MCQ on Amino Acids (Part-3)

Biochemistry MCQ-06: Amino acids are the building blocks of proteins. They are organic compounds that contain both an amino group (-NH2) and a carboxyl group (-COOH) attached to a central carbon atom. There are 22 different types of amino acids that are commonly found in proteins, each with a unique side chain that gives it distinct properties. In addition to their role in protein synthesis, amino acids also play important roles in metabolism, cell signaling, and other biological processes. This the Part-3 of Amino Acids MCQ with answer key and explanations.

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(1). pH below pI amino acids will be___.
a.       Anionic
b.      Cationic
c.       Net charge zero
d.      No charge

(2). Naturally occurring proteins are usually polymers of _____.
a.       D-amino acids
b.      L-amino acids
c.       A mixture of D and L amino acids
d.      Either D amino acids or L- amino acids

(3). At zwitterionic form, an amino acid will act as_____.
a.       Proton donor
b.      Proton acceptor
c.       Proton donor and acceptor
d.      None of these

(4). Which of the following amino acid is more likely to occupy the interior of a globular protein?
a.       Methionine
b.      Aspartate
c.       Lysine
d.      Arginine
e.       All of these

(5). 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

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(6). Glycine is _____.
a.       A stimulatory neurotransmitter
b.      An inhibitory neurotransmitter
c.       Not act acting as a neurotransmitter
d.      Both (a) and (b)
e.       Not a neurotransmitter

(7). Selenocysteine is a rare amino acid which contain ____-
a.       Selenium
b.      Selenium and Sulfur
c.       Sulfur
d.      Selenium and Nickel

(8). Which of the following amino acid contain a thioether group in the side chain?
a.       Cysteine
b.      Cystine
c.       Glycine
d.      Methionine

(9). An amino acid with a sulfhydryl group in the side chain:
a.       Methionine
b.      Cystine
c.       Cysteine
d.      All of these

(10). Which among the following is the largest amino acid?
a.       Phenylalanine
b.      Tyrosine
c.       Tryptophan
d.      Histidine

(11). Which of the following amino acid is completely non-polar?

a.       Phenylalanine
b.      Tyrosine
c.       Tryptophan
d.      All of these
e.       None of these

(12). Which amino acid is known as “the 22nd amino acid”?

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

(13). Cystine is ___.

a.       Highly polar
b.      Highly non polar
c.       Partially polar
d.      Partially non polar

(14). The side chain of Histidine contain____.

a.       Indole ring
b.      Phenol group
c.       Imidazole ring
d.      Guanidino ring

(15). Taurine, the major constituent of bile, is derived from____.

a.       Cysteine
b.      Methionine
c.       Tryptophan
d.      Lysine

(16).  Selenocysteine is a derived from _____.

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

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(17). Example for selenocysteine containing protein:

a.       Glutathione peroxidase
b.      Thioredoxin reductase
c.       Glycine reductase
d.      All of these
e.       None of these

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

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

(19).  ____ is the only amino acid which is optically inactive.

a.       Valine
b.      Glycine
c.       Selenocysteine
d.      Proline

(20). Selenocystein is coded by ________ codon.

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

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Answers with explanations

1). Ans. (b). Cationic

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.

2). Ans. (b). L-amino acids

D and L Forms of Amino AcidsAll amino acid except glycine has one chiral centre and thus can exist in two different optically active forms (enantiomers) designated as D and L amino acids. Naturally occurring proteins almost exclusively composed of L-amino acids because, during protein synthesis, the peptidyl transferase enzyme of ribosome can only recognize L-amino acids. D-amino acids do occur in nature and they usually exist as free amino acids or in short peptide sequences which are not synthesized by regular translation mechanism. Example: the peptide cross links of peptidoglycan cell wall of bacteria are rich in D-amino acids. In animals some D amino acids acts as neurotransmitters such as D-serine.

Enantiomers: Pair of stereoisomers that are non-superposable mirror images of each other. For example D and L Glyceraldehyde, D and L alanine

3). Ans. (c). Proton donor and acceptor

At zwitterionic form, the fully protonated amino group (NH3+) can act as proton donor; whereas deprotonated carboxylic group (COO-) can act as proton acceptor.

4). Ans. (a). Methionine


Hydrophilic and hydrophobic interactions of amino acids play a crucial role in determining protein’s three dimensional conformations. In globular proteins, the hydrophobic residues occupy the interior portion whereas hydrophilic residues stay exterior and form a shell by interacting with water. This hydrophilic and hydrophobic interaction of amino acids with water assists the folding of polypeptide to globular manner in globular proteins.

5). Ans. (d). Tryptophan

Absorption spectra of protein and amino acidsLight 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.

A260/280 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 and 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.

6). Ans. (b). An inhibitory neurotransmitter

7). Ans. (a). Selenium

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.Amino Acids MCQ with Answer Key

8). Ans. (d). Methionine

sulfur sulphur in methionine is as thioetherMethionine (first amino acid in protein synthesis) is another sulfur containing amino acid. Sulfur in methionine is in the form of methyl thioether (C – S – C). Thioether group is highly non polar.

9). Ans. (c). Gysteine

10). Ans. (c). Tryptophan

11). Ans. (a). Phenylalanine

Among aromatic amino acids, phenylalanine is completely non polar. Tyrosine is slightly polar due to the hydroxyl group (-OH) in the side chain. Similarly Tryptophan is also shows some polarity due to the nitrogen in the indole ring.

12). Ans. (b). Pyrrolysine

22nd amino acidPyrrolysine biosynthesis: Two molecules of L-lysine condense together to form pyrrolysine. One molecule of lysine is first catalytically converted (3R)-3-Methyl-D-ornithine, which is then ligated to a second molecule of lysine. During this reaction an NH2 group is eliminated, followed by cyclization and dehydration to produce L-pyrrolysine.

13). Ans. (b). Highly non polar

Cystine is a dimeric amino acid of two cysteine molecules joined through Disulfide Bridge after oxidation. Sulfhydryl group (SH) in cysteine is highly polar; however the disulfide bond is highly non polar making cysteine a nonpolar entity. Disulfide bond formation stabilizes the secondary and tertiary structures of protein.

Disulfide bridge in cystine

14). Ans. (c). Imidazole ring

15). Ans. (a). Cysteine

Taurine (2-aminoethanesulfonic acid) is a major constituent of bile in human. It is very essential for proper cardiovascular functions, development and functions of skeletal muscles, retina and central nervous system.

16). Ans. (b). Serine

Selenocysteine synthesis: Selenocysteine is synthesized by a special mechanism in the cells only when they are required. Since selenocysteine is toxic to cells in higher concentration, unlike other amino acids, cell do not maintain the pool of this amino acid in the cytosol. Cells store selenium in the form of less reactive selenide (H2Se) to reduce the toxicity. Selenocysteine is synthesized on a specialized tRNA molecule and this special tRNA also does the duty of incorporation of selenocysteine in the growing polypeptide chain. The structure of selenocysteine-specific-tRNA differs from those of standard tRNA in many aspects.

Pathway of biosynthesis of amino acid selenocysteine

During the biosynthesis of selenocysteine, the tRNA-Sec is charged by another amino acid serine with the help of an enzyme seryl-tRNA ligase. The elongation factors of prokaryotes and eukaryotes cannot recognize and incorporate Ser-tRNA-Sec in the polypeptide chain. Then the enzyme selenocysteine synthase act on the serine residue attached on tRNA-Sec and converted it to selenocysteine. Then this Sec-tRNA-Sec is incorporated into the growing polypeptide chain with the help of additional elongation factors in the cells.

17). Ans. (d). All of these

Selenocysteine containing proteins are called selenoproteins. So far about 25 selenoproteins are discovered in human cells.

18). Ans. (b). 2

One proton from – NH3+ and another form – COOH group

fully protonated and deprotonated amino acid

19). Ans. (b). Glycine

The side chain (R-group) of glycine is – H and thus it lacks a chiral centre. See the figure.

only optically inactive amino acid

20). Ans. (c). UGA

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.

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