Catalytic Promiscuity of Aromatic Ring-Hydroxylating Dioxygenases and Their Role in the Plasticity of Xenobiotic Compound Degradation

Abstract

Persistent organic pollutants pose one of the most critical challenges to the humankind due to their well-established hazardous effects to the ecosystem and various life forms including the human being. Many of these pollutants are anthropogenic in nature and exhibit the tendency of recalcitrance toward natural biodegradation. Most of these pollutants belong to chemical scaffolds that were never present in the environment in the past. Consequently, the evolution of the enzymes and metabolic pathways for their metabolism and degradation over the short geological time span that these compounds have been present in the environment is considered to be rather challenging. Interestingly, microorganisms belonging to diverse taxonomic groups have been identified and characterized with the metabolic potential to degrade the anthropogenic compounds and utilize them as a source of carbon/energy. Evolution of such degradative potential is widely accepted to have occurred with one of the following mechanisms: (i) horizontal gene transfer and (ii) genome reorganization and domain shuffling. An alternative and recent theory in this regard suggests that the evolution of degradative enzymes for anthropogenic compounds may have exploited the “catalytic promiscuity” of metabolic enzymes evolved for degradation of structurally related yet distinct compounds. Noticeably, such catalytic promiscuity of metabolic enzymes has been reported for a number of enzymes involved in degradation of anthropogenic compounds. “Aromatic ring-hydroxylating dioxygenase” is one prominent group of enzymes which exhibit catalytic promiscuity, a potential that has been exploited for technological application in the field of biocatalysis as well as for enhancing the plasticity of anthropogenic xenobiotic compound degradation. The present book chapter aims to present a comprehensive account of aromatic ring-hydroxylating dioxygenases with respect to their basic introduction, classification, the molecular mechanism of action, structure-function relationship, catalytic promiscuity, and applications with respect to expansion of biocatalysis and biodegradation.