Conformation of corticotropin. An infrared spectrometry study of hydrogen exchange kinetics

Conformational aspects and rotational dynamics of synthetic adrenocorticotropin-(1-24) and glucagon in reverse micelles

The tryptophan (Trp) rotational dynamics and the secondary structure of the peptide hormones adrenocorticotropin-(1-24) [ACTH(1-24)]--the fully active N-terminal fragment of adrenocorticotropin-(1-39)--and glucagon were studied in aqueous solutions and in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water/isooctane, a system selected to mimic the membrane-water interface. In aqueous solutions, the total fluorescence intensity decays of their single Trp residue [Trp-9 and Trp-25 for ACTH(1-24) and glucagon, respectively] are multiexponential. This is also the case for ACTH(5-10), a fragment of the adrenocorticotropin "message" region. Time-resolved fluorescence anisotropy data evidence a high degree of rotational freedom of the single Trp residue. Transfer of these peptides from water to the aqueous core of reverse micelles induces severe restrictions of the Trp internal motion and of its local environment. The results indicate that the Trp-9 residue in ACTH(1-24 is maintained in the close neighborhood of the water-AOT molecular interface where the water molecules are strongly immobilized. By contrast, the Trp residues in ACTH(5-10) and glucagon are likely to be located closer to the center of the micellar aqueous core where the water molecules are in a more mobile state. Furthermore, the above location of Trp can be extended to the peptide chains themselves as evidenced by the overall correlation time values of the peptide-containing micelles. Nevertheless, in all peptides, the indole ring remains susceptible to oxidation by N-bromosuccinimide. Circular dichroism measurements evidence the induction in glucagon of alpha-helices remaining unaffected by the micellar water content. Conversely, beta-sheet structures are favored in ACTH(1-24) at low water-to-surfactant molar ratios (w0) but are disrupted by subsequent additions of water. These results are discussed in terms of the possible role of the micellar interfaces in selecting the preferred peptide dynamical conformation(s)

Conformational changes of adrenocorticotropin peptides upon interaction with lipid membranes revealed by infrared attenuated total reflection spectroscopy

Infrared attenuated total reflection (IR-ATR) spectroscopy was used to study conformational and topological aspects of the interaction between two adrenocorticotropin fragments and dioleoylphosphatidylcholine membranes. Corticotropin-(1-10)-decapeptide, ACTH1-10, was found to exist as a rigid antiparallel pleated sheet structure in dry membranes. In aqueous environment, it completely escaped from the lipid. This dominant preference for the aqueous phase is a possible explanation for the very low biological potency of ACTH1-10 in some assays. On the other hand, the very potent corticotropin-(1-24)-tetracosapeptide, ACTH1-24, was firmly incorporated into dry and wet membranes. Aqueous environment even promoted the peptide-lipid interaction. Under these latter conditions, part of the molecule entered the bilayer and adopted a helical structure with the axis oriented perpendicularly to the bilayer plane. Contact of a 0.1 mM solution of ACTH1-24 in liquid deuterium oxide with the pure lipid membrane system resulted in measurable adsorption of the peptide to the membrane with the same conformational and topological characteristics as described above (perpendicularly oriented helix entering the bilayer). The helical part of the ACTH1-24 molecule entering the bilayer was the quite hydrophobic N-terminal decapeptide unit ("message" segment). The adjacent hydrophilic C-terminal tetradecapeptide unit ("address" segment) remained on the membrane surface. As the message region is essential for triggering corticotropin receptors, its intrusion into the membrane and its adoption of an oriented, helical conformation may facilitate receptor stimulation.

Molecular dynamics of two homologous neurotoxins revealed by 1H-2H exchange: an infrared spectrometry study

Temperature effects on the hydrogen exchange kinetics and the infrared spectra of two homologous snake neurotoxins (Laticauda semifasciata erabutoxin b and Naja nigricollis toxin alpha) were investigated between 10 and 40 degrees C, at their isoionic pH. (1) Erabutoxin b is more accessible to the solvent than toxin alpha. (2) With increasing temperature, both toxin molecules undergo a global transition affecting the most accessible as well as the most buried hydrogens: the overall accessibility changes are more important for erabutoxin b than for toxin alpha. The different conformational stabilities of the toxins are also qualitatively supported by the temperature-induced shifts which affect the infrared amide I band of toxin alpha only. The existence of two conformer families could be responsible for the different conformational stability of these proteins.