Identification and functional characterization of a novel CaSrpA enzyme for selenite reduction and selenium nanoparticle formation.
Academic Article
Overview
abstract
Selenite reductases are widely distributed across various oxidoreductase families (e.g., ThxR, OYE, and FccA enzymes) [1]. The ability of short-chain dehydrogenase/reductase (SDR) family enzymes for selenite reduction remains unknown. Using metagenomic and metatranscriptomic analyses, 40 putative selenite reductases were identified from selenium-rich regions based on catalytic domain homology and transcriptional upregulation. These enzymes mainly belong to the SDR family and metalloenzymes. Enzyme activity analysis indicated that CaSrpA possessed the ability (Vmax, 18.85 μM/min/g) to reduce selenite to selenium nanoparticles (SeNPs). Phylogenetic analysis showed that CaSrpA was clustered in the clade of SDR enzymes, with the typical Rossmann fold. CaSrpA also oxidized S-1-phenylethanol to phenylacetone (Vmax, 15.4 μM/min/mg), sharing 53 % sequence similarity with the alcohol dehydrogenase RasADH. Molecular docking and structural superposition identified sixteen key residues associated with CaSrpA activity. Site-directed mutagenesis revealed that over 14 mutants exhibited a 30-90 % reduction in relative activity. Mutant M206A enhanced catalytic efficiency towards selenite by 2.4-fold and S-1-phenylethanol by 5.4-fold via a lid-opening mechanism. Molecular dynamics simulation elucidated that the mutant M206A utilized lid opening mechanism to accommodate more substrate and co-factor for catalysis via altering the conformation of the α7-α8 loop. This study helps understand the intrinsic connection between the SDR family and selenite-reducing capability, broadening the repertoire of selenite reductases.
