Conformation studies of biologically active fragments of bovine growth hormone

Vibrational spectroscopic studies of solid recombinant bovine growth hormone and related growth hormone analogs

Infrared and Raman spectra have been obtained for lyophilized recombinant bovine growth hormone (r-bGH), partially reduced, and completely reduced r-bGH, plus a tryptic digest fragment of r-bGH. Amide I and II data indicate r-bGH to have substantial helical character. Partially reduced r-bGH, in which the carboxyl terminal disulfide bridge (residues 181, 189) has been cleaved, has slightly less helical content than r-bGH. The spectral data indicate that breaking the carboxyl terminal cystine link produces only localized structural alterations. The additional cleavage of the second disulfide bridge (residues 53,164) leads to a further decrease in helix content, accompanied by increases in beta-sheet and disordered structures. A tryptic digest r-bGH fragment (residues 96-133), which contains a small amount of biological activity (approximately 10%), has predominantly helical structure.

Structural analysis of bovine somatotropin using monoclonal antibodies and the conformation-sensitive immunoassay

Bovine somatotropin was studied with respect to thermal stability, quantitative thermal denaturation kinetics, and refolding potential following thermal denaturation using a panel of 6 monoclonal antibodies and the Conformation-Sensitive Immunoassay (CSI). The antibody panel consisted of 4 conformation-dependent and 2 sequence-specific antibodies. Each of the antibodies revealed unique thermal stability profiles for their respective epitopes suggesting that they each recognize different antigenic determinants. Comparing the thermal stability profiles generated with these antibodies allowed the stability of bovine somatotropin to be "dissected" based on individual structural features. The degree to which bovine somatotropin is stabilized by disulfide bonds was examined using CSI-based quantitative thermal denaturation kinetics profiles generated under reducing and nonreducing conditions. All of the conformational epitopes unfolded faster under reducing conditions indicating that the two disulfide bonds within the somatotropin molecule impart some degree of global stabilization. The ability of bovine somatotropin to refold after reducing or nonreducing thermal denaturation was also examined using the antibody panel and the CSI. The results show that, although significant refolding was evident for some epitopes, bovine somatotropin cannot refold to the native state following thermal denaturation under either reducing or nonreducing conditions.

Activity of growth hormone peptides bGH 96-133 and hGH 95-133 in 3T3-F442A cells

Chemically synthesized bovine growth hormone (bGH) bGH 96-133 and its human homologue, hGH 95-133, have similar in vitro biological activities. Unlike native GH, bGH 96-133 and hGH 95-133 were completely without adipogenic or anti-insulin activity at doses up to 10 microM. bGH 96-133 had insulin-like activity, with a 100% increase in glucose uptake at 10 microM. bGH was anti-mitogenic and bGH 96-133 and hGH 95-133 were mitogenic (EC50 approximately 180 nM and maximal response at 1-2 microM). Only bGH 96-133 and hGH 95-133 displaced [125I]hGH 95-133 binding from 3T3-F442A fibroblasts with a Kd between 60-120 nM. bGH, hGH, insulin and IGF-I were without effect on [125I]hGH 95-133 binding. bGH 96-133 and hGH 95-133 did not significantly inhibit [125I]hGH or [125I]IGF-I binding. These experiments indicate that GH containing peptides bGH 96-133 and hGH 95-133 have mitogenic and insulin-like activity without the adipogenic, anti-insulin or anti-mitogenic activity of bGH. These peptides have a specific binding site which appears to be distinct from the GH, insulin and IGF-I receptors.

Evaluation of peptide-peptide interactions using reversed-phase high-performance liquid chromatography

The separation of peptides during RP-HPLC depends mainly upon differential hydrophobic interactions of the individual peptides being separated with the C18 group of the stationary phase. We have examined the behavior of dimeric disulfide-linked model peptides during RP-HPLC in order to study self-induced conformational effects. A set of 18 analogues of the amphipathic alpha-helical sequence Ac-LKLLKKLLKKLKKLLKKL-NH2 was used for this study. These analogues differed only by the successive replacement of each position with a cysteine. Strong peptide-peptide interactions, occurring through interchain hydrophobic forces, resulted in a presenting face to the C18 group, consisting primarily of lysine residues and, in turn, in early retention times. Three homo-dimers were also found to be strongly alpha-helical in water as determined by circular dichroism spectroscopy.

Structure of the propeptide of prothrombin containing the gamma-carboxylation recognition site determined by two-dimensional NMR spectroscopy

The propeptides of the vitamin K dependent blood clotting and regulatory proteins contain a gamma-carboxylation recognition site that directs precursor forms of these proteins for posttranslational gamma-carboxylation. Peptides corresponding to the propeptide of prothrombin were synthesized and examined by circular dichroism (CD) and nuclear magnetic resonance spectroscopy (NMR). CD spectra indicate that these peptides have little or no secondary structure in aqueous solutions but that the addition of trifluoroethanol induces or stabilizes a structure containing alpha-helical character. The maximum helical content occurs at 35-40% trifluoroethanol. This trifluoroethanol-stabilized structure was solved by two-dimensional NMR spectroscopy. The NMR results demonstrate that residues -13 to -3 form an amphipathic alpha-helix. NMR spectra indicate that a similar structure is present at 5 degrees C, in the absence of trifluoroethanol. Of the residues previously implicated in defining the gamma-carboxylation recognition site, four residues (-18, -17, -16, and -15) are adjacent to the helical region and one residue (-10) is located within the helix. The potential role of the amphipathic alpha-helix in the gamma-carboxylation recognition site is discussed.

Peptide alpha-helicity in aqueous trifluoroethanol: correlations with predicted alpha-helicity and the secondary structure of the corresponding regions of bovine growth hormone

The relationship between trifluoroethanol (TFE) enhancement of peptide alpha-helicity and protein secondary structure has been studied for a series of 11 peptides which span the complete primary sequence of bovine growth hormone (bGH). Ten of these peptides become increasingly alpha-helical as the solution concentration of TFE is increased. The amount of alpha-helicity developed by these peptides plateaus above 10 mol % TFE and ranges from 0 to 71%. The increased alpha-helicity, as determined by CD, closely correlates with the amount of alpha-helix predicted for eight of the eleven peptides analyzed (r = 0.9). Therefore, for this group of peptides, it appears that this technique can be used as a measure of alpha-helical propensity. Inclusion of the remaining three peptides in this analysis significantly lowers the correlation (r = 0.6). The reduced correspondence between TFE-enhanced and predicted alpha-helicity in this latter subset of peptides may be due to their relatively high hydrophobicity. In addition, the relevance of TFE-enhanced peptide alpha-helicity and the secondary structure of the corresponding protein regions was explored. Although the three peptides which form the largest amount of alpha-helicity in the presence of 10 mol % TFE correspond to alpha-helical regions of the protein, the overall correlation is significantly lower than is observed for the TFE-enhanced and predicted alpha-helicity. These findings suggest that the propensity of specific amino acid sequences for alpha-helix formation influences the amount of alpha-helicity which forms in corresponding protein sequences, but that other factors can modify this structure.

Protein fragments as probes in the study of protein folding mechanisms: differential effects of dihydrofolate reductase fragments on the refolding of the intact protein

We describe an approach for investigating the protein folding process, using protein fragments as inhibitory probes of the refolding protein. The refolding of Escherichia coli dihydrofolate reductase (EC 1.5.1.3), reversibly unfolded in 7 M urea, was monitored by the reappearance of enzyme activity after diluting the unfolded enzyme into low urea concentrations (less than or equal to 2 M) in the presence of substrates. Of eight protein fragments produced by limited proteolysis of the 159-residue enzyme, three isolated peptides--Ser-49/Glu-90, Ile-91/Glu-154, and Gln-102/Glu-154--were evaluated for their effects on the recovery of the refolding protein's enzymatic activity. By this criterion, 13 microM peptide Gln-102/Glu-154 inhibits the refolding of 0.015 microM enzyme by approximately 80%, while the related peptide, Ile-91/Glu-154, and peptide Ser-49/Glu-90 at the same concentration inhibit the recoverable activity of the refolding enzyme by less than or equal to 20%. None of these three peptides has any significant effect on the activity of the folded enzyme. Our results indicate that peptides may inhibit refolding differentially and that these effects may be extremely sensitive to fragment sequence and composition. We suggest that peptide specificity in the inhibition of protein folding may be exploited as a structural probe of protein folding mechanisms.

Proton nuclear magnetic resonance study of the histidine residues of pituitary bovine growth hormone

The pKa value of histidines 20, 22 and 170 of pituitary bovine growth hormone (pbGH) and their C2H deuterium exchange rates have been determined by high-resolution 1H-NMR spectroscopy. Partial assignment of these parameters indicate that His-170 is a surface residue with a half-life for deuterium exchange at the C2H position of approx. 36 h and a pKa value of 5.68 +/- 0.11. A reversible conformational change of pbGH has also been characterized in terms of the involvement of either histidines 20 or 22. At or near physiological pH values, one of these residues is expelled from a buried, hydrophobic position to become fully solvent-exposed. The disparity in pKa values between histidines 20 and 22 (4.67 +/- 0.20 and 5.94 +/- 0.10, in some combination) has been shown by computer modeling, to be compatible with these residues residing in an alpha-helical region of the protein.

Preferred conformational state of the N-terminus section of a bovine growth hormone fragment (residues 96-133) in water is an omega loop

The solution structure of a 38-amino-acid-residue, biologically active fragment of bovine growth hormone (bGH96-133) was investigated with a combined nuclear magnetic resonance (NMR) and computer modeling approach. With the distance geometry program DISGEO and distance constraints derived from nuclear Overhauser enhancement (NOE) experiments, it was found that residues Ser-100 to Tyr-110 circumscribe and omega-loop, a recently categorized feature of nonregular secondary protein structure.

Reoxidation of reduced bovine growth hormone from a stable secondary structure

In order to determine solution conditions appropriate for reoxidizing reduced bovine growth hormone (bGH), we have examined the possibility of using a particular denaturant concentration to poise the secondary and tertiary structure of the reduced protein in a stable, nativelike state. It was envisioned that the structure of the reduced molecule would differ from that of the final oxidized molecule solely by the absence of disulfide bonds. Dilution of concentrated samples of reduced and unfolded protein from 6.0 M guanidine into 4.5 M urea followed by air oxidation indicated it was possible to induce refolding and reoxidation to an oxidized monomeric species in high yield (approximately 90%). The choice of solution conditions was based on comparison of urea equilibrium denaturation data for native oxidized protein to those for completely reduced protein and to protein in which sulfhydryl groups had been either partially or completely reduced and subjected to modification with iodoacetamide or methyl methanethiolsulfonate. The denaturation behavior of these species supports the existence of equilibrium folding intermediates for bovine growth hormone and demonstrates that chemical modification of the protein is capable of inducing differences in the denaturation behavior of these intermediates. The changes in the protein absorption spectrum and helix-related circular dichroism signal, along with direct titration of protein sulfhydryl groups, indicated that the refolding/reoxidation of bGH is a multistate process. The ordered nature of the kinetic changes in these probes during reoxidation indicates that disulfide formation is a sequential process, with little mispairing in 4.5 M urea, and that it proceeds through one or more obligatory kinetic folding events. The equilibrium denaturation behavior of the oxidized molecule and the various chemically modified forms, together with the reoxidation data, indicated that the protein maintains a high degree of secondary structure without intrachain disulfide bonds. The formation of these disulfide bonds is a discrete process which occurs after a framework of protein secondary structure is established.

Characterization of an associated equilibrium folding intermediate of bovine growth hormone

In the preceding paper [Havel, H. A., Kauffman, E. W., Plaisted, S. M., & Brems, D. N. (1986) Biochemistry (preceding paper in this issue)], an associated intermediate was shown to be highly populated during the equilibrium denaturation of bovine growth hormone. In this paper, we describe its partial characterization and propose a mechanism for association. The associated equilibrium intermediate is populated under conditions that induce partial denaturation and at protein concentrations greater than 0.2 mg/mL. The remaining nativelike helical structure present in the partially denatured species is implicated in the mechanism of association as demonstrated by similar concentration dependencies and thermal stabilities of the helix and the associated equilibrium intermediate. Furthermore, it is suggested that a putative amphiphilic helix from residues 110-127 plays a critical role in the association as demonstrated by a diminution of the associated equilibrium intermediate when mixed with the peptide fragment 96-133. A model is proposed to account for these results in which partial denaturation exposes the segment of the protein corresponding to the hydrophobic face of the putative amphiphilic helix 110-127. This metastable form is the species from which association occurs. Association is stabilized by the hydrophobic interactions resulting from intermolecular packing of the lipophilic faces of the helices. The implications of these results to protein folding studies are described.

Covalent cross-linking of the bovine somatotropin dimer. Effects on growth-promoting, receptor-binding and immunological activities and preliminary characterization of the self-association

Bovine somatotropin, at pH 8.5 in 0.02 M-Bicine [NN-bis-(2-hydroxyethyl)glycine]/0.09M-NaCl, showed by frontal analysis the characteristics of a rapid monomer-dimer equilibrium whose dissociation constant was estimated to be 6.6 X 10(-6)M. Reaction of the hormone with dimethyl suberimidate lead to covalent cross-linking of the dimeric species. Under the conditions chosen (0.4 mg of bifunctional imidate and 1 mg of protein/ml at room temperature for 1 h) the cross-linked dimers accounted for 26% of the total protein, and these were isolated by molecular sieving in 0.29M-NH3/0.12M-NaCl. Covalent stabilization greatly diminished the growth-promoting activity and the ability to interact with somatogenic sites in both rat liver in vivo and rabbit liver microsomal fractions. Evidence indicating a non-critical role for amino groups involved in the covalent cross-linking was provided by a nearly equivalent derivative obtained after reaction with 3,3'-dithiobispropionimidate, which had substantial hormonal activity upon cleavage of the disulphide links. Conversely, immunological reactivity as demonstrated by radioimmunoassay was not affected by cross-linking. Details of the least-squares procedure employed to evaluate the self-association equilibrium constant has been deposited as Supplement SUP 50115 (7 pages) with the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies may be obtained on the terms indicated in Biochem.J. (1981) 193,5.

Intramolecularly hydrogen-bonded peptide conformations

Over the past few years the possible occurrence of intramolecularly hydrogen-bonded structures in linear and cyclic peptides has attracted increasing attention. In this review emphasis is given to solid-state studies, particularly by X-ray diffraction and infrared absorption techniques. Conformational energy calculations are also considered. The discussion is focused both on model peptides and biological activity polypeptide molecules. The tetrapeptide system (Formula: see text), examined allows one to discuss the extended C5 structure and the various folded conformations, namely the C7 (gamma-turn), C8, C10 (beta-turn), C11, and C13 conformations. The four latter forms may include cis peptide configurations. The oxy-analogs to the C7, C10, and C13 conformations and structures containing bifurcated hydrogen bonds are also discussed. The last sections describe intramolecularly hydrogen-bonded peptide structures involving: (1) a side-chain group, (2) the N-protecting group (in synthetic model compounds), and (3) a beta-amino acid.