Acetylcholine interactions with tryptophan-184 of the alpha-subunit of the nicotinic acetylcholine receptor revealed by transferred nuclear Overhauser effect

Solvent-Induced Molecular Folding and Self-Assembled Nanostructures of Tyrosine and Tryptophan Analogues in Aqueous Solution: Fascinating Morphology of High Order

Hydrophobic derivatives of tyrosine and tryptophan, viz. octyl and dodecyl esters of tyrosine and octyl ester of tryptophan, are synthesized, and the interfacial and bulk properties in aqueous media are investigated as models for the membrane proteins. Molecular modeling by the density functional theory method is carried out to understand the molecular conformation and geometry for the purpose of determining the packing parameters. Water-induced molecular folding of the esters of both tyrosine and tryptophan, as observed using rotating frame nuclear Overhauser effect spectroscopy, indicates that the segregation of the hydrophobic and hydrophilic blocks in water is the key to the development of fascinating interfacial property displayed by the aromatic amino acid esters. The unusually high-order morphology of the aggregates, as observed using high-resolution transmission electron microscopy, is highly uncommon for single-chain amphiphiles and points to the fact that the self-assembly behavior of the present systems resembles that of block copolymers. The study of the growth of mesosized hollow aggregates with internal bilayer structures from tyrosine and tryptophan-based model systems would add to the understanding of biochemistry and biotechnology relevant to the cell membrane. The potential of biocompatible nanostructured motifs as the drug carriers is discussed. The highly functional role played by the aromatic amino acids at the membrane-water interface will be considered with the present amphiphilic models for future perspective.

Conformation of a novel tetrapeptide inhibitor NH2-D-Trp-D-Met-Phe(pCl)-Gla-NH2 bound to farnesyl-protein transferase

Farnesyl-protein transferase (FPTase) catalyzes the posttranslational farnesylation of the cysteine residue located in the C-terminal tetrapeptide of the Ras oncoprotein. Prenylation of this residue is essential for membrane association and cell-transforming activities of ras. Inhibitors of FPTase have been demonstrated to display antitumor activity in both tissue culture and animal models, and thus represent a potential therapeutic strategy for the treatment of human cancers. A synthetic tetrapeptide library, which included an expanded set of 68 L-, D- and noncoded amino acids, has been screened for inhibitors of FPTase activity. The tetrapeptide, NH2-D-Trp-D-Met-L-Phe(pCl)-L-Gla-NH2 was shown to be competitive with the isoprenyl cosubstrate, farnesyl diphosphate (FPP) but not with the peptide substrate, the C-terminal tetrapeptide of the Ras protein. The FPTase-bound conformation of the inhibitor, NH2-D-Trp-D-Met-L-Phe(pCl)-L-Gla-NH2 was determined by NMR spectroscopy. Distance constraints were derived from two-dimensional transferred nuclear Overhauser effect (TRNOE) experiments. Ligand competition experiments identified the NOEs that originate from the active-site conformation of the inhibitor. Structures were calculated using a combination of distance geometry and restrained energy minimization. The peptide backbone is shown to adopt a reverse-turn conformation most closely approximating a type II' beta-turn. The resolved conformation of the inhibitor represents a distinctly different structural motif from that determined for Ras-competitive inhibitors. Knowledge of the bound conformation of this novel inhibitor provides a template and future direction for the design of new classes of FPTase antagonists.

Cation-pi interactions in chemistry and biology: a new view of benzene, Phe, Tyr, and Trp

Cations bind to the pi face of an aromatic structure through a surprisingly strong, non-covalent force termed the cation-pi interaction. The magnitude and generality of the effect have been established by gas-phase measurements and by studies of model receptors in aqueous media. To first order, the interaction can be considered an electrostatic attraction between a positive charge and the quadrupole moment of the aromatic. A great deal of direct and circumstantial evidence indicates that cation-pi interactions are important in a variety of proteins that bind cationic ligands or substrates. In this context, the amino acids phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) can be viewed as polar, yet hydrophobic, residues.

Nuclear magnetic resonance (NMR) analysis of ligand receptor interactions: the cholinergic system--a model

Elucidation of the molecular mechanisms that govern ligand-receptor recognition is essential to the rational design of specific pharmacological reagents. Whereas often the receptor and its binding site are the target of investigation, study of the ligand in its free and bound state can also reveal important information regarding this recognition process. Nuclear magnetic resonance (NMR) spectroscopy can be extremely useful for such studies. In this review, we discuss the attributes of NMR in the study of ligand receptor interactions. The cholinergic receptor and its binding to the neurotransmitter, acetylcholine, and cholinergic antagonists serve as a model system, illustrating the power of ligand analysis by NMR. The results discussed prove that the region of residues alpha 180-205 of the nicotinic acetylcholine receptor are an essential component of the cholinergic binding site and that ligand binding involves a positively charged hydrophobic motif.

NMR analysis reveals a positively charged hydrophobic domain as a common motif to bound acetylcholine and d-tubocurarine

A complete 1H assignment of d-tubocurarine was carried out using 1D and 2D NMR techniques. Geometries of free acetylcholine (ACh) and d-tubocurarine were compared with those of the ligands bound to a recombinant cholinergic binding site (T alpha 184-200 expressed as a fusion protein in Escherichia coli). The conformations of the free ligands were determined by NOESY experiments while those of the bound molecules were obtained by transferred NOESY. The complete relaxation matrix was solved yielding distance constraints which were further refined by a sigma back-calculation. ACh bound to recombinant T alpha 184-200 closely resembled the conformation previously reported for ACh bound to the intact receptor. d-Tubocurarine in the bound state undergoes extensive induced conformational rearrangements generating a "cup"-shaped structure. A unique positively charged hydrophobic domain is identified as characteristic of both bound cholinergic ligands.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Nine residues influence the binding of alpha-bungarotoxin in alpha-subunit region 185-200 of human muscle acetylcholine receptor

Identification of residues in the skeletal muscle nicotinic acetylcholine receptor (AChR) that bind snake venom alpha-neurotoxin antagonists of acetylcholine [e.g., alpha-bungarotoxin (alpha-BTx)] provides structural information about the neurotransmitter binding region of the receptor. Using synthetic peptides of the human AChR alpha-subunit region 177-208, we previously localized a pharmacologically specific binding site for alpha-BTx in segment 185-199. To define in more detail the residues that influence the binding of alpha-BTx to this region, we prepared 16 peptide analogues of the alpha-subunit segment 185-200, with the amino acid L-alanine sequentially replacing each native amino acid. Circular dichroism spectroscopy did not reveal changes in the secondary structure of the peptides except for the analogue in which Pro194 was substituted with alanine. This implies that any change in alpha-BTx binding could be attributed to replacement of the native residue's side chain by alanine's methyl group, rather than to a change in the structure of the peptide. The influence of each substitution with alanine was determined by comparing the analogue to the parental sequence alpha 185-200 in solution-phase competition with native human AChR for binding of 125I-labeled alpha-BTx. The binding of alpha-BTx by analogue peptides with alanine substituted for Tyr190, Cys192, or Cys193 was greatly diminished. Binding of alpha-BTx to peptides containing alanine replacements at Val188, Thr189, Pro194, Asp195, or Tyr198 was also reduced significantly (p < 0.003). An unanticipated finding was that substitution of alanine for Ser191 significantly increased alpha-BTx binding (p < 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)