Free Classification of Proteins MCQs with Answers

11. 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.

12. 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.

13. 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.

14. Proteins are described as polymers because they are

• A. large molecules built from repeating amino acid units
• B. made of two identical halves
• C. always branched
• D. insoluble in all solvents

Explanation: A polymer is any large molecule assembled from many small repeating units, and in a protein those units are amino acids joined by peptide bonds. Unlike a synthetic polymer, the sequence is precisely specified rather than random, which is what allows a protein to have a defined function. Polysaccharides and nucleic acids are the other biological polymers.

15. The number of amino acids commonly found in natural proteins is

• A. 4
• B. 20
• C. 64
• D. 100

Explanation: Twenty amino acids account for almost all natural protein, and their enormous number of possible sequences is what allows such variety of structure and function. The figure 64 is the number of codons in the genetic code, which is why several codons specify the same amino acid. Four is the number of bases in DNA.

16. A zwitterion of an amino acid carries

• A. no charged groups
• B. a positive charge on the amino group and a negative charge on the carboxyl group at the same time
• C. two positive charges
• D. a single negative charge only

Explanation: The carboxyl group donates its proton to the amino group within the same molecule, so the ion has both charges and a net charge of zero at the isoelectric point. This internal ionisation explains why amino acids are crystalline solids with high melting points rather than the low melting organic liquids their size would suggest. They are also very soluble in water.

17. Which of the following is a conjugated protein?

• A. albumin
• B. haemoglobin
• C. keratin
• D. fibroin

Explanation: Haemoglobin carries a non protein haem prosthetic group that does the actual oxygen binding, which makes it conjugated. Albumin, keratin and fibroin consist of polypeptide alone and yield only amino acids on hydrolysis. Glycoproteins and lipoproteins are conjugated in the same sense, with carbohydrate or lipid attached.