Ghrelin-induced stimulation of colonic propulsion is dependent on hypothalamic neuropeptide Y1- and corticotrophin-releasing factor 1 receptor activation.

Peptides participating in the hypothalamic control of feeding behaviour are also involved in the central autonomic control of gastrointestinal functions, such as secretion and motility. An anatomical interaction and functional relationship in the central nervous system between the feeding-related peptides neuropeptide Y and ghrelin is well documented. Furthermore, it has been shown that feeding-related peptides can influence digestive function via central corticotrophin-releasing factor (CRF) pathways. In the present study, we investigated the role of ghrelin in the central autonomic control of colonic motility. Furthermore, we addressed the hypothesis that ghrelin is involved in the hypothalamic control of colonic motor function, utilizing central neuropeptide Y receptors and hypothalamic CRF pathways. Ghrelin (0.03, 0.06 and 0.12 nmol) bilaterally microinjected into the paraventricular nucleus (PVN) induced a significant stimulation of colonic propulsion. In particular, the colonic transit time decreased from 312+/-7 min to 198+/-12 min. Microinjection of the neuropeptide Y1 receptor antagonist, BIBP-3226 (200 pmol), or the nonselective CRF receptor antagonist, astressin (30 pmol), into the PVN abolished the stimulatory effect of ghrelin injected into the PVN on colonic transit time, whereas pretreatment with the selective CRF2 receptor, antisauvagine-30 (28 pmol), failed to affect the effect of PVN-ghrelin injection on colonic propulsion. These results suggest that ghrelin can act as central modulator of gastrointestinal motor functions at the level of the PVN via neuropeptide Y1- and CRF1 receptor-dependent mechanisms.

The NMR solution structure of the pheromone Er-1 from the ciliated protozoan Euplotes raikovi.

The 3-dimensional structure of the pheromone Er-1 isolated from the ciliated protozoan Euplotes raikovi has been determined in aqueous solution by 1H NMR spectroscopy. The structure of this 40-residue protein was calculated with the distance geometry program DIANA on the basis of 503 upper distance constraints derived from nuclear Overhauser effects and 77 dihedral angle constraints derived from spin-spin coupling constants, and refined by restrained energy minimization with the program OPAL. The Er-1 solution structure is represented by a group of 20 conformers with an average RMS deviation relative to the mean structure of 0.55 A for the backbone atoms N, C alpha, and C', and 0.93 A for all heavy atoms of the complete polypeptide chain, residues 1-40. The molecular architecture is dominated by an up-down-up bundle of 3 alpha-helices formed by residues 2-9, 12-19, and 24-33. Although this core part coincides closely with the previously determined structure of the homologous pheromone Er-10, the C-terminal peptide segment adopts a novel conformation. This is of interest in view of previous suggestions, based on sequence comparisons, that this molecular region may be important for the different specificity of receptor recognition by different pheromones.

Structure comparison of the pheromones Er-1, Er-10, and Er-2 from Euplotes raikovi.

The NMR structures of the homologous pheromones Er-1, Er-10, and Er-2 from the ciliated protozoan Euplotes raikovi are compared. For all 3 proteins the molecular architecture is made up of an antiparallel 3-helix bundle. The preservation of the core part of the structure is directly manifested by similar patterns of slowed backbone amide proton exchange rates, hydrogen bond formation, and relative solvent accessibility. To align the 6 half-cystine residues in the individual sequences within the preserved 3-dimensional core structure, several deletions and insertions had to be introduced that differ from those previously proposed on the basis of the primary structures. Of special interest is a deletion in the second helix of Er-2, which is accommodated by a transition from an alpha-helix in Er-1 and Er-10 to a 3(10)-helix in Er-2. The most significant structural differences are located in the C-terminal part of the proteins, which may have an important role in specific receptor recognition.

The pseudo-beta I-turn. A new structural motif with a cis peptide bond in cyclic hexapeptides.

Synthesis and conformational analysis of three cyclic hexapeptides cyclo(-Gly1-Pro2-Phe3-Val4-Xaa5-Phe6), Xaa = Phe (I), D-Phe (II) and D-Pro (III), were carried out to examine the influence of proline on the formation of reverse turns and the dynamics of hydrophobic peptide regions. Assignment of all 1H and 13C resonances was achieved by homo- and heteronuclear 2D-NMR techniques (TOCSY, ROESY, HMQC, HMQC-TOCSY and HMBCS-270). The conformational analysis is based on interproton distances derived from ROESY spectra and homo- and heteronuclear coupling constants (E.COSY, HETLOC and HMBCS-270). For structural refinements restrained molecular dynamics (MD) simulations in vacuo and in DMSO were performed. Each peptide exhibits two conformations in DMSO solution due to cis-trans isomerism about the Gly-Pro peptide bond. Surprisingly the cis-Gly-Pro segment in the minor isomers is not involved in a beta VI-turn, but forms a turn structure with cis-Gly-Pro in the i and i + 1 positions. Although no stabilizing hydrogen bond is found in this turn, the phi- and psi-angles closely correspond to a beta I-turn [Pro2: phi(i + 1) -60 degrees, psi(i + 1) -30 degrees; Phe3: phi(i + 2) -100 degrees, psi(i + 2) -50 degrees]. Hence we call this structural element a pseudo-beta I-turn. As expected, in the dominating all-trans isomers proline occupies the i + 1 position of a standard beta I-turn. Therefore, cis-trans isomerization of the Gly1-Pro2 amide bond only induces a local conformational rearrangement, with minor structural changes in other parts of the molecule. However, the geometry of the other regions is affected by the chirality of the i + 1 amino acid for both isomers (beta I for Phe5, beta II' for D-Phe5 or D-Pro5).

Nuclear magnetic resonance solution structure of the pheromone Er-10 from the ciliated protozoan Euplotes raikovi.

The three-dimensional structure in solution of the pheromone Er-10 from the ciliated protozoan Euplotes raikovi has been determined by nuclear magnetic resonance spectroscopy. The structure of this 38-residue protein was obtained from 384 nuclear Overhauser enhancement distance constraints and 78 dihedral angle constraints using the distance geometry program DIANA for the structure calculation and the program AMBER for energy minimization. For a group of 20 conformers used to characterize the solution conformation, the average root-mean-square distance calculated for the backbone heavy atoms relative to the mean structure was 0.33 A. The structure includes three short helices of residues 2 to 8, 12 to 19 and 24 to 33, and a turn in the carboxy-terminal region of residues 34 to 38. These structural elements are held together by three disulfide bridges. The structure is quite stable relative to heat denaturation, since at both pH 4.6 and pH 6.0 only minor changes in the circular dichroism and nuclear magnetic resonance spectra were observed over the temperature range 20 to 80 degrees C. The surface of the Er-10 structure shows an asymmetric charge distribution that results in a predominantly apolar surface on one side of the molecule. There is also a deep cleft in the structure with an asymmetric distribution of charged and apolar residues on the two walls. These surface features may be important for the homologous (autocrine) and heterologous binding of the pheromone to receptors.

Conformational analysis of a IgG1 hinge peptide derivative in solution determined by NMR spectroscopy and refined by restrained molecular dynamics simulations.

The hinge region links the antigen binding Fab part to the constant Fc domain in immunoglobulins. For the hinge peptide derivative [AcThr(OtBu)-Cys-Pro-Pro-Cys-Pro-Ala-ProNH2]2 the assignment of the 1H and 13C resonances was achieved by two-dimensional nmr techniques: total correlation spectroscopy (TOCSY), nuclear Overhauser enhancement spectroscopy (NOESY), rotating frame nuclear Overhauser enhancement spectroscopy (ROESY), heteronuclear multiple quantum coherence (HMQC) transfer, and a HSQC (modified Overbodenhausen experiment) with high resolution in F1, which was several times folded in F1 but still phase correctable. Conformational relevant parameters (78 nuclear Overhauser effect distance restraints, 3JHH for prochiral assignments, temperature gradients) were determined by nmr and served as input data for molecular dynamics (MD) structure refinement. A simulated model compound corresponding to the [Cys-Pro-Pro-Cys]2 core elongated by the peptide chains in the Fab and Fc direction served as a starting structure for the final MD run. The conformation calculated in in vacuo does not agree with the C2 symmetry required from nmr data, but the structure obtained by a water simulation fulfills the requirement. Here the core of the hinge peptide derivative adopts a polyproline II double helix as in the x-ray structure of IgG1. Hence, segments responsible for the internal flexibility are located outside the core as confirmed by the flexibility of the solvent exposed C termini.