Q.54 Hydrophobic nature of amino acids affects the tertiary structure of proteins.
Which of the following option shows the given amino acids in decreasing order
of their hydrophobic character?

A. Leucine
B. Phenylalanine
C. Glycine
D. Histidine
E. Lysine

Choose the correct answer from the options given below:

1. A, C, E, D, B
2. B, A, C, D, E
3. B, E, D, C, A
4. A, B, D, C, E

   Phenylalanine and leucine are the most hydrophobic among the listed amino acids, followed by glycine (neutral), then histidine and lysine (hydrophilic). The correct order of decreasing hydrophobic character is A (Leucine), B (Phenylalanine), no—B, A, C, D, E is the accurate sequence based on standard scales.​

   Correct Option Explained

   Option: B, A, C, D, E (Phenylalanine > Leucine > Glycine > Histidine > Lysine)

   Standard hydrophobicity scales (e.g., at pH 7) rank phenylalanine highest (index ~100) due to its large aromatic ring, followed closely by leucine (~97) with its aliphatic isobutyl chain. Glycine (~0) serves as the neutral reference with no side chain. Histidine (~8, but often hydrophilic due to imidazole) and lysine (negative, charged side chain) prefer water.

   Why Other Options Are Incorrect

   • A, C, E, D, B: Starts with leucine over phenylalanine (reversed) and places hydrophilic lysine before histidine.

   • B, E, D, C, A: Positions lysine (very hydrophilic) too high, after phenylalanine.

   • A, B, D, C, E: Correctly prioritizes leucine and phenylalanine first but inserts histidine before glycine, ignoring glycine’s neutral benchmark.

   Introduction to Hydrophobicity

   The hydrophobic nature of amino acids drives protein tertiary structure by burying nonpolar side chains in the core, stabilizing folds via hydrophobic interactions. Ranking them correctly—phenylalanine, leucine, glycine, histidine, lysine—helps predict folding in genetics and biotech studies.

   Hydrophobicity Ranking

   Standard scales (e.g., Kyte-Doolittle or pH-adjusted indices) classify:

   Amino Acid	Symbol	Hydrophobicity Index (pH 7)	Key Feature
   Phenylalanine	B (Phe)	100 ​	Aromatic ring, very hydrophobic
   Leucine	A (Leu)	97 ​	Aliphatic chain, highly hydrophobic
   Glycine	C (Gly)	0 ​	No side chain, neutral reference
   Histidine	D (His)	~8 (variable) ​	Imidazole ring, weakly hydrophilic
   Lysine	E (Lys)	-23 ​	Charged amine, hydrophilic

   This order reflects burial preference: Phe/Leu form cores; Gly is flexible; His/Lys stay surface-exposed.​

   Impact on Protein Folding

   In plant proteins or enzymes, hydrophobic cores (Phe, Leu) resist water, while hydrophilic Lys/His enable solubility. For GATE Life Sciences prep, memorize this for questions on biochemistry pathways or molecular techniques.​