Based on the uncertain notion of reversal of peptide bond direction causing mirror imaging of the molecule mentioned above, another assumption has been made to suggest that the mirror image of a retro sequence as represented by a retro-inverso sequence may return side chains to their original positions in space [7] (Supplementary Fig. 1). Indeed, there have been many instances where retro-inverso sequences have shown promise in mimicking the parent peptide [1] and [25]. Guptasarma has suggested that since a retro-inverso peptide is mirror image of the corresponding retro-peptide, it should have a conformation topologically similar to the pro-peptide because mirror image of mirror image should resemble the original [7]. However, it must be remembered that these arguments of structural mimicry apply only at the primary structure level and do not necessarily operate at secondary and tertiary structural levels. In order to investigate if the stringency of recognition across the trio of peptide/retro-peptide/retro-inverso peptide could vary from system to system, we set out to explore the Ribonuclease S peptide to understand the potential of cross-recognition in systems as diverse as an enzyme,
Sulfadimethoxine and T cells. Our structural studies involving NMR (Fig. 1 and Fig. 2), chromatography and CD (Supplementary Fig. 2) indicate that these peptides have unique structural distinctions. Nevertheless, it was important to know if upon interaction with binding partners viz. the S protein, antibody or T cell receptor, modulation of structural differences could facilitate an induced fit with payment of energy penalties. However, as shown in Fig. 3 and Fig. 4, neither RS nor RIS peptides could complement the S protein either structurally or functionally. This inability of RS and RIS peptides to mimic S peptide was not restricted to an enzyme system alone. The exquisite biological specificity for recognition of the native S peptide was also seen in antibody recognition and T cell stimulation. Ours is perhaps the first study where the pro-, retro- and retro-inverso peptides have been studied in the context of reconstitution of enzyme activity, recognition by
antibodies and for signaling through the T cell receptor. The interesting conclusion from our study is that the thresholds of biological recognition are pretty much constant across systems as diverse as enzymes and antigen specific receptors. Thus even as the recognition motifs employed by S protein, the antibodies and the T cell receptors are quite different, they all show strictly stringent recognition for the S peptide and no recognition for either of the two retro-peptides.
