An enzyme's substrate specificity is one of its most important characteristics. The quantitative comparison of broad-specificity enzymes requires the selection of a homogenous set of substrates for experimental testing, determination of substrate-specificity data and analysis using multivariate statistics. We describe a systematic analysis of the substrate specificities of nine wild-type and four engineered haloalkane dehalogenases. The enzymes were characterized experimentally using a set of 30 substrates selected using statistical experimental design from a set of nearly 200 halogenated compounds. Analysis of the activity data showed that the most universally useful substrates in the assessment of haloalkane dehalogenase activity are 1-bromobutane, 1-iodopropane, 1-iodobutane, 1,2-dibromoethane and 4-bromobutanenitrile. Functional relationships among the enzymes were explored using principal component analysis. Analysis of the untransformed specific activity data revealed that the overall activity of wild-type haloalkane dehalogenases decreases in the following order: LinB~DbjA>DhlA~DhaA~DbeA~DmbA>DatA~DmbC~DrbA. After transforming the data, we were able to classify haloalkane dehalogenases into four SSGs (substrate-specificity groups). These functional groups are clearly distinct from the evolutionary subfamilies, suggesting that phylogenetic analysis cannot be used to predict the substrate specificity of individual haloalkane dehalogenases. Structural and functional comparisons of wild-type and mutant enzymes revealed that the architecture of the active site and the main access tunnel significantly influences the substrate specificity of these enzymes, but is not its only determinant. The identification of other structural determinants of the substrate specificity remains a challenge for further research on haloalkane dehalogenases.

Structural comparison of three different haloalkane dehalogenases suggested that substrate specificity of these bacterial enzymes could be significantly influenced by the size and shape of their entrance tunnels. The surface residue leucine 177 positioned at the tunnel opening of the haloalkane dehalogenase from Sphingomonas paucimobilis UT26 was selected for modification based on structural and phylogenetic analysis; the residue partially blocks the entrance tunnel, and it is the most variable pocket residue in haloalkane dehalogenase-like proteins with nine substitutions in 14 proteins. Mutant genes coding for proteins carrying all possible substitutions in position 177 were constructed by site-directed mutagenesis and heterologously expressed in Escherichia coli. In total, 15 active protein variants were obtained, suggesting a relatively high tolerance of the site for the introduction of mutations. Purified protein variants were kinetically characterized by determination of specific activities with 12 halogenated substrates and steady-state kinetic parameters with two substrates. The effect of mutation on the enzyme activities varied dramatically with the structure of the substrates, suggesting that extrapolation of one substrate to another may be misleading and that a systematic characterization of the protein variants with a number of substrates is essential. Multivariate analysis of activity data revealed that catalytic activity of mutant enzymes generally increased with the introduction of small and nonpolar amino acid in position 177. This result is consistent with the phylogenetic analysis showing that glycine and alanine are the most commonly occurring amino acids in this position among haloalkane dehalogenases. The study demonstrates the advantages of using rational engineering to develop enzymes with modified catalytic properties and substrate specificities. The strategy of using sitedirected mutagenesis to modify a specific entrance tunnel residue identified by structural and phylogenetic analyses, rather than combinatorial screening, generated a high percentage of viable mutants.Haloalkane dehalogenases are microbial enzymes acting on haloorganic compounds. The enzymes cleaves the carbon-halogen bond and replaces a halogen with a hydroxyl group from a water molecule (1). Activity and specificity of haloalkane dehalogenases is not optimal for industrial applications (2), and numerous studies have been conducted to improve their catalytic properties using in vitro techniques (3-13). Engineered enzymes can be used in biotechnology applications, such as detoxification of environmental pollutants and bioorganic synthesis. Such technologies are already in use (14) or are under development (2, 15, 16). Furthermore, haloalkane dehalogenases has become an important model system for in silico study of molecular principles of enzymatic catalysis (12,(17)(18)(19)(20)(21)(22)(23)(24).Haloalkane dehalogenase LinB (25) is the enzyme isolated from a ␥-hexachlorocyclohexane-degrading bacterium Sphingomonas paucimob...

Haloalkane dehalogenases are bacterial enzymes capable of carbon-halogen bond cleavage in halogenated compounds. To obtain insights into the mechanism of the haloalkane dehalogenase from Sphingomonas paucimobilis UT26 (LinB), we studied the steady-state and presteady-state kinetics of the conversion of the substrates 1-chlorohexane, chlorocyclohexane, and bromocyclohexane. The results lead to a proposal of a minimal kinetic mechanism consisting of three main steps: (i) substrate binding, (ii) cleavage of the carbon-halogen bond with simultaneous formation of an alkyl-enzyme intermediate, and (iii) hydrolysis of the alkyl-enzyme intermediate. Release of both products, halide and alcohol, is a fast process that was not included in the reaction mechanism as a distinct step. Comparison of the kinetic mechanism of LinB with that of haloalkane dehalogenase DhlA from Xantobacter autotrophicus GJ10 and the haloalkane dehalogenase DhaA from Rhodococcus rhodochrous NCIMB 13064 shows that the overall mechanisms are similar. The main difference is in the rate-limiting step, which is hydrolysis of the alkylenzyme intermediate in LinB, halide release in DhlA, and liberation of an alcohol in DhaA. The occurrence of different rate-limiting steps for three enzymes that belong to the same protein family indicates that extrapolation of this important catalytic property from one enzyme to another can be misleading even for evolutionary closely related proteins. The differences in the rate-limiting step were related to: (i) number and size of the entrance tunnels, (ii) protein flexibility, and (iii) composition of the halide-stabilizing active site residues based on comparison of protein structures.

Haloalkane dehalogenases are key enzymes for the degradation of halogenated aliphatic pollutants. Two rhizobial strains, Mesorhizobium loti MAFF303099 and Bradyrhizobium japonicum USDA110, have open reading frames (ORFs), mlr5434 and blr1087, respectively, that encode putative haloalkane dehalogenase homologues. The crude extracts of Escherichia coli strains expressing mlr5434 and blr1087 showed the ability to dehalogenate 18 halogenated compounds, indicating that these ORFs indeed encode haloalkane dehalogenases. Therefore, these ORFs were referred to as dmlA (dehalogenase from Mesorhizobium loti) and dbjA (dehalogenase from Bradyrhizobium japonicum), respectively. The principal component analysis of the substrate specificities of various haloalkane dehalogenases clearly showed that DbjA and DmlA constitute a novel substrate specificity class with extraordinarily high activity towards ␤-methylated compounds. Comparison of the circular dichroism spectra of DbjA and other dehalogenases strongly suggested that DbjA contains more ␣-helices than the other dehalogenases. The dehalogenase activity of resting cells and Northern blot analyses both revealed that the dmlA and dbjA genes were expressed under normal culture conditions in MAFF303099 and USDA110 strain cells, respectively.Haloalkane dehalogenases are key enzymes for the degradation of halogenated aliphatic compounds that occur as soil pollutants (1,17,38). Haloalkane dehalogenases catalyze the hydrolytic cleavage of the carbon-halogen bond(s) and produce the corresponding alcohols, halide ions, and protons. These enzymes belong to the ␣/␤-hydrolase superfamily (39).Haloalkane dehalogenases are attractive targets for proteinengineering studies aimed at improving catalytic efficiency and at broadening the range of substrate specificity for important environmental pollutants. To date, the three-dimensional structures of three haloalkane dehalogenases have been determined by protein crystallography: DhlA from Xanthobacter autotrophicus GJ10 (48), DhaA from Rhodococcus sp. (25, 35), and LinB from Sphingomonas paucimobilis UT26 (29). The differences in the substrate specificities of these three haloalkane dehalogenases can be accounted for on the basis of their three-dimensional structures (11). Comparison of the kinetic mechanisms of DhlA, DhaA, and LinB showed that the overall reaction mechanisms are similar but that the rate-limiting steps differ, i.e., halide release in the case of DhlA (44), liberation of an alcohol in the case of DhaA (4), and hydrolysis of an alkyl-enzyme intermediate in the case of LinB (41). Partial improvement in the catalytic properties and modification of the substrate specificities of haloalkane dehalogenases by rational design (5, 34) and directed evolution approaches (3, 40) have recently been reported. However, it remains difficult to construct mutant enzymes with entirely new capabilities using only protein-engineering techniques, and therefore, the isolation of new family members is still desirable.For quite some time, haloalkane ...