Document Detail

Conformers of acetylcholinesterase: a mechanism of allosteric control.
MedLine Citation:
PMID:  8302283     Owner:  NLM     Status:  MEDLINE    
Rate control in acetylcholinesterase (AChE) involves a single anionic site whose anionic center controls rate-related biochemical and conformational changes in the E (free enzyme) and EA (acylated enzyme) conformers. Change in conformer structure and biochemistry affect binding, acylation, and hydrolysis. It is significant that the anionic-esteratic intersite distance is not altered during conformer change as E is converted to EA. In this enzyme system, cationic acetylcholine and anionic AChE are true structural, functional, and biochemical counterparts. The anionic center in the E conformer lies at the bottom of a sterically restricted, hydrophobic cleft < 8 A wide at the top and > 3 A wide at the bottom, while the anionic center in the EA conformer is relatively open. It is characterized by a decrease in the relative binding of hydrophobic cations and by an ability to bind large organic cations. Binding of acetylcholine, H+, or organic cations at the anionic site controls k2(acylation) in the E conformer and k3(hydrolysis) in the EA conformer. Acetylcholine binding forms the ES complex in which the cation maximizes k2. In the EAS complex, the cation reduces k3 and provides allosteric control. Anionic site structure and biochemistry and the effect of pH on k2 and k3 differentiates AChE from butyrylcholinesterase. This comprehensive study of kinetic and thermodynamic processes in AChE was made possible by the synthesis and/or use of families of over 30 cationic and acylation probes of known stereochemistry. They act as rulers of the E and EA conformers of AChE and provide comparative data on kinetic-based and thermodynamic-based constants. Cationic inhibitors affect decarbamylation rates in AChE and provide an additional set of comparative data related to the mechanism of substrate hydrolysis by AChE. Acridine araphanes are unique neural receptor and cholinergic enzyme probes. Their parallel plane and coplanar conformations are related to bridge length. Two parallel plane acridine araphanes are pure uncompetitive inhibitors of AChE. Scatchard plots of the binding of methylacridinium and 9-aminoacridine with the E conformer and 9-aminoacridine with the EA conformer indicate binding at a single anionic site. No ternary complex (EII or EAII) from two-site binding was detected. In AChE, nonspecific, low-level binding at surface ionic and hydrophobic areas is ubiquitous. Binding affinity differences greater than two orders of magnitude distinguish binding at the anionic site from low level binding at surface moieties. Surface binding provides environmental and stability changes in the enzyme but does not modify the fundamental biochemistry of the E and EA conformers.
J L Taylor; R T Mayer; C M Himel
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Molecular pharmacology     Volume:  45     ISSN:  0026-895X     ISO Abbreviation:  Mol. Pharmacol.     Publication Date:  1994 Jan 
Date Detail:
Created Date:  1994-03-10     Completed Date:  1994-03-10     Revised Date:  2007-11-14    
Medline Journal Info:
Nlm Unique ID:  0035623     Medline TA:  Mol Pharmacol     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  74-83     Citation Subset:  IM    
Department of Entomology, University of Georgia, Athens 30602.
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MeSH Terms
Acetylcholinesterase / chemistry,  metabolism*
Allosteric Regulation
Aminoacridines / metabolism
Binding Sites
Cholinesterase Inhibitors / pharmacology
Protein Conformation
Pyridines / metabolism
Quinolines / metabolism
Grant Support
Reg. No./Substance:
0/Aminoacridines; 0/Anions; 0/Cholinesterase Inhibitors; 0/Pyridines; 0/Quinolines; EC

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