Thermodynamic analysis of halide binding to haloalkane dehalogenase suggests the occurrence of large conformational changes.
Haloalkane dehalogenase (DhlA) hydrolyzes short-chain haloalkanes to produce the corresponding alcohols and halide ions. Release of the halide ion from the active-site cavity can proceed via a two-step and a three-step route, which both contain slow enzyme isomerization steps. Thermodynamic analysis of bromide binding and release showed that the slow unimolecular isomerization steps in the three-step bromide export route have considerably larger transition state enthalpies and entropies than those in the other route. This suggests that the three-step route involves different and perhaps larger conformational changes than the two-step export route. We propose that the three-step halide export route starts with conformational changes that result in a more open configuration of the active site from which the halide ion can readily escape. In addition, we suggest that the two-step route for halide release involves the transfer of the halide ion from the halide-binding site in the cavity to a binding site somewhere at the protein surface, where a so-called collision complex is formed in which the halide ion is only weakly bound. No large structural rearrangements are necessary for this latter process.
Department of Biochemistry, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, The Netherlands.
G H
GH
Krooshof
R
R
Floris
A W
AW
Tepper
D B
DB
Janssen
D001965:Bromides / Q000032:analysis; D004926:Escherichia coli / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006867:Hydrolases / Q000378:metabolism*; D007700:Kinetics; D011487:Protein Conformation; D013696:Temperature; D013816:Thermodynamics; D013997:Time Factors
D001965:Bromides; D006867:Hydrolases; C046137:haloalkane dehalogenase