Enzyme haloalkane dehalogenase

Figure C2.7.8. Catalytic cycle indicating the working of tire enzyme haloalkane dehalogenase [11],...

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... 

Ring opening of epoxides with nucleophiles other than water (Cl, BrT I, NOz , CN ) can also be catalyzed by halohydrin dehalogenase enzymes (EC 3.8.1.5, also named haloalkane dehalogenase or haloalcohd dehalogenase) (Figure 6.74) [197]. 

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). 

Amino acid sequence relationships have suggested a number of HYL families based on percent identity, enzymes with >40% identity belonging to the same family [48]. Families so identified include the mammalian microsomal EH (HYL1), the mammalian cytosolic EH (HYL2), the plant cytosolic EH (HYL3), and bacterial C-X bond hydrolases (haloacid dehydrogenases, HAD, and haloalkane dehalogenases, HLD). 

Some examples of emerging enzyme classes that are rapidly making an impact in synthetic organic chemistry are shown in Figure 16.4 alcohol dehydrogenase for the reduction of ketones [30], transaminase [31], and haloalkane dehalogenase... 

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... 

Epoxide hydrolase phosphatase/ kinase haloalkane dehalogenase Hydrolysis of epoxides, phosphate esters, haloalkanes Chemocatalysis fails Few enzymes... 

Several membrane-bound and soluble epoxide hydrolases from mammalian origin have been purified and (at least partially) sequenced. Some of them have also been cloned and overexpressed, which is the case for the soluble EH from rat liver which has been overexpressed in Escherichia cob 54, 55. This enzyme (as well as its microsomal analog) was shown to share an amino acid sequence similarity to a region around the active center of a bacterial haloalkane dehalogenase 56, an enzyme with known three-dimensional structure that belongs to the a/(3-hydrolase fold-family 571. Rat soluble EH forms a dimer from two complete structural monomeric units, both possessing a distinct active site. The EH activity is known to be located close to the C-terminal unit, while the function of the N-terminal unit remains unknown 581. 

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