Question

A tetrapeptide is made of naturally occurring alanine, serine, glycine, and valine. If the C-terminal amino acid is alanine and the N-terminal amino acid is chiral, the number of possible sequences of the tetrapeptide is: 
 

• A. 4
• B. 8
• C. 6
• D. 12

Hint:

Fixing the C-terminal amino acid reduces the number of possible arrangements.

Answer 1

The Correct Option is A

Step 1: Defining the Tetrapeptide Structure
- A tetrapeptide is composed of four amino acids. 
- The amino acid at the C-terminus is specified as alanine. 
- The N-terminus must be a chiral amino acid, specifically valine or serine. 
Step 2: Enumerating Potential Sequences 
Given that glycine is achiral, the available choices for the N-terminal amino acid are valine or serine. This results in four possible sequence arrangements:
1. Val-Gly-Ser-Ala 
2. Val-Ser-Gly-Ala 
3. Ser-Gly-Val-Ala 
4. Ser-Val-Gly-Ala 
Conclusion: There are 4 distinct sequences that meet the specified criteria.