I’ve seen tyrosine classified as a hydrophobic amino acid due to its aromatic ring in some textbooks and as hydrophilic due to its hydroxyl group in other textbooks. How does tyrosine actually function in a peptide under physiological conditions (aqueous polar medium and pH~7)?


2 Answers 2


The answer to this question emerges from an examination of the structure of tyrosine — or, more strictly, the tyrosyl residue, which is how it exists in proteins, the concern of the question:

It has both hydrophobic and hydrophilic features and can exhibit both behaviours depending on the circumstances.

The ring is aromatic and hydrophobic, but the hydroxyl substituent is hydrophilic.

It is important not to try to fit things into rigid classifications that may not be appropriate for them. Indeed, one should ask how useful a particular classification is in any circumstance, and be aware of its limitations.

The classic illustration of the importance of these dual properties of tyrosine is in Perutz’s description of the differences between the oxy- and deoxy- structures of haemoglobin, where oxygenation of the haem cause the movement of tyrosine from the hydrophobic interior of the protein to the hydrophilic surface. This movement is associated with breaking of ionic bonds between the subunits in the overall change in quaternary structure:

Role of tyrosine in change in Hb structure

(This is a modified version of Fig. 3 of the original, produced many years ago for teaching purposes. The wiggly line is a cartoon representation of the polypeptide chain of the α-subunit of haemoglobin between His-F8 and the C-terminal Arg residue. For clarity, the preceding region is not shown but its continuation indicated by an arrow head.)

  • $\begingroup$ Hi, @David. I was wondering why the arrows in the figure were so curly and convoluted? What's this meant to represent? $\endgroup$
    – Jam
    Jul 5, 2018 at 19:59
  • $\begingroup$ @Jam — The arrrow heads just indicate the continuation of the polypeptide chain. The wigglyness is just meant to indicate the omplexity of the structure, with the key point being the helices in which the tyrosine is embedded move. I was a believer in giving students diagrams that they could reasonably attempt to reproduce in a written examination question, rather than using elaborate computer representations where this was impossible. For this forum, the latter type of diagram would be better, I admit. $\endgroup$
    – David
    Jul 6, 2018 at 0:59

Tyrosine (Tyr or Y, 4-hydroxyphenilalanin) is usually reffered as polar amino acid because its hydroxyl groupe (polar is rather hydrophilic), but there is a catch with the benzen ring and stacking pi-pi interactions.

So many authors just put this aa in the "middle". But the other parth of your question is much more complicated.

First of all you need to know that amino acids can behave as zwitterionts. Basicly this is state where is chared amino groupes and karboxyl groupes in amino acid equaly. This state descriped the isoelectric point which is value Ph where is amino acid occure as zwitter ion. The isoelectric point od Tyr is 5.66 Ph. So in 7 Ph it will be charged anyway.

Quick summary chemistry: Tyrosin is polar amino acid which is hydrophilic or hydrophobic up to conditions.

For biological use tyrosine is good adept for phosphorylation and others modifications of -OH group on C4 benzens ring. But basicly is up to circumstances how tyrisyl in peptideds will behave.

  • $\begingroup$ @David your answer is excellent (and I upvoted it), and you're right that the OP was interested in the behavior in peptides, but your aside here is wrong. Free tyrosine is of great interest in biology and medicine. It's a metabolic intermediate in a number of important pathways, include catecholamine synthesis. $\endgroup$
    – De Novo
    Jul 2, 2018 at 18:02
  • 3
    $\begingroup$ The comment made by @DanHall is correct, so I have withdrawn my original comment. The point I wanted to make was that in considering amino acid residues in proteins, the zwitterionic nature of the free amino acid is irrelevant. Some of the free amino acids (tyrosine happens to be an example) have other roles in the cell, and in those the zwitterionic nature ensures their solubility in the aqueous milieu. However their side chain is key to their function. $\endgroup$
    – David
    Jul 2, 2018 at 20:37

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