MCQ on aminoacids with explanation set 1

MCQ on aminoacids with explanation

1) Among the 20 standard proteins coding amino acids, which one is least occurs in proteins?     

a) Glycine
b) Alanine
c) Tryptophan
d) Methionine

Answer: Option C

2) A fully protonated glycine (NH3+ – CH2 – COOH) can release ____ protons.

a) 1
b) 2
c) 3
d) 4

Answer: option B

One proton released from –NH3+

Second proton released from –COOH group

3) Which amino acid is known as “the 22nd amino acid”?

a) Selenocysteine
b) Pyrrolysine
c) N-formylmethionine
d) Selenomethionine

Answer: Option B

Pyrrolysine biosynthesis: Pyrrolysine is formed by the condensation of two molecules of L-lysine.

Step 1: One molecule of lysine is first catalytically converted (3R)-3-Methyl-D-ornithine.

Step 2: Ligation of 3-Methyl-D-ornithine to a second molecule of lysine.

During this reaction an NH2 group is eliminated, followed by cyclization and dehydration to produce L-pyrrolysine.

4) Which among the following is the largest amino acid?

a) Phenylalanine
b) Tyrosine
c) Tryptophan
d) Histidine

Answer: Option C

5) Example for selenocysteine containing protein:

a) Glutathione peroxidase
b) Thioredoxin reductase
c) Glycine reductase
d) All of these

Answer: Option D

Selenoproteins: selenocysteine containing proteins

So far about 25 selenoproteins are discovered in human cells.

6) Selenocystein is coded by ________ codon.

a) UAA
b) UAG
c) UGA
d) AUG

Answer: Option C

UGA is a stop codon. Cell uses UGA codon to incorporate both selenocysteine and pyrrolysine (21st and 22nd amino acids) in the polypeptide by a special mechanism called translational recoding.

7) Selenocysteine is a derived from _____.

a) Cysteine
b) Serine
c) Methionine
d) Cystine

Answer: Option B

8) Selenocysteine is a rare amino acid which contain ____-

a) Selenium
b) Selenium and Sulfur
c) Sulfur
d) Selenium and Nickel

Answer: Option A

Selenocysteine contain selenium as selenol (SeH) same as thiol (Sulfhydril, SH) group in Cysteine. Both the selenol and thiol group are highly polar in nature.

9) Which of the following amino acid is critical in the folding of proteins due to the slow isomerization of the peptide bond preceding this amino acid depending upon the solvent environment?

a) Tryptophan
b) Leucin
c) Proline
d) Histidine

Answer: Option C

Proline is an iminoacid. Proline is the only amino acid with a secondary amine. The alpha-amino group in proline is attached directly to the side chain, making the alpha carbon a direct substituent of the side chain.

10) Which of the following amino acid is coded by maximum number of codons?

a) Leucine
b) Tryptophan
c) Valine
d) Alanine

Answer: Option A

Leucine can be coded by 6 codons
Methionine and Tryptophan are coded by only one codon (Least number of codons)

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