at which an amino acid exists mainly as a zwitterion and does not migrate in an electric field is called its

• A. neutral point
• B. isoelectric point
• C. equivalence point
• D. melting point

Explanation: At the isoelectric point the positive and negative charges within the molecule exactly balance, so the net charge is zero and the amino acid is at its least soluble and does not move towards either electrode. The value differs for each amino acid according to its side chain. Electrophoresis separates amino acids and proteins by exploiting these differences.

4. Proteins are classified as fibrous or globular. A fibrous protein such as keratin is

• A. insoluble in water and structural in function
• B. soluble in water and catalytic in function
• C. always an enzyme
• D. made of a single amino acid repeated

Explanation: Fibrous proteins consist of long parallel chains held in bundles, which makes them tough, insoluble and suited to structural roles in hair, nails, tendons and skin. Globular proteins fold into compact soluble balls with hydrophilic groups outside, which is what enzymes, antibodies and haemoglobin require. Both types are built from the same twenty amino acids in different sequences.

5. The secondary structure of a protein, such as the alpha helix, is maintained by

• A. peptide bonds between amino acids
• B. ionic bonds between side chains only
• C. hydrogen bonds between the carbonyl and amide groups of the backbone
• D. disulphide bridges only

Explanation: Regular coiling or pleating arises because each carbonyl oxygen hydrogen bonds to an amide hydrogen further along the same chain, giving the alpha helix and the beta pleated sheet. Peptide bonds create the primary sequence, and side chain interactions such as disulphide bridges belong to the tertiary level. Heat breaks the weak hydrogen bonds and the structure unwinds, which is denaturation.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.