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Proceedings of the 31 st European Peptide SymposiumMichal Lebl, Morten Meldal, Knud J. Jensen, Thomas Hoeg-Jensen (Editors)European Peptide Society, 2010Conformational Behaviour of Vasopressin-like Peptides in theMembrane-Mimetic EnvironmentEmilia Sikorska and Anna KwiatkowskaFaculty of Chemistry, University of Gdańsk, Sobieskiego 18, 80-952, Gdańsk, PolandIntroductionIt is believed that lipids are implicated in interaction of peptide hormones with theirmembrane receptors. This interaction, through the change of peptide conformation, is likelyto facilitate the entry of the hormone into the microenvironment of the receptor [1]. In thepresent study, we used a combined experimental and computational approach to learn howvasopressin-like peptides interact with a dodecylphosphocholine (DPC) micelle. Themicelle simulates eukaryotic cell membranes that are generally rich in zwitterionicphospholipids. We use NMR spectroscopy and molecular modelling to examine twovasopressin analogues, Aca[cis-Apc 2 ,Val 4 ]AVP (I) and [Nmp 2 ,D-Arg 8 ]VP (II) (Aca:1-adamantanecarboxylic acid; cis-Apc: cis-1-amino-4-phenyl-cyclohexanecarboxylic acidand Nmp: (2S,4R)-4-(naphthalene-2-ylmethyl)pyrrolidine-2-carboxylic acid) embedded inthe dodecylphosphocholine (DPC) micelle. Both analogues exhibit antioxytocic activity.With [Nmp 2 ,D-Arg 8 ]VP (II), a weak antipressor activity has also been reported(unpublished).Previous studies have shown that C -C cyclized residues, such as cis-Apc, impartsconsiderable stereochemical rigidity to peptide backbones and are constrained to adoptconformations in the 3 10 / -helical regions of the and spaces. They can beaccommodated at either the i+1 position of type III (III')-turns or at the i+2 position oftype II (II')-turns. Furthermore, they display the tendency to induce - or inverse -turns(C7-conformation) [2-4]. In turn, the proline ring of Nmp has significant effect on the anddihedral angles. The former should be relatively fixed at about -60 , whereas the latermay reach two minima, 40 and 150 [5]. Consequently, Nmp, similar to proline, is likelyto exhibit a strong preference for the i+1 position of -turn. Moreover, in the case ofpeptides containing proline-like residues the cis/trans isomerization of peptide bond cantake place. The differences in conformational behaviour arising from cis/transisomerization are crucial for biological profile of peptides.Results and DiscussionThe three-dimensional structures of both analogues studied were determined by timeaveragedmolecular dynamics in an explicit DPC micelle with the parm99 force field inAMBER11.0 [6] package. The NMR spectra of peptide II indicate a cis Cys 1 -Nmp 2 peptidebond, which was included during MD simulations. The cis peptide bond between residuesat positions 1 and 2 was also found for native vasopressin in SDS micelle [7].The analogue I adopts I’ or III’ -turns at positions 2,3 and 3,4. The former isstabilized by HN 4 -CO 1 hydrogen bond, whilst the later is not tight enough and is nothydrogen bonded. In turn, analogue II reveals the tendency to create the III’- or II-turnsat positions 4,5 and 7,8 respectively. The -turn at position 7,8 is closed by HN 9 -CO 6hydrogen bond. Moreover, the side chains of Gln 4 and Asn 5 of analogue II are involved inhydrogen bonds with oxygen atoms of Cys 1 and Gly 9 , respectively.The average radii of gyration (Rg) calculated with all of the heavy atoms for bothanalogue (7.1 and 6.9 Å for analogues I and II, respectively) indicate similar sizes of themolecules, despite the fact that analogue I is additionally acylated with Aca at itsN-terminus.It is believed that mutual arrangement of the aromatic side chains of a ligand is likelyto play a crucial role in specific receptor affinity. Therefore, the relationship between theflat angle between the planes of aromatic rings (F X-Phe ) and dihedral angle between twoplanes, where the former is determined by the centre of mass of aromatic part ofcis-Apc/Nmp 2 , C of cis-Apc/Nmp 2 and C of Phe 3 , whereas the latter by C ofcis-Apc/Nmp 2 , C of Phe 3 and the centre of mass of aromatic ring of Phe 3 (D X-Phe ) wasestablished. The dihedral angle values, D X-Phe , are clustered about horizontal lines, D X-Phe =-50 for both peptides. In turn, the average values of the flat angle, F X-Phe , are 50 and 125 ,450