Haloalkane dehalogenase mechanism

Verschueren K H G, Seljee F, Rozeboom J, Kalk K H and Dijkstra B W 1993 Crystallographic analysis of the catalytic mechanism of haloalkane dehalogenase Nature 363 693-8... 

Nucleophilic substitution is one of a variety of mechanisms by which living systems detoxify halogenated organic compounds introduced into the environment Enzymes that catalyze these reactions are known as haloalkane dehalogenases The hydrolysis of 1 2 dichloroethane to 2 chloroethanol for example is a biological nude ophilic substitution catalyzed by a dehalogenase... 

The haloalkane dehalogenase DhlA mechanism takes place in two consecutive Sn2 steps. In the first, the carboxylate moiety of the aspartate Aspl24, acting as a nucleophile on the carbon atom of DCE, displaces chloride anion which leads to formation of the enzyme-substrate intermediate (Equation 11.86). That intermediate is hydrolyzed by water in the subsequent step. The experimentally determined chlorine kinetic isotope effect for 1-chlorobutane, the slow substrate, is k(35Cl)/k(37Cl) = 1.0066 0.0004 and should correspond to the intrinsic isotope effect for the dehalogenation step. While the reported experimental value for DCE hydrolysis is smaller, it becomes practically the same when corrected for the intramolecular chlorine kinetic isotope effect (a consequence of the two identical chlorine labels in DCE). 

Scheme 6. Mechanism of microsomal epoxide hydrolase and of haloalkane dehalogenase...

Haloacid dehalogenase(s) 590 mechanism of 590 Haloalkane dehalogenase(s) 591 active site structure 591 Halocyanin 883 Haloperoxidases 855, 889 Hammerhead ribozyme 649, 651s mechanism of action 651 Hammett equation 308... 

It is well established that the same three-dimensional scaffolding in proteins often carries constellations of amino acids with diverse enzymatic functions. A classic example is the large family of a/jS, or TIM, barrel enzymes (Farber and Petsko, 1990 Lesk et ai, 1989). It appears that lipases are no exception to date five other hydrolases with similar overall tertiary folds have been identified. They are AChE from Torpedo calif arnica (Sussman et al., 1991) dienelactone hydrolase, a thiol hydrolase, from Pseudomonas sp. B13 (Pathak and Ollis, 1990 Pathak et al, 1991) haloalkane dehalogenase, with a hitherto unknown catalytic mechanism, from Xanthobacter autotrophicus (Franken et al, 1991) wheat serine carboxypeptidase II (Liao et al, 1992) and a cutinase from Fusa-rium solani (Martinez et al, 1992). Table I gives some selected physical and crystallographic data for these proteins. They all share a similar overall topology, described by Ollis et al (1992) as the a/jS hydrolase... 

Edmond Y. Lau, Kalju Kahn, Paul A. Bash and Thomas C. Bruice, The importance of reactant positioning in enzyme catalysis a hybrid quantum mechanics/molecular mechanics study of a haloalkane dehalogenase, Proceedings of the National Academy of Sciences USA, 97 (2000), 9937-9942. 

This haloalkane dehalogenase is believed to act by using one of its side-chain carboxylates to displace chloride by an Sn2 mechanism. (Recall the reaction of carboxylate ions with alkyl halides from Table 8.1.)... 

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