Crystal structure and site 1 binding energetics of @entity1 XXXX .

@entity1 @entity8862 @entity219 @entity4151 @entity5804 @entity493
In primates, @entity5804 ( @entity4151 ) is a pituitary hormone with fundamental roles during pregnancy involving fetal growth, metabolism, and stimulating lactation in the mother. @entity1 @entity5804 ( @entity4151 ) is highly conserved with @entity1 growth hormone (hGH) and both hormones bind to the hPRLR extracellular domain (ECD), the first step in receptor homodimerization, in a @entity8862 -dependent manner. A modified surface plasmon resonance method was developed to measure the kinetics for @entity4151 and hGH binding to the hPRLR ECD, with and without @entity8862 and showed that @entity4151 has about a tenfold higher affinity for the hPRLR ECD1 than hGH. The crystal structure of the free state of @entity4151 has been determined to 2.0 A resolution showing the molecule possesses an overall structure similar to other long chain four-helix bundle cytokines. Comparison of the free @entity4151 structure with the 1:1 complex structure of hGH bound to the hPRLR ECD1 suggests that two surface loops undergo conformational changes >10 A upon binding. An 18 residue Ala-scan was used to characterize the binding energy epitope for the site 1 interface of @entity4151 . Individual @entity493 substitutions at five positions reduced binding affinity by a DeltaDeltaG > or = 3 kcal mol(-1). A comparison of the @entity4151 site 1 epitope with that previously determined for hGH indicates contributions of individual residues track reasonably well between @entity4151 and hGH. In particular, residues involved in the @entity219 -binding site and Lys172 constitute the principal binding determinants for both hormones. However, several residues that are identical between @entity4151 and hGH contribute quite differently to the binding of the hPRLR ECD1. Additionally, the overall magnitudes of the DeltaDeltaG changes observed from the Ala-scan of @entity4151 were markedly larger than those determined in the comparative scan of hGH to the hPRLR ECD1. The structural and biophysical data presented here show that subtle changes in the structural context of an interaction can lead to significantly different effects at the individual residue level.
