This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Amino-acid substitute for tryptophan without altering proteins characteristics ? 
Select and explain. 
A. Tyrosine, B. Phenylalanine, C. Methionine, D. Alanine
B: Phenylalanine
Answer: B: Phenylalanine 
Explanation: Ans. is `b' i.e., Phenylalanine Tryptophan is an aromatic amino acid, thus it can only be replaced with other aromatic amino acid.Other two aromatic amino acids are :?PhenylalanineTyrosineAmong these two, phenylalanine can substitute tryptophan but not tyrosine becausePhenylalanine and tryptophan both are hydrophobic (non-polar).Whereas, tyrosine is hydrophilic (Polar)."The only other aromatic residue that can substitude for tryptophan is phenylalanine. Tyrosine cannot substitute since it has hydrophilic propey"