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.