X. autotrophicus

Increase in water activity has an opposite effects on activity and stability of whole cells. Both for R. eiythropolis and X. autotrophicus an increase in dehalogenation was noticed on raising water activity [14, 15]. Interestingly, a minimum water activity of 0.4 was required in both cases to observe hydrolysis of 1-chlorobutane, while Dravis et al. [47] showed that the isolated haloalkane dehalogenase of R. 

Figure .6 (a) Effect of water activity on the relative dehalogenase activity of dehydrated R. erythropolis cells maintained at 40°C (D)or (b) X. autotrophicus cells maintained at 30°C ( ) in a solid/gas reactor. The maximal dehalogenase activity obtained with 0.8 water... 

For both bacteria, the increase in activity was concomitant with the decrease in the catalyst s stability. At high temperatures, this phenomenon can be explained by thermal denaturation. At low temperatures (lower than 40 °C for R. erythropolis and 30 °C for X. autotrophicus) the drop in activity could be attributed to the accumulation of acid, more rapidly produced at high water activity. 

To prepare the catalyst, cells are suspended in a buffer and then lyophilized. The pH has to be controlled to maintain activity after drying. For R. erythropolis, using Tris/HCl as buffer, an optimal pH of 9.0 was found, in agreement with data reported for the isolated enzyme in the aqueous [52] or gas phase [47]. For dehydrated X. autotrophicus cells the optimal pH for activity in the gas phase was 8.5 (Tris/HCl buffer), whereas an optimal pH of 8.2 has been determined for the isolated enzyme by Keuning et al. [53] as shown in Figure 11.7. 

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