Investigating Ligand Binding of the Orthophthalic Acid Binding Protein: OpaL

OPA (orthophthalic acid) is a plasticiser used to increase the flexibility of plastics. OPA is hydrogen bonded to the plastic polymer, allowing it to leach out into the environment. Plastic itself poses major environmental concerns, OPA additionally has a myriad of adverse effects on human health. Investigating microbial degradation of OPA is a bioremediation approach to remove this pollutant from the environment. The OpaL TAXI-TRAP transporter transports OPA inside the cell, therefore this research aimed to characterise OpaL’s binding site to understand how OPA binds, and which amino acid residues are important for binding. WT and eight mutant amino acids were selected from the predicted binding site (H179A, G224A, T282A, S101A, W175A, Q55A, D133A and F284A), the purified protein was used in DSF (Differential Scanning Fluorimetry) and Intrinsic Fluorescence assays. DSF revealed that OPA is highly specific to OpaL as nine similarly structured ligands did not specifically bind to OpaL. A Tm was calculated for WT and its mutants, where five of the eight mutant Tm values were statistically significantly reduced compared to WT (G224A, T284A, W175A, F284A and S101A). The W175A mutant was most impacted as there was a -16.228 °C difference in Tm compared to WT OpaL at 5 mM OPA concentration, with a significant p value of <0.0001 for the 0-5 mM OPA titration. W175A’s reduced Tm highlights its importance in OpaL’s binding site and fits with the knowledge that tryptophan is found in biologically essential areas of proteins. Finally, intrinsic fluorescence was used to obtain a binding affinity of WT and mutants’ binding of OPA, although the results were inconclusive. Characterising the binding site of OpaL and investigating how OPA binds contributes to the work on biodegradation of this plastic-derived pollutant, as well as expanding the knowledge of the poorly characterised TAXI-TRAP transporter group.