Conformational similarity of ovine prolactin and bovine growth hormone

Ovine placental lactogen and ovine prolactin: partial proteolysis and conformational stability

The high-resolution structure of ovine placental lactogen (oPL) and ovine prolactin (oPRL), not yet established in detail, was probed by limited proteolysis with the Glu-specific protease from Staphylococcus aureus V8. While in hGH there were no cleavage sites inside of any of the four alpha-helices, the analysis of the fragments obtained after partial proteolysis of oPL showed a site of cleavage at the putative third helix, suggesting that this helix is partially unwound at this point. The partial proteolysis of the rest of the molecule was compatible with a similar folding pattern for oPL, hGH and pGH, on the basis of the crystal structure of these last hormones. In the case of oPRL, proteolytic cleavage occurred at Glu residues which would be located at the end of the first helix and the beginning of the second in the hGH folding model, suggesting that these helices are shorter in oPRL than in hGH. In order to gain further insight on the folding of these molecules, circular dichroism and intrinsic fluorescence measurements were used to examine the effect of denaturing conditions on oPL and oPRL. After exposure to 6 M guanidine the unfolding of both proteins was completely reversed upon elimination of the denaturing agent. In contrast, exposure to pH 3.0 caused an irreversible decrease in the alpha-helical content in both hormones, most striking for oPL, indicating that this hormone is less stable than oPRL or hGH.

pH-induced transition and Zn2+-binding properties of bovine prolactin

A pH-induced conformational transition was found in bovine prolactin within the physiologically significant pH region from 6.5 to 8.5. The thermal stability of prolactin at pH 6.5 is essentially higher than at pH 8.5. Bovine prolactin binds zinc ions with an apparent association constant of 2 x 10(5) M(-1) at pH 6.5 and 1 x 10(4) M(-1) at pH 8.5. The pH dependence of both thermal stability and zinc binding surrounding the pKa of histidine suggests that these residues plays a key role in the structural integrity of bovine prolactin.

Detection and characterization of an ovine placental lactogen stable intermediate in the urea-induced unfolding process

The urea-induced equilibrium unfolding of ovine placental lactogen, purified from ovine placenta, was followed by size-exclusion chromatography, far-UV CD, and intrinsic tryptophan fluorescence. The data obtained by each of these methods showed a poor fit to a two-state model involving only a native and an unfolded form. A satisfactory fit required, instead, a model that involved a stable, partially folded form in addition to the native and unfolded ones. The results obtained from the best-fitting theoretical curves for the three-state model indicated that this intermediate state, which is the predominant species in solution at 3.6 M of urea activity, is compact, largely alpha-helical, and changes considerably the native-like tertiary packing around its tryptophan residues. These findings suggest that this stable intermediate exhibits properties similar to those that characterize the molten globule state.

Secondary structure prediction of anterior pituitary hormones. Lack of correlation between predicted values and circular dichroism data

The secondary structures of human somatotropin, human choriomammotropin, ovine and porcine prolactin, human, ovine and porcine beta-lipotropin, human and ovine lutropin, human thyrotropin, human corticotropin, alpha-melanotropin and human beta-melanotropin have been predicted by the method of Chou & Fasman. Predicted contents of alpha-helix and beta-sheet do not correspond well with values estimated from circular dichroism spectra.

The relation of polypeptide hormone structure and flexibility to receptor binding: the relevance of X-ray studies on insulins, glucagon and human placental lactogen

Thr relevance of the crystal structure of the polypeptide hormones, insulin, glucagon and human placental lactogen to conformation and flexibility in solution and to receptor binding is considered. X-ray studies for crystal forms of glucagon, human placental lactogen and three insulin derivatives (A1 acetyl insulin, A1-t-butoxy carbonyl insulin and A1 2,2-dimethyl-3-formyl-L-thiazolidine-4-carbonyl insulin) are reported. Neither glucagon nor human placental lactogen are as ordered as insulin in the crystal form. Glucagon crystals undergo distinct transformations on changing the pH of the mother liquor from pH 9.5 to pH 6, indicating that the glucagon molecule is flexible in the crystal, as it is in solution. On the other hand all insulin analogues have a similar three dimensional structure to that of native insulin. Three dimensional difference Fourier studies of two insulin derivatives at 3 A resolution indicate the position of the modifying groups and define the small conformational changes which have occurred. The in vitro biological activity and receptor binding decrease with the increasing size of the group added to A1. The correlation of the structure analysis with the biological data strongly implicate a region close to A1 in receptor binding. Insulin appears to bind to the receptor in a specific conformation similar to that observed in the crystal structure and in solution; amino acid residues which are separated in the primary structure but brought into close juxtaposition in the tertiary structure are important for full potency.