Overexpression of the nitroreductase NfsB in an E. coli strain as a whole-cell biocatalyst for the production of chlorinated analogues of the natural herbicide DIBOA.

Benzohydroxamic acids, such as DIBOA (2,4-dihydroxy-2 H)-1,4-benzoxazin-3(4 H)-one), are plant products that exhibit interesting herbicidal, fungicidal and bactericidal properties. A feasible alternative to their purification from natural sources is the synthesis of analogous compounds such as D-DIBOA (2-deoxy-DIBOA) and their chlorinated derivatives. Their chemical synthesis has been simplified into two steps. However, the second step is an exothermic reaction and involves hydrogen release, which makes this methodology expensive and difficult to scale up. The study reported here concerns the possibility of producing chlorobenzoxazinones by a whole-cell biocatalytic process using the ability of the engineered E. coli nfsB-/pBAD-NfsB to catalyse the synthesis of 6-Cl-D-DIBOA and 8-Cl-D-DIBOA from their respective precursors (PCs). The results show that this strain is able to grow in media that contain these compounds and to produce the target molecules with 59.3% and 46.7% biotransformation yields, respectively. Moreover, the strain is capable of processing non-purified PCs from the first chemical step to give similar yields to those obtained from the purified PCs. The kinetics of the reaction in vitro with purified recombinant NfsB nitroreductase were studied to characterise the catalysis further and evaluate the effects that several components of the non-purified PCs have on the process. The results revealed that the kinetics are that of an allosteric enzyme. The inhibitory effect of the substrate of the first step of the chemical synthesis, which is present in some non-purified PCs, was also demonstrated.