Polyaromatic Hydrocarbon Specific Ring Hydroxylating Dioxygenases: Diversity, Structure, Function, and Protein Engineering

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitously present in the environment. These compounds have demonstrated both mutagenic and carcinogenic properties. In the past few decades, scientists have constantly been looking for a possible route to their biological degradation. Bacterial ring hydroxylating dioxygenases (RHDs) implicated in the polycyclic aromatic hydrocarbon degradation comprise a large family of enzymes. RHD catalyzes the stereospecific oxidation of PAHs by incorporating molecular oxygen into inert aromatic nuclei. These biocatalysts hold the potential to completely transform and mineralize toxic forms of these compounds into non-toxic forms. RHDsmediated oxygenation produces cis-dihydrodiols, a chiral compound used in pharmaceutical industries. The Molecular investigation of 16S rRNA and key functional genes involved in pollutant degradation have revealed the dominant occurrence of phylum proteobacteria and actinobacteria in hydrocarbonpolluted environments. The present review is aimed at narrating the diversity, distribution, structural and functional characteristics of RHDs. The review further highlights key amino acids participating in RHDs catalysis. It also discusses the robustness of protein engineering methods in improving the structural and functional activity of the ring hydroxylating dioxygenases.