This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Biomolecules – Proteins – 1”.
1. Proteins are polymers of ______
a) α-amino acids
b) β-amino acids
c) γ-amino acids
d) δ-amino acids
Explanation: Amino acids contain amino (NH2) and carboxyl (COOH) functional groups. α-amino acids contain the NH2 group on the carbon adjacent to the COOH group. Proteins on hydrolysis yield only α-amino acids.
2. If the basic formula of an α-amino acid is R-CH(NH2)-COOH, where R is the side chain, what is the primary point of distinction between any two proteins?
a) Number of amino groups
b) Number of carboxyl groups
c) The side chain R
d) Relative positions of amino, carboxyl groups and R
Explanation: α-amino acids are the constituents of proteins. Different proteins are formed by polymerisation of different α-amino acids, which are formed due to the difference in the side chain substituted group R, which may be as simple as hydrogen or as complex as imidazole.
3. Which of the following amino acids is optically inactive?
Explanation: Glycine is the only naturally occurring α-amino acid that is optically inactive. This is because the α-carbon of glycine has two hydrogen atoms attached to it.
4. What is the one letter code for tyrosine?
Explanation: Tyrosine is a natural amino acid first obtained from cheese, hence the name. It has a 4-hydroxyphenylmethyl side chain. Its three-letter symbol is Tyr and its code is Y.
5. Which of the following amino acids are aromatic in nature?
Explanation: Amino acids can be aromatic only if their side chain consists of an aromatic compound. Methionine has a sulphur containing straight chain. Isoleucine has an isobutyl side group. Proline has a complex cyclic side chain. Histidine has a basic aromatic side group, imidazole.
6. Which of the following is a non-essential amino acid?
Explanation: Threonine, phenylalanine and valine along with seven other amino acids cannot be produced by the human body and must be obtained through diet. These are essential amino acids. Other amino acids like glutamine are synthesized by the human body and are therefore called non-essential amino acids.
7. Which of the following is a neutral amino acid?
Explanation: Neutral amino acids contain equal number of amino and carboxyl groups. Lysine, arginine and histidine contain two NH2 groups are one COOH group, and are hence basic amino acids.
8. Cysteine is a/an ________ amino acid.
d) sulphur containing
Explanation: Cysteine is a semi-essential, neutral amino acid. Its side chain is HS-CH2 (thiol), which is aliphatic and contains sulphur. Its symbol is Cys and its code is C.
9. Which of the following amino acids contains only one amino group?
Explanation: Lysine, asparagine and glutamine all contain two amino (NH2) groups, of which one is a part of the side chain. Leucine has a purely alkyl side chain and the only amino group it has is one the α-carbon.
10. What is the one letter code for asparagine?
Explanation: For the sake of simplicity, each amino acid has been given an abbreviation which is either a three-letter symbol or a one-letter code. Asparagine is represented as Asn or N.
11. Which of the following is incorrect regarding tryptophan?
a) It is an essential amino acid
b) It is a basic amino acid
c) It has an aromatic side chain
d) It is a non-polar amino acid
Explanation: Tryptophan is an essential amino acid with bicyclic aromatic side chain, indole. It cannot be synthesized by the human body and must be taken for nitrogen balance in the body. It has one amino and carboxyl group each and is a neutral amino acid.
12. The structure shown below is ______
a) Side chain of histidine
b) Side chain of tryptophan
c) Side chain of proline
Explanation: Proline is a neutral aliphatic amino acid, with cyclic pyrrolidine side chain, making it non-polar. It is non-essential as the human body can synthesize it.
13. Identify the amino acid with the formula HOOC-CH2-CH2-CH(NH2)-COOH.
a) Glutamic acid
b) Aspartic acid
Explanation: Glutamic acid is an amino substituted dicarboxylic acid which can be synthesized by the body. Its IUPAC name is 2-aminopentanedioic acid. Since it has two COOH groups (one more than the number of NH2 groups), it is acidic in nature.
14. Amino acids behave like carboxylic acids
Explanation: Generally, amino acids behave like salts rather than simple amines or carboxylic acids. This is due to the presence of both acidic (COOH) and basic (NH2) groups in the same molecule.
15. Amino acids can show amphoteric behaviour.
Explanation: In an aqueous solution of amino acid, COOH loses a proton and NH2 accepts a proton, giving rise to a dipolar zwitter ion. This is neutral but contains both positive and negative terminals. In this form, amino acids react both with acids and bases, and hence show amphoteric behaviour.
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