We are evaluating a facilitative transport strategy to move oximes across the blood brain barrier (BBB) to reactivate inhibited brain acetylcholinesterase (AChE). We selected glucose (Glc) transporters (GLUT) for this purpose as these transporters are highly represented in the BBB. Glc conjugates have successfully moved drugs across the BBB and previous work has shown that Glc-oximes (sugar-oximes, SOxs) can reduce the organophosphonate induced hypothermia response. We previously evaluated the reactivation potential of Glc carbon C-1 SOxs. Here we report the reactivation parameters for VX- and GB-inhibited human (Hu) AChE of the best SOx (13c) and our findings that the kinetics are similar to those of the parent oxime. Although crystals of Torpedo californica AChE were produced, neither soaked or co-crystallized experiments were successful at concentrations below 20mM 13c, and higher concentrations cracked the crystals. 13c was non-toxic to neuroblastoma and kidney cell lines at 12-18mM, allowing high concentrations to be used in a BBB kidney cell model. The transfer of 13c from the donor side was asymmetric with the greatest loss of 13c from the apical- or luminal-treated side. There was no apparent transfer from the basolateral side. The 13cPapp results indicate a 'low' transport efficiency; however, mass accounting revealed only a 20% recovery from the apical dose in which high concentrations were found in the cell lysate fraction. Molecular modeling of 13c through the GLUT-1 channel demonstrated that transport of 13c was more restricted than Glc. Selected sites were compared and the 13c binding energies were greater than two times those of Glc.
Title: Kinetics of Torpedo californica acetylcholinesterase inhibition by bisnorcymserine and crystal structure of the complex with its leaving group Bartolucci C, Stojan J, Yu QS, Greig NH, Lamba D Ref: Biochemical Journal, 444:269, 2012 : PubMed
Natural and synthetic carbamates act as pseudo-irreversible inhibitors of AChE (acetylcholinesterase) as well as BChE (butyrylcholinesterase), two enzymes involved in neuronal function as well as in the development and progression of AD (Alzheimer's disease). The AChE mode of action is characterized by a rapid carbamoylation of the active-site Ser(200) with release of a leaving group followed by a slow regeneration of enzyme action due to subsequent decarbamoylation. The experimental AD therapeutic bisnorcymserine, a synthetic carbamate, shows an interesting activity and selectivity for BChE, and its clinical development is currently being pursued. We undertook detailed kinetic studies on the activity of the carbamate bisnorcymserine with Tc (Torpedo californica) AChE and, on the basis of the results, crystallized the complex between TcAChE and bisnorcymserine. The X-ray crystal structure showed only the leaving group, bisnoreseroline, trapped at the bottom of the aromatic enzyme gorge. Specifically, bisnoreseroline interacts in a non-covalent way with Ser(200) and His(440), disrupting the existing interactions within the catalytic triad, and it stacks with Trp(84) at the bottom of the gorge, giving rise to an unprecedented hydrogen-bonding contact. These interactions point to a dominant reversible inhibition mechanism attributable to the leaving group, bisnoreseroline, as revealed by kinetic analysis.
Title: Probing Torpedo californica acetylcholinesterase catalytic gorge with two novel bis-functional galanthamine derivatives Bartolucci C, Haller LA, Jordis U, Fels G, Lamba D Ref: Journal of Medicinal Chemistry, 53:745, 2010 : PubMed
N-Piperidinopropyl-galanthamine (2) and N-saccharinohexyl-galanthamine (3) were used to investigate interaction sites along the active site gorge of Torpedo californica actylcholinesterase (TcAChE). The crystal structure of TcAChE-2 solved at 2.3 A showed that the N-piperidinopropyl group in 2 is not stretched along the gorge but is folded over the galanthamine moiety. This result was unexpected because the three carbon alkyl chain is just long enough for the bulky piperidine group to be placed above the bottleneck (Tyr121, Phe330) midway down the gorge. The crystal structure of TcAChE-3 at 2.2 A confirmed that a dual interaction with the sites at the bottom, and at the entrance of the gorge, enhances inhibitory activity: a chain of six carbon atoms has, in this class of derivatives, the correct length for optimal interactions with the peripheral anionic site (PAS).
Ganstigmine is an orally active, geneserine derived, carbamate-based acetylcholinesterase inhibitor developed for the treatment of Alzheimer's disease. The crystal structure of the ganstigmine conjugate with Torpedo californica acetylcholinesterase (TcAChE) has been determined at 2.40 A resolution, and a detailed structure-based analysis of the in vitro and ex vivo anti-AChE activity by ganstigmine and by new geneserine derivatives is presented. The carbamoyl moiety is covalently bound to the active-site serine, whereas the leaving group geneseroline is not retained in the catalytic pocket. The nitrogen atom of the carbamoyl moiety of ganstigmine is engaged in a key hydrogen-bonding interaction with the active site histidine (His440). This result offers an explanation for the inactivation of the catalytic triad and may account for the long duration of action of ganstigmine in vivo. The 3D structure also provides a structural framework for the design of compounds with improved binding affinity and pharmacological properties.
Title: Three-dimensional structure of a complex of galanthamine (Nivalin) with acetylcholinesterase from Torpedo californica: implications for the design of new anti-Alzheimer drugs Bartolucci C, Perola E, Pilger C, Fels G, Lamba D Ref: Proteins, 42:182, 2001 : PubMed
The 3D structure of a complex of the anti-Alzheimer drug galanthamine with Torpedo californica acetylcholinesterase is reported. Galanthamine, a tertiary alkaloid extracted from several species of Amarylidacae, is so far the only drug that shows a dual activity, being both an acetylcholinesterase inhibitor and an allosteric potentiator of the nicotinic response induced by acetylcholine and competitive agonists. The X-ray structure, at 2.5A resolution, shows an unexpected orientation of the ligand within the active site, as well as unusual protein-ligand interactions. The inhibitor binds at the base of the active site gorge, interacting with both the acyl-binding pocket and the principal quaternary ammonium-binding site. However, the tertiary amine group of galanthamine does not directly interact with Trp84. A docking study using the program AUTODOCK correctly predicts the orientation of galanthamine in the active site. The docked lowest-energy structure has a root mean square deviation of 0.5A with respect to the corresponding crystal structure of the complex. The observed binding mode explains the affinities of a series of structural analogs of galanthamine and provides a rational basis for structure-based drug design of synthetic derivatives with improved pharmacological properties. Proteins 2001;42:182-191.
Title: Accurate prediction of the bound conformation of galanthamine in the active site of Torpedo californica acetylcholinesterase using molecular docking Pilger C, Bartolucci C, Lamba D, Tropsha A, Fels G Ref: J Mol Graph Model, 19:288, 2001 : PubMed
The alkaloid (-)-galanthamine is known to produce significant improvement of cognitive performances in patients with the Alzheimer's disease. Its mechanism of action involves competitive and reversible inhibition of acetylcholinesterase (AChE). Herein, we correctly predict the orientation and conformation of the galanthamine molecule in the active site of AChE from Torpedo californica (TcAChE) using a combination of rigid docking and flexible geometry optimization with a molecular mechanics force field. The quality of the predicted model is remarkable, as indicated by the value of the RMS deviation of approximately 0.5A when compared with the crystal structure of the TcAChE-galanthamine complex. A molecular model of the complex between TcAChE and a galanthamine derivative, SPH1107, with a long chain substituent on the nitrogen has been generated as well. The side chain of this ligand is predicted to extend along the enzyme active site gorge from the anionic subsite, at the bottom, to the peripheral anionic site, at the top. The docking procedure described in this paper can be applied to produce models of ligand-receptor complexes for AChE and other macromolecular targets of drug design.
The crystal structure of Torpedo californica (Tc) acetylcholinesterase (AChE) carbamoylated by the physostigmine analogue 8-(cis-2,6-dimethylmorpholino)octylcarbamoyleseroline (MF268) is reported at 2.7 A resolution. In the X-ray structure, the dimethylmorpholinooctylcarbamic moiety of MF268 is covalently bound to the catalytic serine, which is located at the bottom of a long and narrow gorge. The alkyl chain of the inhibitor fills the upper part of the gorge, blocking the entrance of the active site. This prevents eseroline, the leaving group of the carbamoylation process, from exiting through this path. Surprisingly, the relatively bulky eseroline is not found in the crystal structure, thus implying the existence of an alternative route for its clearance. This represents indirect evidence that a "back door" opening may occur and shows that the release of products via a "back door" is a likely alternative for this enzyme. However, its relevance as far as the mechanism of substrate hydrolysis is concerned needs to be established. This study suggests that the use of properly designed acylating inhibitors, which can block the entrance of catalytic sites, may be exploited as a general approach for investigating the existence of "back doors" for the clearance of products.
