Among the 20 standard protein coding amino acids, which was the last discovered amino acid?

Options

A. Leucine
B. Isoleucine
C. Threonine
D. Serine

Answer

To answer the question regarding which was the last discovered amino acid among the 20 standard protein-coding amino acids, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Standard Amino Acids**: Recognize that there are 20 standard amino acids that are coded for proteins.

2. **Research the Discovery Dates**: Look into the historical context of the discovery of these amino acids to find out when each was discovered.

3. **List Key Discoveries**:
- Leucine was discovered by Proust from fermented milk curd.
- Isoleucine was discovered by Lewis in 1905.
- Threonine was discovered by William C. Rose in 1936.
- Serine was discovered by Amyl Kramer in 1865 from silk protein.

4. **Compare Discovery Dates**: Analyze the dates of discovery:
- Serine (1865)
- Threonine (1936)
- Isoleucine (1905)
- Leucine (date not specified in the transcript but is earlier than the 1900s)

5. **Determine the Last Discovered Amino Acid**: From the comparison, it is clear that threonine, discovered in 1936, is the last among the standard amino acids.

6. **Conclusion**: Therefore, the last discovered amino acid among the 20 standard protein-coding amino acids is **Threonine**.