The structure of H-2K b /S598 Q600Y consists of four heterodimers in the asymmetric unit (rmsd of 0.09 Å for heavy chain (HC) Ca's), the four copies of the peptide adopting virtually identical conformations (rmsd of 0.12 Å for all peptide atoms and 0.05 Å for Ca atoms). S598 Q600Y displays the same conformation as the S598 determinant (the average rmsd for peptide Ca atoms between the two structures is 0.24 Å ) and forms equivalent interactions with the MHC. The only prominent structural difference is observed at the mutated position (Q3RY3) ( Figure 5B,E, 6A,B) . In contrast to Gln-3, the Tyr-3 side chain is oriented towards the a-2 helix rather Figure 5E ). The side chain of Tyr-3 also forms a number of close contacts with the residues in its immediate environment, specifically HC residues Gln-114, Arg-155, Leu-156 and Tyr-159, as well as intramolecular interactions with Phe-5 (Table S2 ). In addition to Tyr-3, deviations of potential significance (.0.5 Å ) in the peptide structures that are attributable to the Q660Y mutation are observed at Ile-4 and Phe-5.