Tyrosine / TYR / Y (EnzDes coloring)
Tyrosine (TYR or Y) is one of the more hydrophobic amino acids. Its shape in Foldit is similar to that of phenylalanine, but it has a "stick" on top, consisting of an oxygen atom.
In the Foldit Lua interface, the functions structure.GetAminoAcid and structure.SetAminoAcid use lowercase "y" for tyrosine.
Tyrosine's sidechain has an oxygen atom that can act as both a hydrogen bond donor and acceptor.
The an oxygen atom in tyrosine's sidechain can act as a hydrogen bond acceptor or donor. See the Sidechain Bonding Gallery and the Sidechain Bonding Table for other sidechain donors and acceptors.
As with phenylalanine, the six-sided ring is related to benzene. The addition of the oxygen atom changes the ring structure to phenol, a common example of a phenyl group, where one of the hydrogen atoms in benzene is replaced by a different atom.
For more, see tyrosine on wikipedia.