Small structural changes probably account for super-agonist binding properties. Figures a-d are presented as wall-eyed stereo images. (a) Magnification (stereo image) of the region around Thr13 and Phe82 of wild type IL-4, a 2F
obs – F
calc electron density map is shown at a level of 1.5σ. (b) The same area is shown for the super-agonistic IL-4 variant T13D; the exchange of Thr13 for an aspartate leads to a change in side chain conformation of Arg85, which exhibits a bi-dentate hydrogen bond between the carboxylate group of Asp13 and the guanidinium group of Arg85. (c) Area shown for the super-agonist IL-4 F82D; as in (b) a hydrogen bond between Asp82 and Arg85 leads to a change in side chain conformation of Arg85. However, two alternative side chain conformations can be observed for Arg85, one that is "bound" to Asp82 and a second where the Arg85 side chain is oriented towards the solvent. (d) The double variant IL-4 T13D/F82D shows similar side chain orientations to those in IL-4 F82D, but only a single side chain conformation is observed for Arg85.