Fig. 3.

GCha and GHle effectively catalyze peptide exchange on K^b. K^b–FAPGNYPAL and K^b–FAPGNYPAA were prepared by folding K^b/hβ₂m with the indicated peptides. To these complexes, 10 mM dipeptides were added as indicated, and exchange was monitored by measuring the association rate (k_on) of SIINFEK_TAMRAL (100 nM) peptide in a fluorescence anisotropy assay. (A) A representative experiment shows exchange of the K^b-bound high-affinity peptide FAPGNYPAL for SIINFEK_TAMRAL in the presence and absence of indicated dipeptides. (B) Structure of dipeptides with nonnatural amino acids GCha and GHle. (C, Left) Representative experiment showing exchange of FAPGNYPAA bound to K^b in the presence of indicated dipeptides and in the absence of any dipeptide. (Right) k_on values (average ± SEM of three or more independent experiments). All association rates are shown in Table S2.