Biochemical Characterization of Recombinant Cu–Zn SOD from Citrus limon Fused to Gliadin Peptides

Two fusion proteins of SOD Citrus limon (SOD_Cl) with gliadin peptides, QQPYPQPQPF (GliSOD_P61) and LGQ-QQPFPPQQPYPQPQPF (GliSOD_P51) were constructed to enhance their permeability through epithelial cells. The introduction of gliadin peptide might alter the biochemical properties of the protein. In the present study, the effect of using synthetic codon-optimized gene to the level of SODs production in Escherichia coli BL21(DE3) and the effects of the introduction of gliadin peptides to the biochemical character of SOD_Cl were investigated including their enzyme kinetics and stability. All proteins were overproduced and affinity purified into homogeneity. Residual SOD activity was determined based on nitro blue tetrazolium-riboflavin oxidation-reduction. All recombinant SODs shared similar enzymatic properties including activity over a broad pH range from 2.0–8.0 and were relative tolerance to chemical agents tested at low concentration. Inhibition by KCN and H2O2 confirmed that SOD_Cl belonged to Cu/ Zn-SOD. The stability of the SODs increased with increasing concentrations of KCl, MgCl2 and MgSO4. Of the three enzymes, SOD_Cl exhibited the highest specific activity and its kinetic parameters (Km and Vmax). In conclusion, the introduction of gliadin peptides to SOD_Cl decreases specific activity, thermal stability and stability at alkaline pH (9-11).