Steady-State and Pre-Steady-State Kinetic Analysis of Halopropane Conversion by a Rhodococcus Haloalkane Dehalogenase

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Abstract
Haloalkane dehalogenase from Rhodococcus rhodochrous NCIMB 13064 (DhaA) catalyzes the hydrolysis of carbon−halogen bonds in a wide range of haloalkanes. We examined the steady-state and pre-steady-state kinetics of halopropane conversion by DhaA to illuminate mechanistic details of the dehalogenation pathway. Steady-state kinetic analysis of DhaA with a range of halopropanes showed that bromopropanes had higher kcat and lower KM values than the chlorinated analogues. The kinetic mechanism of dehalogenation was further studied using rapid-quench-flow analysis of 1,3-dibromopropane conversion. This provided a direct measurement of the chemical steps in the reaction mechanism, i.e., cleavage of the carbon−halogen bond and hydrolysis of the covalent alkyl−enzyme intermediate. The results lead to a minimal mechanism consisting of four main steps. The occurrence of a pre-steady-state burst, both for bromide and 3-bromo-1-propanol, suggests that product release is rate-limiting under steady-state conditions. Combining pre-steady-state burst and single-turnover experiments indicated that the rate of carbon−bromine bond cleavage was indeed more than 100-fold higher than the steady-state kcat. Product release occurred with a rate constant of 3.9 s-1, a value close to the experimental kcat of 2.7 s-1. Comparing the kinetic mechanism of DhaA with that of the corresponding enzyme from Xanthobacter autotrophicus GJ10 (DhlA) shows that the overall mechanisms are similar. However, whereas in DhlA the rate of halide release represents the slowest step in the catalytic cycle, our results suggest that in DhaA the release of 3-bromo-1-propanol is the slowest step during 1,3-dibromopropane conversion.