Phenylalanine
In the genetic code set, phenylalanine is an amino acid with the most hydrophobic side chain. However, this hydrophobicity is different. The classical hydrophobic amino acids, Val, Leu, Ile, have their side chain branched and therefore protruding in different directions. The side chain in Phe is a single benzene ring, which is flat. Therefore, Phe is hydrophobic, but the topology of the side chain is drastically different. As the result, Phe has hydrophobic, but also compact side chain, which may occupy specific places in hydrophobic cores of proteins.
The π-system created by the benzene ring may have a special use in interaction of proteins with other species. This is called π-stacking interaction. Although, Phe is usually not as good in this interaction as Tyr or Trp, it can still donate some of the π-density to counterparts with reduced electron density. This could be for example, cations, nucleobases, or specific enzyme substrates.
Finally, Phe is a precursor to tyrosine, and a number of neurotransmitter and hormones, adrenaline, dopamine, phenetylamine, etc. Phenylalanine is a parent structure in diverse chemical pathways. Therefore, it is difficult to underestimate importance of this simple basic structure.
Interesting readings:
- Blau, N. Genetics of Phenylketonuria: Then and Now. Hum. Mutat., 37, 2016, 508-515, doi: 10.1002/humu.22980
- Wang, F. and Feng, C.-L. Metal-Ion-Mediated Supramolecular Chirality of l-Phenylalanine Based Hydrogels. Angew. Chem. Int. Ed., 2018, 57, 5655-5659, doi: 10.1002/anie.201800251
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