Previous year GATE questions solved Aminoacids and proteins set: 1

Previous year GATE questions solved Aminoacids and proteins set: 1

Q) Chymotrypsin and lysozymes are involved respectively in

P) Removal of successive carboxyl terminal residues

Q) Hydrolytic cleavage of peptide bond

R) Cleavage of glycosidic C-O bond

S) Oxygen transport in blood

A) P, Q

B) Q, R

C) Q, S

D) R, S

Answer: Option B

Explanation:

Chymotrypsin is responsible for cleaving peptide bonds following a bulky hydrophobic amino acid residue. Preferred residues include phenylalanine, tryptophan, and tyrosine.

Lysozyme, also known as muramidase attacks the peptidoglycans found in the cell walls of bacteria, especially Gram-positive bacteria. It hydrolyses the glycosidic bond that connects N-acetylmuramic acid with the fourth carbon atom of N-acetylglucosamine.

Q) The rise per residue of α-helix is about 1.5 Å. A protein spans 2 nm bilayer, 7 times through its trans-membrane α-helical domain. Approximately, how many amino acid residues constitute the trans-membrane domain of the protein.

a) 105
b) 450
c) 30
d) 190

Answer: Option D

Explanation:

 The bilayer is 4nm thick that is 40 A0 thick (As 10 A0 = 1nm).

The protein travels through it 7 times that is it covers a distance of 40 x 7 =280 A0.

The rise per AA residue is 1.5 A0.

Hence the number of amino acids present in this domain is 280/1.5 = 190

Q) A DNA binding motif is

A) Helix-loop-helix

B) Helix-turn-helix

C) Helical wheel

D) Loop-helix-loop

Answer: Option A and B

Explanation: Within the DNA binding domain there is at least one motif which recognizes the DNA and they determine which of the gene has to be transcribed. Four major motifs present in the DNA binding proteins are Helix-loop-helix, Helix-turn-helix, Leucine zipper motif and Zinc finger motif.

Q) Amino acids responsible for N-linked and O-linked glycosylation of proteins are

A) Asparagine and aspartic acid

B) Glutamic acid and serine

C) Glutamine and serine

D) Asparagine and Threonine

Answer: option D

Explanation: All N-linked glycosylation involves the addition of N – Acetyl glucosamine to the amino acid asparagine. The N-linked amino acid consensus sequence is Asn-any AA Ser or Thr. The middle amino acid cannot be proline (Pro). Most O-linked glycosylation involves the addition of N- Acetyl galactosamine to the amino acids serine or threonine.

Q) One of the following is not a neurotransmitter

A) Dopamine

B) Glutamic acid

C) Histidine

D) Glycine

Answer: option C

Explanation: Amino acids like glutamic acid, aspartic acid, glycine, D-serine and biogenic amines like Dopamine, epinephrine, norepinephrine act as neurotransmitters.

Q) Approximate molecular weight (kDa) of the product after translation of a 390 bases mRNA will be

A) 48

B) 26

C) 39

D) 14

Answer: Option D

Explanation: There are 390 bases in mRNA and they lead to the synthesis of 390/3 = 130amino acids after translation.

Average weight of an amino acid is 110 Daltons.

Hence the weight of translated product is 130*110 = 14300 Daltons or ~14KDa.

Q) The advantage of hemoglobin having high histidine content is

A) Histidine binds to oxygen.

B) Histidine carries oxygen to the tissues.

C) Histidine imparts buffering capacity to hemoglobin.

D) R group of histidine has low pKa.

Answer: Option C

Explanation: As Histidine has a pKa near to the blood pH, it possesses a significant buffering action in blood. Hemoglobin contains a large number of histidine subunits in its structure.

Q) The method used for prediction of three dimensional structure of a protein from known structure(s) of one or more related proteins is

A) Multiple sequence alignment

B) Homology modeling

C) Phylogeny

D) Docking

Answer: Option B

Explanation: Homology modeling, also termed comparative modeling or knowledge-based modeling, develops a three-dimensional model from a protein sequence based on the structures of homologous proteins.

Q) pH below pI amino acids will be___.        

A) Anionic
B) Cationic
C) Net charge zero
D) No charge

Answer: Option B

Explanation:

At pH below pI, amino acid will be fully protonated with protonated – NH3+ and intact COOH, with a net positive charge and thus the amino acid will be cationic.

At pH above pI amino acid will be completely deprotonated with –NH2 and deprotonated COO-, with a net negative charge and thus the amino acid will be anionic.

At isoelectric pH (pI), the amino group will be protonated (NH3+) and the carboxylic group will be deprotonated (COO-) with net charge zero and the amino acid will be zwitterionic.

Q) Proteins absorb UV light at 280 nm and show a characteristic peak at this wavelength. Which amino acid residue in the protein is responsible for this absorption?

A) Methionine
B) Valine
C) Glutamic acid
D) Tryptophan

Answer: Option D

Explanation:

Light wavelength used for protein quantification: 280 nm

Light wavelength used for DNA and RNA quantification: 260 nm

Tyrosine and phyenylalanine can also absorb light at 280 nm at a lesser extent.

A 260/280 ratio: In laboratories a ratio of the absorption of 260/280 is used to assess the purity of extracted nucleic acid (DNA and RNA) from tissue samples.

For a pure DNA, the ratio will be ~1.8

For pure RNA the ratio will be ~2.0.

Ratio values below 1.8 show protein contamination in the sample. As you know the reduced ratio is due to the higher values of denominator which is the absorption shown by protein in the sample.

 Absorption spectra of protein and amino acids

 

 

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