X-RAY STRUCTURE OF AGED DI-ISOPROPYL-PHOSPHORO-FLUORIDATE (DFP) BOUND TO ACETYLCHOLINESTERASE
(see also AChE inhibitors and substrates, 1cfj, and 1som)
Publication Abstract from PubMed
Organophosphorus acid anhydride (OP) nerve agents are potent inhibitors which rapidly phosphonylate acetylcholinesterase (AChE) and then may undergo an internal dealkylation reaction (called "aging") to produce an OP-enzyme conjugate that cannot be reactivated. To understand the basis for irreversible inhibition, we solved the structures of aged conjugates obtained by reaction of Torpedo californica AChE (TcAChE) with diisopropylphosphorofluoridate (DFP), O-isopropylmethylphosponofluoridate (sarin), or O-pinacolylmethylphosphonofluoridate (soman) by X-ray crystallography to 2.3, 2.6, or 2.2 A resolution, respectively. The highest positive difference density peak corresponded to the OP phosphorus and was located within covalent bonding distance of the active-site serine (S200) in each structure. The OP-oxygen atoms were within hydrogen-bonding distance of four potential donors from catalytic subsites of the enzyme, suggesting that electrostatic forces significantly stabilize the aged enzyme. The active sites of aged sarin- and soman-TcAChE were essentially identical and provided structural models for the negatively charged, tetrahedral intermediate that occurs during deacylation with the natural substrate, acetylcholine. Phosphorylation with DFP caused an unexpected movement in the main chain of a loop that includes residues F288 and F290 of the TcAChE acyl pocket. This is the first major conformational change reported in the active site of any AChE-ligand complex, and it offers a structural explanation for the substrate selectivity of AChE.
Crystal structures of aged phosphonylated acetylcholinesterase: nerve agent reaction products at the atomic level., Millard CB, Kryger G, Ordentlich A, Greenblatt HM, Harel M, Raves ML, Segall Y, Barak D, Shafferman A, Silman I, Sussman JL, Biochemistry. 1999 Jun 1;38(22):7032-9. PMID:10353814
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
Acetylcholinesterase (AChE) hydrolysizes the neurotransmitter , producing group. directly binds catalytic (via its nucleophilic Oγ atom). , O-(1,2,2-trimethylpropyl) methylphosphonofluoridate (fluorine atom is colored violet and phosphorus atom is colored darkmagenta), is one of the most toxic organophosphate compounds (OPs). Soman inhibits AChE by to catalytic Ser200, . This process implicates nucleophilic attack of the Ser200 nucleophilic Oγ atom on the phosphorus atom of soman, with concomitant departure of its fluoride atom. After that AChE catalyzes the of the soman or other OP. This causes irreversible inhibition of AChE, "aged" soman/AChE conjugate can not be reactivated. However, before “aging”, at the step of , AChE can be by nucleophiles, such as pralidoxime (2-PAM), resulting in of the phosphonyl adduct from Ser200 Oγ.
At the (2wfz) the catalytic His440 forms hydrogen bonds with Ser200 Oγ and Glu327 Oε1 via its Nε2 and Nδ1 nitrogens, respectively. The O2 atom of soman is within hydrogen bonding distance of His440 Nε2. Soman O1 mimicks carbonyl oxygen of ACh. A water molecule 1001 interacting with soman O2 is represented as a red ball. The active site residues of the nonaged soman/TcAChE are colored yellow. The O2 atom of the (2wg0) forms a salt bridge with His440 Nε2. The active site residues of the aged soman/TcAChE are colored pink. of the structures of the nonaged (2wfz) and aged (2wg0) conjugates reveals a small, but important, change within the active site - the imidazole ring of His440 is tilted back to a native-like conformation after dealkylation. The water molecule 1001, which interacts with soman O2 in the nonaged crystal structure, is not within hydrogen bonding distance of O2 in the aged crystal structure. 2-PAM binds poorly to the nonaged phosphonylated enzyme (its electron density was not found) and binds in an after soman aging to TcAChE (2wg1).
DFP, diisopropylphosphorofluoridate, is an other toxic OP nerve agent. It is also inhibits AChE by covalent binding to the catalytic Ser200. There are four hydrogen bond donors (dotted lines) to the anionic phosphonyl oxygen atoms: the backbone amide nitrogen atoms of Ala201, Gly118, and Gly119, as well as His440 Nε2 at the of aged DFP-TcAChE (2dfp). Phosphorylation with DFP caused an unexpected distortion in the main chain of a loop that includes residues F288 and F290 of the TcAChE acyl binding pocket. F288 and F290 move significantly in the DFP-TcAChE structure (lime), in comparison to their positions in the native enzyme (2ace). This is the first major conformational change reported in the active site of any AChE−ligand complex, and it offers a structural explanation for the substrate selectivity of AChE.
About this Structure
2DFP is a 1 chain structure of sequence from Torpedo californica. Full crystallographic information is available from OCA.
Additional Resources
For additional information, see: Alzheimer's Disease
