Step 1: Understanding the Question:
Amino acids are classified as acidic, basic, or neutral based on the relative number of amino (\( -NH_{2} \)) and carboxyl (\( -COOH \)) groups in their structure.
Step 2: Detailed Explanation:
- Thr (Threonine): Contains one amino and one carboxyl group. It is a neutral amino acid.
- Trp (Tryptophan): Contains one amino and one carboxyl group. It is a neutral amino acid.
- His (Histidine): Contains more amino groups than carboxyl groups. It is a basic amino acid.
- Glu (Glutamic acid): Contains two carboxyl groups and one amino group. Due to the extra carboxyl group, it is an acidic amino acid.
Step 3: Final Answer:
Glu (Glutamic acid) is the acidic amino acid.