Bacterial production of maize and human serine racemases as partially active inclusion bodies for d-serine synthesis

Recombinant protein overexpressed in Escherichia coli is often less folded, leading to form the insoluble aggregates also called as inclusion bodies (IBs). IBs are classified as active and inactive ones, and enzyme produced as active IBs is the novel carrier-free immobilized material. In this study, we determined that His6-tagged serine racemase (SR) from maize or human produced as partially active IBs maintained the activities for reversible racemization of l-serine to d-serine but lost the activities for irreversible β-elimination of both enantiomers, in contrast to the soluble one displaying all activities. Fourier transform infrared spectroscopy analysis showed structural changes between the soluble and insoluble SR. Compared with the soluble SR with attachment of the N-terminal cellulose-binding module via the oriented immobilization of the regenerated amorphous cellulose, the insoluble SR with the fusion of the N-terminal aggregation-inducible tag GFIL8 displayed higher production and usage efficiency, lower leaky capacity, more stability at 4 °C storage with the prolonged time, less sensitivity to the limited proteolysis mediated by trypsin, and higher yield of the synthesized d-serine. These advantages allow the SRs as partially active IBs to synthesize d-serine for medical and agricultural applications.