Using phage display mutagenesis selections, a variant of hGH that binds > 100-fold more tightly to hGH-R at site 1 was produced [41,42]. This variant (hGHv) has 15 mutations localized in its site 1 binding site and is fully biologically active but is totally specific to hGH-R, losing its ability to bind to hPRL-R. (This is a clinically relevant, second-generation hGH used for treating acromegaly). The recent high-resolution X-ray analysis of the ternary complex of hGHv bound to two copies of hGH-R  indicates global similarity to the wt-hGH complex but major important structural differences in the binding interfaces . These changes are exemplified in the finding that of the 17 H bonds that are formed between the hormone and receptors at sites 1 and 2, only two correspond to H bonds in the wt complex. This demonstrates the inherent plasticity of the protein-protein interfaces in this system where new contacts can be formed and still remain specific to hGHR in a biologically relevant way.
While it was anticipated that site 1 interactions would be altered as a result of the mutations at this site in the hormone, it was surprising that the largest changes in structural conformation were found in the site 2 interface, where no mutations were made . This interface in hGHv has only limited structural relationship to its wt counterpart. The same sets of hormone and receptor residues are used, but their stereochemical relationships are completely different, with several groups differing by more than 10 Á. Interestingly, this new, reconfigured hGHv-ECD2 interface has a binding association comparable to that found in the wt complex. This structure is an excellent example of the structural coopera-tivity that exists between binding sites in these systems. Another important aspect of this structure is that the distribution of binding energy among the residues energy at sites 1 and 2 is different than their counterparts in the wt ternary complex (Walsh and Kossiakoff, unpublished results).
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