Abstract
This work investigated the potential for biocatalytic degradation of micropollutants, focusing on chlorine oxyanions as model contaminants, by mining biology to identify promising biocatalysts. Existing isozymes of chlorite dismutase (Cld) were characterized with respect to parameters relevant to this high volume, low-value product application: kinetic parameters, resistance to catalytic inactivation, and stability. Maximum reaction velocities (Vmax) were typically on the order of 104 μmol min-1 (μmol heme)-1. Substrate affinity (Km) values were on the order of 100 μM, except for the Cld from Candidatus Nitrospira defluvii (NdCld), which showed a significantly lower affinity for chlorite. NdCld also had the highest susceptibility to catalytic inactivation. In contrast, the Cld from Ideonella dechloratans was least susceptible to catalytic inactivation, with a maximum turnover number of approximately 150,000, more than sevenfold higher than other tested isozymes. Under non-reactive conditions, Cld was quite stable, retaining over 50% of activity after 30 days, and most samples retained activity even after 90–100 days. Overall, Cld from I. dechloratans was the most promising candidate for environmental applications, having high affinity and activity, a relatively low propensity for catalytic inactivation, and excellent stability.
Highlights
Micropollutants and other emerging contaminants are challenging to remediate due to their diverse chemical structures, their persistence in the environment, the toxicity of some degradation products, and their ability to cause toxic effects at low concentrations
When the stability of perchlorate-reducing biocatalysts was examined under nonreactive conditions, chlorite was the first intermediate to accumulate, suggesting that chlorite dismutase (Cld) has the shorter shelf life of the two enzymes (Hutchison et al, 2013). These previous observations on catalytic inactivation and shelf life suggest that Cld would be the limiting component in biocatalytic treatment of perchlorate and chlorate, in addition to being the only biocatalyst required for chlorite removal
The results presented here demonstrate favorable activity and stability in soluble protein fractions as well as significant variation in catalytic life, affecting which Cld isozymes would be best suited for water treatment and other environmental applications
Summary
Micropollutants and other emerging contaminants are challenging to remediate due to their diverse chemical structures, their persistence in the environment, the toxicity of some degradation products, and their ability to cause toxic effects at low concentrations. These previous observations on catalytic inactivation and shelf life suggest that Cld would be the limiting component in biocatalytic treatment of perchlorate and chlorate, in addition to being the only biocatalyst required for chlorite removal. We targeted three characteristics of Cld relevant to large-scale application: kinetic parameters in soluble protein fractions, catalytic inactivation, and shelf life.
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