Diisopropyl-fluorophosphatase (EC 3.8.2.2, DFPase) refers to any enzyme able to catalyzes the reaction

This hydrolyses the nerve agent, diisopropyl fluorophosphate (DFP), to diisopropylphosphate and fluoride ion. Many examples are able to defeat a range of similar nerve agents used in chemical warfare.

This enzyme is a hydrolase, specifically one acting on ester bonds phosphoric-triester hydrolases. The systematic name is diisopropyl-fluorophosphate fluorohydrolase. It requires a divalent cation such as calcium as a cofactor and is inhibited by chelating agents. Other names in use include: Phrasing variants: diisopropylphosphofluoridase, dialkyllfluorolphosphatase, diisopropyl phosphorofluoridate hydrolase, isopropyllphosphorolfluoridase, and diisopropyllfluorolphosphonate dehalogenase. Referring to other specific agents: tabunase, somanase. More general terminology: organophosphorus acid anhydrolase (OPAA), organophosphate acid anhydrase, OPA anhydrase (see also EC 3.8.2.-, "Hydrolases; Acting on halide bonds; In P-halide compounds").

The kind of activity is found in a diverse range of organisms, including protozoa, squid and clams, mammals,Little, J. S., C. A. Broomfield, L. J. Boucher, and M. K. Fox-Talbot. 1986. Partial characterization of a rat liver enzyme that hydrolyzes sarin, soman, tabun and DFP. Fed. Proc. 45:791. and soil bacteria. Most examples of the enzyme also double as an aryldialkylphosphatase (EC 3.1.8.1), an enzyme that degrades arylated organophosphate compounds. DFPase was formally assigned EC 3.1.8.2 and aryldialkylphosphatase formally EC 3.8.2.1 – demonstrative of the close relationships of these enzymes.

Because DFP (and other nerve agents) is not a natural compound, the ability to break it down is most likely an accident of evolution and not the native biological role of these enzymes. Some examples are apparently involved in cellular dipeptide metabolism.

Examples The enzyme from Loligo vulgaris (European squid; ) head ganglion has been shown to detoxify a wide variety of similar nerve agents.

A highly active form of the enzyme (pepQ; )is typically isolated from the marine bacteria Alteromonas undina for laboratory study. This form is both halophilic and thermophilic, making it particularly useful for detoxification applications. Its main in vivo function seems to be that of a C-terminal Xaa-Pro dipeptidase (EC 3.4.13.9). It also has the ability to degrate non-halogenated organophosphates, activity.

Less active variants of OPAA have also been isolated in slime mold, duckweed, and mung beans.

Structure

The Alteromonas version, pepQ (also known as opaA), is a single polypeptide composed of 517 amino acids, with a molecular weight of 58 kDa.Cheng, T-c, Harvey, SP, and Chen, GL, Cloning and Expression of a Gene Encoding a Bacterial Enzyme for Decontamination of Organophosphorus Nerve Agents and Nucleotide Sequence of the Enzyme, Applied and Environmental Microbiology, 62 (1996) 1636-1641.Cheng, T-c, Liu, L, DeFrank, JJ, Anderson, D, and Hamilton, AB, Bacterial Nerve Agent Degrading Enzymes that are Similar to Prolidase Protein Sequence and Activity, Proceedings of the 1995 ERDEC Scientific Conference on Chemical Defense Research, 14–17 November 1995, AD-A315 812. The three-dimensional crystal structure of pepQ is a trimer, as seen in Figure 2. The enzyme has a 22% amino acid homology with human prolidase (PEPD) and a 30% homology to E. coli aminopeptidase P (). pepQ is unstable under harsh conditions, losing its activity in the presence of organic solvents, at elevated temperature, and over long-term storage. Unprotected pepQ is also vulnerable to inhibition from other enzymes. InterPro classifies it as a member of the Xaa-Pro dipeptidase family, IPR022846.

The squid version has also been crystallized. Unlike pepQ it is a monomeric protein, a beta-propeller that comprises six blades: these two proteins have arrived at the same catalytic activity from very different origins. InterPro classifies this enzyme as a member of the "SMP-30/Gluconolactonase/LRE-like region" family, which includes human regucalcin and PON1, two other animal enzymes known to break down nerve agents. Its endogenous function is unclear.

Structural studies As of late 2007, 16 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , , , , , , , , , and .

Mechanism

Although the exact mechanism of pepQ's nerve-agent detoxification is unclear, researchers have deduced that the basic reaction follows a general base mechanism with a simple in-line displacement of fluoride at the phosphorus center using an activated water molecule, as seen in Figure 1.

pepQ activity is enhanced by reducing agents such as dithiothreitol (DTT) and beta-mercaptoethanol.Elashvili, I, DeFrank, JJ, and Culotta, VC, Purification and characterization of DFPase from Alteromonas haloplanktis ATCC 14393, Proceedings of the 1998 ERDEC Scientific Conference on Chemical and Biological Defense Research, 17–20 November 1998, AD-A375 171. pepQ is also catalytically active over a wide pH range between 6.5 and 9.5 and temperature range between 10 and 65 °C, and is stimulated by manganese.

Industrial relevance

OPAA’s susbstrate-specificity to organophosphorus-containing compounds, especially fluoride-containing G-type nerve agents such as sarin, cyclosarin, tabun, and soman,Cheng, T-c and Calomiris, JJ Stability of a nerve-agent-degrading enzyme in blast containment and firefighting foams, Proceedings of the 1994 ERDEC Scientific Conference on Chemical Defense Research, 15–18 November 1994, ADE 479952-001Cheng, T-c, DeFrank, JJ, Rastogi, VK, Alteromonas prolidase for the organophosphorus G-agent decontamination, Chemico-Biological Interactions 119-120(1999) 455-462. have drawn the interest of the U.S. Army. That attention has resulted in the establishment of a biodegradation program in search of safe and effective means of disposal for chemical weapons in the 1980s; the program is currently called the Advanced Catalytic Enzyme System (ACES).DeFrank, JJ, Cheng, T-c, Harvey, SP, and Rastogi, VK, Advanced catalytic enzyme system (ACES)-dual use capabilities, Proceedings of the 2002 Joint Service Scientific Conference on Chemical & Biological Defense Research, 19–21 November 2002, AD-M001 523.

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