Given below are statements related to protein structures.

A. The dihedral angles of an amino acid X in Acetyl-X-NMethyl amide in the Ramachandran plot, occur in very small but equal areas in the left and right quadrants. It can be concluded that X is not one of the 20-coded amino acids.

B. The dihedral angles of a 20-residue peptide are represented in the Ramachandran plot. It is possible to conclude that the peptide does not have a proline.

C. Two proteins can have a similar fold even if they do not share significant similarity in their primary structure.

D. On denaturation of a protein by urea, the interaction that would be disrupted are ionic bonds and van der Waal's interaction but not disulfide bonds.

Choose the combination with ALL the correct answers:

• A, B, C

• A, C, D

• B, C, D

• A, B, D

Various modifications of nucleotides occur in nucleic acids. Which of the following combinations contains at least one modification that does not occur in nucleic acids?

• N, N-dimethylguanosine, pseudouridine, 2'O-methyluridine

• 2-thiouridine, dihydrouridine, N-isopentenyladenine

• 5-methyldeoxycytosine, 5-thiouridine, pseudouridine

• dihydrouridine, 4-thiouridine, 2'O-methyluridine

Given below are statements that may or may not be correct.

A. Fructose 2,6-biphosphate is an allosteric inhibitor of phosphofrucokinase 1.

B. The TCA cycle intermediates, succinate and oxaloacetate can both be derived from amino acids.

C. A diet rich in cysteine can compensate for a methionine deficient diet in humans.

D. dTTP for DNA synthesis can be obtained from UTP.

E. In the fatty acid biosynthetic pathway, the carbon atom from HCO₃^- in the synthesis of malonyl CoA is not incorporated into palmitic acid.

Choose the option that represents the combination of all the CORRECT statements.

• A, B, C, and E

• B, D, and E

• A, D, and E

• Only B and C

Three electron acceptors 'X', 'Y' and 'Z' have redox potential (E₀') of +0.15V, +0.05V and -0.1 V, respectively. For a reaction

B + 2H + 2e^- → BH₂   E₀^' = +0.05V

Which of these three electron acceptors are appropriate?

[useful equation: $∆$G₀^' = -nFE₀^']

$∆$G₀^' = free energy change; n = number of electrons; F = Faraday constant

• X and Y

• Only X

• Y and Z

• Only Z

A serine protease was tested for its activity on the following peptide substrates of different lengths and sequences. The obtained kinetic parameters of the protease are shown along with the peptide.

Peptide substrate	Kcat(S-1)	Km(mM)
Ac-X-Ala-$\mathrm{CO}\stackrel{\downarrow }{-}{\mathrm{NH}}_2$	0.01	100
Ac-Y-X-Ala-$\mathrm{CO}\stackrel{\downarrow }{-}{\mathrm{NH}}_2$	0.10	4.0
Ac-Z-Y-X-Ala-$\mathrm{CO}\stackrel{\downarrow }{-}{\mathrm{NH}}_2$	8.0	4.0
Ac-Y-X-Val-$\mathrm{CO}\stackrel{\downarrow }{-}{\mathrm{NH}}_2$	6.0	35.0

Arrow denotes the site of cleavage. Based on the above data, the following statements are made:

A. Catalytic efficiency (K_cat/K_m) increases with the size of the peptide.

B. Amino acid at the hydrolytic cleavage position of the peptide is critical for binding of the peptide with the protease.

C. Catalytic efficiency decreases from three amino acid peptide to four amino acid peptide.

Which of the following combinations of the above statements is correct?

• A and B

• A and C

• B and C

• A, B, and C

A membrane associated protein is composed of seven "α-helices", with each helix containing 19 hydrophobic residues. While treating the membrane with all kinds of proteases, a major portion of this protein remains intact. Treatment with high salt (till 1.5M NaCl) and buffer with pH 5.0 failed to dissociate this protein from the membrane. Predict the most appropriate nature and orientation of this protein in the membrane.

• Peripheral glycoprotein

• Integral protein with seven membrane spanning regions.

• Peripheral protein with both N and C-terminals remains exposed to the outer surface of the cell membrane.

• Peripheral protein with both N and C-terminal remain exposed to cytosolic surface of the cell membrane.

When the cholera toxin (protein of Mr 90,000Da) gains access to the human intestinal tract, it binds tightly to specific receptors in the plasma membrane of the epithelial cells lining the small intestine, causing membrane bound adenylyl cyclase to undergo prolonged activation resulting in extensive loss of H₂O and Na⁺. Pretreatment of the epithelial cells with various phospholipases and proteases failed to inhibit the binding of cholera toxin to its receptor and the fluid loss but treatment with exoglycosidase, prior to binding, significantly reduces these effects. Which of the following molecule could be the receptor for this toxin?

• Phosphatidylcholine

• Sodium-potassium ATPase

• Ganglioside

• Chloride-bicarbonate exchanger

Chloramphenicol is a "broad spectrum" antibiotic which inhibits protein synthesis in prokaryotes. Given below are a few statements regarding the mode of action of chloramphenicol.

A. Chloramphenicol inhibits the peptidyltransferase activity of ribosomes.

B. Chloramphenicol can be used to treat moderate to severe infections, because mitochondrial ribosomes are not sensitive to chloramphenicol.

C. Chloramphenicol binds to one of the domains of 23S rRNA.

D. Chloramphenicol competes for binding with the E-site tRNA.

Which of the following options describes correctly the mechanism of action of chloramphenicol?

• B and D only

• A and C only

• A, C, and D

• B, C, and D

The molar function of hydrochloric acid in an extremely dilute aqueous solution is doubled. The pH of the resulting solution is

• approximately doubled

• approximately halved

• increased

• reduced

Choose the correct statement from the following:

• Disulfide bonds in a 20-residue peptide can be formed only if the cysteines are adjacent to each other.

• The amino acid isoleucine has only one chiral centre.

• Both bases and sugar contribute to the chirality of nucleic acids.

• The pI of aspartic acid is less than that of glutamic acid.