Proteins - Amino Acids, Peptide Bond - UNSOLVED PRACTICE SET
Chapter: Biomolecules | Topic: Proteins Amino Acids Peptide Bond
PROTEINS - AMINO ACIDS, PEPTIDE BOND - UNSOLVED PRACTICE SET
Topic: Proteins Amino Acids Peptide Bond
Multiple Choice Questions
Q1. Amino acids are the building blocks of:
- Carbohydrates
- Proteins
- Nucleic acids
- Lipids
Q2. The general formula of an Ξ±-amino acid is:
- R-CH(NHβ)-COOH
- R-CHβ-NHβ
- R-CO-NHβ
- R-CH(OH)-COOH
Q3. The peptide bond is formed by:
- The reaction between two -COOH groups
- The reaction between -NHβ of one amino acid and -COOH of another
- The reaction between two -NHβ groups
- The reaction between -OH and -COOH groups
Q4. Which of the following is an essential amino acid?
- Glycine
- Alanine
- Valine
- Serine
Q5. The peptide bond has partial double bond character due to:
- Hydrogen bonding
- Resonance
- Ionic bonding
- Van der Waals forces
Q6. The isoelectric point (pI) of an amino acid is:
- The pH at which it exists as a cation
- The pH at which it exists as an anion
- The pH at which it exists as a zwitterion with no net charge
- The pH at which it is most soluble
Short Answer Questions
Q7. What is a zwitterion? Draw the zwitterionic form of glycine.
Q8. Distinguish between essential and non-essential amino acids. Give two examples of each.
Q9. Write the reaction showing the formation of a dipeptide from two molecules of glycine. Name the bond formed.
Q10. Why do amino acids have high melting points and are soluble in water? Explain based on their structure.
Q11. What is the isoelectric point? Why does the solubility of an amino acid become minimum at its isoelectric point?
Q12. Your mother makes dal (lentils) and rice for dinner. Why is this combination considered a complete protein source in Indian vegetarian cuisine?
Long Answer Questions
Q13. (a) Describe the structure of an Ξ±-amino acid. What are the essential features?
(b) Explain the amphoteric nature of amino acids with suitable reactions.
(c) What is a zwitterion? Draw the zwitterionic form of alanine at its isoelectric point.
(d) Why do amino acids not migrate in an electric field at their isoelectric point?
Q14. (a) Explain the formation of a peptide bond with a suitable example. Why is it called an amide linkage?
(b) Describe the resonance in a peptide bond. How does this resonance affect the geometry around the peptide bond?
(c) A tripeptide contains three amino acids: glycine, alanine, and valine. How many different tripeptides can be formed using these three amino acids? (Consider all possible sequences.)
Q15. (a) Classify amino acids based on the nature of their R-groups (side chains). Give one example of each type:
(i) Acidic amino acid
(ii) Basic amino acid
(iii) Neutral amino acid with non-polar R-group
(iv) Neutral amino acid with polar R-group
(b) Explain why the sequence of amino acids in a protein is called its primary structure and why it is so important.
Numerical / Application-Based Problems
Q16. A tetrapeptide is hydrolyzed and found to contain 2 moles of glycine, 1 mole of alanine, and 1 mole of valine.
(a) How many different tetrapeptides are possible with this amino acid composition? (Consider that glycine appears twice.)
(b) If the N-terminal amino acid is glycine and the C-terminal is valine, how many different sequences are now possible?
(c) Calculate the molecular mass of the tetrapeptide. (Molecular masses: glycine = 75, alanine = 89, valine = 117. Account for loss of water during peptide bond formation.)
(d) When this tetrapeptide is completely hydrolyzed, how many moles of water are consumed?
(e) Why is it difficult to determine the exact sequence of a peptide with more than 10 amino acids using simple chemical methods?
Q17. The following data relates to amino acid analysis:
| Amino Acid | pKa (-COOH) | pKa (-NHββΊ) | pKa (R-group) | pI |
|---|---|---|---|---|
| Glycine 2.34 | 2.34 | 9.60 | - | ? |
| Aspartic acid | 1.88 | 9.60 | 3.65 | ? |
| Lysine | 2.18 | 8.95 | 10.53 | ? |
(a) Calculate the isoelectric point (pI) for each amino acid using the appropriate formula.
(b) At pH 7, predict whether each amino acid will migrate towards the anode or cathode during electrophoresis.
(c) A mixture of these three amino acids is subjected to electrophoresis at pH 6. Predict the direction of migration for each.
(d) Why is the isoelectric point of lysine much higher than that of glycine?
Q18. In a school biology-chemistry integrated project, students study protein digestion.
(a) When you eat a boiled egg (rich in protein), it enters your stomach where pepsin breaks it down. What type of bond does pepsin cleave? Name the smaller units formed.
(b) In the small intestine, trypsin and chymotrypsin further break down the protein. What is the final product of complete protein digestion?
(c) A student with phenylketonuria (PKU) cannot metabolize the amino acid phenylalanine. Why must such a patient avoid foods containing the artificial sweetener aspartame? (Hint: Aspartame is a dipeptide of aspartic acid and phenylalanine.)
(d) During a blood test, a doctor measures the levels of amino acids in a patient's blood. If the level of essential amino acids is low, what dietary advice would you give?
(e) Athletes often consume whey protein supplements after workouts. Why is the timing of protein intake important for muscle recovery?