Biomolecules – Proteins: NEET MCQs with Answers

Explanation: The residue marked D carries a free —COOH group at the end of its side chain, which marks it as an acidic amino acid, glutamic acid. A has a —CH₂OH side chain (serine), not the sulphur-containing side chain of methionine. C has a phenol ring with an —OH group (tyrosine), not tryptophan. The chain runs from the amino end at A to the carboxyl end at D, so A is the N-terminal residue and D is the C-terminal residue, not the reverse.

Explanation: The primary structure is the sequence of the polypeptide chain (A-III), the secondary structure includes α-helix and β-sheet (B-IV), the tertiary structure is stabilised by bonds such as disulphide bonds (C-II), and human haemoglobin with four subunits shows quaternary structure (D-I).

Explanation: Collagen is the most abundant protein in the animal world (A-III), GLUT-4 enables glucose transport into cells (B-IV), trypsin is an enzyme (C-I) and RuBisCO is the most abundant protein/enzyme in the biosphere (D-II).

Explanation: Amino acids have an ionisable amino group and carboxyl group, so they can exist as a zwitterion carrying both positive (–NH₃⁺) and negative (–COO⁻) charges. Monosaccharides, nucleic acids and fatty acids do not form zwitterions.

Explanation: The –NH₂ and –COOH groups of amino acids are ionisable, so an amino acid has different structures in solutions of different pH, which makes Statement I correct. The zwitterion is the form NCERT shows between the fully protonated form in strong acid and the deprotonated form in strong base, and it persists over a wide pH range on both the mildly acidic and mildly basic sides of neutrality, so Statement II is also accepted as correct.

Explanation: Proteins are polypeptides because their amino acid monomers are linked successively by peptide bonds. They are polymers of amino acids, not of 'peptide monomers'; being linear or able to fold into conformations does not explain the name.

Explanation: Lysine has an extra amino group in its side chain, making it basic; glutamic acid is acidic, valine neutral and tyrosine aromatic.

Explanation: GLUT-4, present in muscle and adipose tissue, is recruited to the plasma membrane in response to insulin and is therefore the insulin-dependent transporter.

Explanation: Tryptophan is hydroxylated and decarboxylated to serotonin, which is then converted to melatonin in the pineal gland.

Explanation: Conjugated proteins whose prosthetic group is a carbohydrate are glycoproteins (e.g. mucin, many membrane proteins, antibodies). Lipoproteins carry lipid, nucleoproteins carry nucleic acid and chromoproteins carry a pigmented prosthetic group.

Explanation: Essential or indispensable amino acids are those that humans and other vertebrates cannot synthesise and must obtain from the diet. There are nine such amino acids — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine — and phenylalanine is one of them.

Explanation: Collagen is a major fibrous protein of connective tissue, occurring as the white fibres produced by fibroblasts. Its long parallel polypeptide chains give it high tensile strength, in contrast to the compact folded shape of globular proteins.

Explanation: Proteins are polymers of amino acids joined by peptide bonds. Glycine is the simplest amino acid because its side chain, the R group, consists of only a single hydrogen atom.

Explanation: Tryptophan is the precursor for the synthesis of the indoleamines melatonin and serotonin. Thyroxine and triiodothyronine are produced from tyrosine in the thyroid gland, while oestrogen, progesterone, cortisol and cortisone are steroids derived from cholesterol.

Explanation: The tertiary, that is, three-dimensional, structure of a protein is stabilised by hydrogen bonds, ionic and electrostatic bonds, van der Waals interactions, covalent disulphide bonds and hydrophobic interactions. Ester bonds are formed between sugar and phosphate in a nucleotide and play no part in stabilising a polypeptide chain.