Biochemistry MCQ-05: 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-2 of Amino Acids MCQ with answer key and explanations.
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(1). Which amino acid, among the 20 standard protein coding amino acids, is most abundantly occurs in proteins?
a. Glycine
b. Methionine
c. Serine
d. Leucine
(2). Which of the following amino acid is having more polarity?
a. Lysine
b. Arginine
c. Histidine
d. Aspartate
(3). Majority of enzyme’s active site usually contain one or more _____ residues.
a. Glycine
b. Tryptophan
c. Histidine
d. Arginine
(4). The following is the amino acid sequence of an alien polypeptide: ‘AGQHIKXSKPWYVGLBUFF’. The ‘X’ in the given sequence represents ___________.
a. Asparagine
b. Isoleucine
c. An ambiguous amino acid
d. An unknown amino acid
(5). Amino acid acting as a defensive molecule in plants:
a. Canavanine
b. Con-canavalin
c. Proline
d. All of these
(6). Which amino acid act as the precursor for the synthesis of the neurotransmitter serotonin?
a. Tyrosine
b. Tryptophan
c. Glycine
d. Arginine
(7). _____ is an exclusively ketogenic amino acid in human.
a. Valine
b. Methionine
c. Leucine
d. Proline
(8). pI of ___________ is near to physiological pH.
a. Lysine
b. Histidine
c. Arginine
d. Glutamic acid
(9). Which amino acid act as the precursor of dopamine?
a. Glycine
b. Aspartate
c. Valine
d. Tyrosine
(10). What is the molecular weight of Tryptophan?
a. 202 g.mol-1
b. 204 g.mol-1
c. 206 g.mol-1
d. 208 g.mol-1
(11). Amino acid which act as the precursor of IAA (indole 3-acetic acid) biosynthesis in plants is_____.
a. Tyrosine
b. Phenylalanine
c. Tryptophan
d. Methionine
(12). Amino acid involved in urea cycle____.
a. Ornithine
b. Citrulline
c. Arginine
d. Both (a) and (b)
e. All of these
(13). Which of the following amino acid have the buffering capacity at physiological pH?
a. Lysine
b. Arginine
c. Histidine
d. Aspartic acid
(14). Amino acid which act as the precursor of epinephrine synthesis is_____.
a. Glycine
b. Aspartate
c. Tyrosine
d. Valine
(15). Which amino acid act as the precursor of nitric oxide biosynthesis in animals?
a. Phenyl alanine
b. Arginine
c. Aspartate
d. Ornithine
(16). Which of the following is an essential amino acid?
a. Aspartic acid
b. Alanine
c. Leucine
d. Asparagine
e. All of these
(17). Which of the following is a non-essential amino acid?
a. Alanine
b. Histidine
c. Lysine
d. Methionine
e. All of these
(18). Which of the following amino acid is coded by a single codon?
a. Lysine
b. Arginine
c. Tryptophan
d. Phenylalanine
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(19). First discovered amino acid is ____.
a. Asparagine
b. Aspartate
c. Glutamate
d. Glutamine
(20). Among the 20 standard protein coding amino acids, which was the last discovered amino acid?
a. Leucine
b. Isoleucine
c. Threonine
d. Serine
Answers with explanations
(1). Ans. (d). Leucine
(2). Ans. (b). Arginine
(3). Ans. (c). Histidine
Histidine has its pI near to physiological pH. The best buffering activity of an amino acid is at a pH which is equal to or near to its pI value (= isoelectric pH). Furthermore, an amino acid at its pI will be in zwitter ionic form and there it can act as both proton donor (acid) and proton acceptor. Since histidine’s pI is near to physiological pH, it can act as a good proton donor and proton acceptor at this pH. This is why the active site of many enzymes contains one or more histidine residues since most of the enzymatic reactions involve exchange of protons and electrons between enzyme and substrates.
Diethylpyrocarbonate (DEPC) which is used in laboratories to inactivate RNase enzyme will inactivate histidine residues in the active site and thereby it deactivate the enzyme.
(4). Ans. (d). An unknown amino acid
(5). Ans. (a). Canavanine
Concanavalin A (ConA) is a lectin isolated from a legume, jack bean (Canavalia ensiformis)
Lectins are protein which specifically binds to carbohydrates.
(6). Ans. (b). Tryptophan
(7). Ans. (c). Leucine
Leucine and lysine are the two amino acids in human which are exclusively ketogenic.
(8). Ans. (b). Histidine
(9). Ans. (d). Tyrosine
(10). Ans. (b). 204 g.mol-1
Tryptophan is the largest amino acid among ‘magic 20’.
Pyrrolysine is the largest proteinogenic amino acid with a molecular mass 255 g.mol-1
(11). Ans. (c). Tryptophan
Indole-3-acetic acid is the naturally occurring auxin in plants. Several pathways of auxin biosynthesis in plants are known and in most of them, tryptophan acts as the precursor. Tryptophan independent biosynthesis of auxin is also known to science now. IAA is synthesized from the indole ring of tryptophan in the young leaves and shoots.
(12). Ans. (e). All of these
Urea cycle (also called ornithine cycle): a biochemical cycle in many animals including human to produce urea from ammonia. Urea cycle was the first discovered metabolic cycle by Hans Krebs and Kurt Henseleit in1932.
(13). Ans. (c). Histidine
Amino acids have best buffering activity in a pH equal to its pI. At this pH amino acids will be zwitter ionic in nature (with net charge zero) and they can act as both proton donor (acid) and proton acceptor (base). The pI of histidine is near to physiological pH and thus histidine can act as a good buffer at physiological pH (7.20 – 7.40).
(14). Ans. (c). Tyrosine
(15). Ans. (b). Arginine
Nitric oxide (NO) is one of the gaseous bio-signaling molecules in the cell. It is also called as Endothelium Derived Relaxing Factor (EDRF). NO is synthesized from the amino acid arginine by the enzyme nitric oxide synthase in the endothelial cells of blood vessels. NO is a vasodilating agent and it relaxes the smooth muscle cells and thereby enhances blood flow through blood vessels.
(16). Ans. (c). Leucine
(17). Ans. (a). Alanine
(18). Ans. (c). Tryptophan
Methionine is also coded by a single codon (click here to know number of codons for each amino acid)
(19). Ans. (b). Aspartate
Aspartate was discovered from Asparagus racemosus by Plisson in 1827.
(20). Ans. (c). Threonine
Threonine was discovered in 1930 by William Cumming Rose
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