Hopeahainol A binds reversibly at the acetylcholinesterase (AChE) peripheral site and inhibits enzyme activity with a novel higher order concentration dependence.
Natural product inhibitors of AChE are of interest both because they offer promise as inexpensive drugs for symptomatic relief in Alzheimer's disease and because they may provide insights into the structural features of the AChE catalytic site. Hopeahainol A is an uncharged polyphenol AChE inhibitor from the stem bark of Hopea hainanensis with a constrained, partially dearomatized bicyclic core. Molecular modeling indicates that hopeahainol A binds at the entrance of the long but narrow AChE active site gorge because it is too bulky to be accommodated within the gorge without severe distortion of the gorge as depicted in AChE crystal structures. We conducted inhibitor competition experiments in which AChE inhibition was measured with hopeahainol A together with either edrophonium (which binds at the base of the gorge) or thioflavin T (which binds to the peripheral or P-site near the gorge mouth). The results agreed with the molecular modeling and indicated that hopeahainol A at lower concentrations (<200 μM) bound only to the P-site, as hopeahainol A and thioflavin T were unable to form a ternary complex with AChE while hopeahainol A and edrophonium did form a ternary complex with essentially no competition between them. Inhibition increased to a striking extent at higher concentrations of hopeahainol A, with plots analogous to classic Dixon plots showing a dependence on hopeahainol A concentrations to the third- or fourth order. The inhibition at higher hopeahainol A concentrations was completely reversed on dilution and blocked by bound edrophonium. We hypothesize that bound hopeahainol A induces conformational changes in the AChE active site that allow binding of additional hopeahainol A molecules, a phenomenon that would be unprecedented for a reversible inhibitor that apparently forms no covalent bonds with AChE.
Mayo Clinic College of Medicine, Departments of Neuroscience and Pharmacology, Jacksonville, FL, 32224, USA. Electronic address: rosenberry@mayo.edu.
Terrone L
TL
Rosenberry
Mayo Clinic College of Medicine, Departments of Neuroscience and Pharmacology, Jacksonville, FL, 32224, USA.
Patricia K
PK
Martin
Mayo Clinic College of Medicine, Departments of Neuroscience and Pharmacology, Jacksonville, FL, 32224, USA.
A Jeremy
AJ
Nix
Carbone Cancer Center, University of Wisconsin, Madison, WI, 53705, USA.
Scott A
SA
Wildman
New York Structural Biology Center, New York, NY, 10027, USA.
Jonah
J
Cheung
Department of Chemistry, University of Chicago, Chicago, IL, 60637, USA.
Scott A
SA
Snyder
State Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing 210093, PR China.
Ren Xiang
RX
Tan
D000110:Acetylcholinesterase / Q000737:chemistry / Q000378:metabolism*; D052160:Benzothiazoles; D001665:Binding Sites; D020134:Catalytic Domain; D002800:Cholinesterase Inhibitors / Q000737:chemistry / Q000378:metabolism*; D053958:Dipterocarpaceae / Q000737:chemistry / Q000378:metabolism; D004491:Edrophonium / Q000737:chemistry / Q000378:metabolism; D006576:Heterocyclic Compounds, 4 or More Rings / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D062105:Molecular Docking Simulation; D024301:Plant Bark / Q000737:chemistry / Q000378:metabolism; D013379:Substrate Specificity; D013816:Thermodynamics; D013844:Thiazoles / Q000737:chemistry / Q000378:metabolism
D052160:Benzothiazoles; D002800:Cholinesterase Inhibitors; D006576:Heterocyclic Compounds, 4 or More Rings; D013844:Thiazoles; C526863:hopeahainol A; C009462:thioflavin T; D004491:Edrophonium; D000110:Acetylcholinesterase
Acetylcholinesterase; Enzyme mechanism; Hopeahainol A; Natural product inhibitors; Peripheral site