Maria Pellegrini

Parathyroid Hormone-Receptor Interactions Identified Directly by Photocross-linking and Molecular Modeling Studies

Direct mapping of the interface between parathyroid hormone (PTH) and its receptor (hPTH1-Rc) was carried out by photoaffinity scanning studies. Photoreactive analogs of PTH singularly substituted with a p-benzoylphenylalanine (Bpa) at each of the first six N-terminal positions have been prepared. Among these, the analog [Bpa1,Nle8,18,Arg13,26,27,l-2-Nal23,Tyr34]bPTH-(1–34)NH2(Bpa1-PTH-(1–34)) displayed in vitroactivity with potency similar to that of PTH-(1–34). The radioiodinated analog 125I-Bpa1-PTH-(1–34) cross-linked specifically to the hPTH1-Rc stably expressed in human embryonic kidney cells. A series of chemical and enzymatic digestions of the hPTH1-Rc–125I-Bpa1-PTH-(1–34) conjugate suggested that a methionine residue (either Met414 or Met425) within the contact domain hPTH1-Rc-(409–437), which includes the transmembrane helix 6 and part of the third extracellular loop, as the putative contact point. Site-directed mutagenesis (M414L or M425L) identified Met425 as the putative contact point. Molecular modeling of the hPTH1-Rc together with the NMR-derived high resolution structure of hPTH-(1–34), guided by the cross-linking data, strongly supports Met425, at the extracellular end of transmembrane helix 6, as the residue interacting with the N-terminal residue of the hPTH-(1–34). The photocross-linking and molecular modeling studies provide insight into the topologic arrangement of the receptor-ligand complex.

Addressing the Tertiary Structure of Human Parathyroid Hormone-(1–34)

Parathyroid hormone (PTH) regulates mineral metabolism and bone turnover by activating specific receptors located on osteoblastic and renal tubular cells and is fully functional as the N-terminal 1–34 fragment, PTH-(1–34). Previously, a “U-shaped” conformation with N- and C-terminal helices brought in close proximity by a turn has been postulated. The general acceptance of this hypothesis, despite limited experimental evidence, has altered the direction of the design of PTH-analogs. Examining the structure of human PTH-(1–34) under conditions that encompass the different environments the hormone may experience in the approach to and interaction with the G-protein-coupled receptor (including benign aqueous and saline solutions and in the presence of dodecylphosphocholine), we observe no evidence for a U-shape conformation or any tertiary structure. Instead, the N- and C-terminal helical domains, which vary in length and stability depending on the conditions, are separated by a highly flexible region of undefined conformation. These observations are in complete accord with recent conformational studies of PTH-related protein analogs containing lactams (Mierke, D. F., Maretto, S., Schievano, E., DeLuca, D., Bisello, A., Mammi, S., Rosenblatt, M., Peggion, E., and Chorev, M. (1997) Biochemistry 36, 10372–10383) or a model amphiphilic α-helix (Pellegrini, M., Bisello, A., Rosenblatt, M., Chorev, M., and Mierke, D. F. (1997) J. Med. Chem. 40, 3025–3031). Reliable structural data from different environmental conditions are absolutely requisite for the next step in the design of non-peptide PTH analogs.

Receptor–Ligand Interactions

Receptor–ligand interaction is the fundamental step in the initiation of a signal from outside to inside the cell, a process often called signal transduction. The fidelity of this signaling, and therefore the interaction between the receptor and specific ligand, is integral to cellular vitality and function. Defining the elements required for receptor–ligand interactions provides novel insight into the development of therapeutic agents to control (enhance or inhibit) the signaling event and therefore treat various disease states.

Conformational characterization of a peptide mimetic of the third cytoplasmic loop of the G‐protein coupled parathyroid hormone/parathyroid hormone related protein receptor

The third‐cytoplasmic loop of the G‐protein coupled receptor responsible for the activity of parathyroid hormone and parathyroid hormone‐related protein has been structurally characterized in aqueous solution in the presence of sodium dodecylsulfate and dodecylphosphocholine micelles. The high‐resolution conformation of the 29‐amino acid peptide containing the sequence of the cytoplasmic loop was obtained by CD and nmr. The structure was refined using a two‐step distance geometry based method that first includes the removal of all side chains to quickly locate the globular fold of the peptide. After a simulated annealing protocol, the side chains are added in a random fashion. The resulting conformation was further refined with nuclear Overhauser enhancement restrained molecular dynamics using a two‐phase simulation cell consisting of carbon tetrachloride and water as a mimetic of the biphasic, hydrophobic/hydrophilic character of the micelles in which the experimental measurements were carried out. The topological orientation of the cycloplasmic loop within the micelle was determined by addition of 5‐doxylstearate and monitoring the decrease of nmr signal intensities from the radical‐induced relaxation. The conformation and relative orientation of the peptide provided insight into the mechanism by which the cytoplasmic portion of the receptor activates the heterotrimeric, guanine nucleotide‐binding regulatory protein, one of the first steps in signal transduction.

Threonine6‐bradykinin: Conformational study of a flexible peptide in dimethyl sulfoxide by NMR and ensemble calculations

The conformation of the natural peptide threonine6 (Thr6)‐bradykinin, Arg1‐Pro2‐Pro3‐Gly4‐Phe5‐Thr6‐Pro7‐Phe8‐Arg9, was investigated in DMSO by nmr spectroscopy and computer simulations. The structural analysis of the Thr6‐peptide is made particularly interesting by the fact that despite the high sequence homology with native bradykinin (only one conservative substitution: Ser6/Thr6) there is a marked and significant difference in the biological profiles of the two peptides.

Structure, dynamics, and topological orientation of the polyether, ionophore antibiotic monensin, in a micellar environment

The structure and dynamics of the ionophoric antibiotic monensin in the presence of micelles have been determined. The conformation of monensin was derived from 50 nuclear Overhauser enhancement (NOE) derived distance restraints and metric-matrix based distance geometry calculations. The conformation was further refined with extensive NOE restrained molecular dynamics simulations carried out in a biphasic simulation cell. From the addition of doxylstearate and monitoring of the induced relaxation of the nmr signals, the relative topological orientation of the molecule within the micelle was ascertained. The results indicate two dihedral angles that act as hinge regions allowing the molecule to adopt a wide range of conformations. Considering the biological activity of monensin, i.e., the capture and transport of cations across cell membranes, an open and closed form of monensin have been postulated. The identification of these hinge regions, which are only observed in the membrane-like environment of the detergent micelles, provides insight into the mechanism of action and can serve as targets for modification to alter the biological profile of monensin.

1,2,5-Trisubstituted 1,4-diazepine-3-one: A novel dipeptidomimetic molecular scaffold

The continuing effort to transform bioactive peptides into non-peptide peptidomimetics of therapeutic potential requires a diversity of tools such as molecular scaffolds, pseudopeptide modifications, and conformation mimetics. To this end, a novel polyfunctional monoheterocyclic system, 1,2,5-trisubstituted hexahydro-3-oxo-1H-1,4-diazepine ring (DAP), was designed. The linear precursor for the DAP was generated through a reductive alkylation step including a modified side chain and an α-amino function of two amino acid derivatives. Structural analysis of model diastereomeric DAPs, employing1H and13C NMR and computer simulation, revealed the conformational preferences of this system. The structural similarities to the 1,4-benzodiazepine, a common molecular scaffold for many non-peptidic peptidomimetic agents, and the pronounced dipeptidomimetic character of the DAP system offer a new powerful tool to medicinal chemists engaged in rational peptide-based drug design.

Conformational investigation of a novel dipeptide based molecular scaffold

The conformational features of a novel, dipeptide-based molecular scaffold are described. Four model systems of a trisubstituted 1,4-diazepine-3-one system, varying in the chirality and amino acid within the ring system, have been investigated by high-resolution NMR and metric-matrix distance geometry calculations. Because of the small number of protons within the scaffold, nuclear Overhauser effects provide only limited conformational information. Instead, extensive use of scalar1H−H1 and1H−13C coupling constants was utilized in the refinement. The resulting conformations of the model systems provide insigh into the expected topological orientation of the amino acids or chemical functionalities and attached to the seven-membered ring system, the first step of the utilization of this scaffold in the rational design of peptidomimetics.