Expression and Enzyme Kinetics of Fused Plasmodium falciparum Orotate Phosphoribosyltransferase and Orotidine 5′-monophosphate Decarboxylase in Different Escherichia Coli

Fusion of the last two enzymes in the pyrimidine biosynthetic pathway in the inverse order by having COOH-terminal orotate phosphoribosyltransferase (OPRT) and NH₂-terminal orotidine 5′-monophosphate decarboxylase (OMPDC), as OPRT-OMPDC, has been described in many organisms.

The study aimed to select the optimum host cell and temperature for expressing the recombinant fusion OMPDC-OPRT having the enzymatic activity.

We constructed gene fusions of the human malaria parasite Plasmodium falciparum OMPDC-OPRT (1,836 bp) in the pTrcHisA vector and expressed it as a 6xHis-tag bifunctional protein in three Escherichia coli strains (BL21(DE3), TOP10, Rosetta) at 18°C and 25°C. The recombinant bifunctional protein was partially purified by Ni-nitrilotriacetic acid affinity chromatography and confirmed via Western blot and LC-MS/MS. The enzyme kinetics of OPRT and OMPDC was assessed.

Specific enzymatic activities of both OPRT and OMPDC domains expressed in E. coli BL21(DE3) cells were approximately eight-to-nine-fold higher than those in the TOP10 cells at 18°C. However, the specific activities of both domains expressed in the TOP10 cells were twice higher than those of the BL21(DE3) cells at 25°C. Very low and no enzymatic activities were observed when the constructed vector was expressed in the Rosetta cells at both induction temperatures. The bifunctional enzyme had specific activities of the OPRT and OMPDC domains in a ratio of 1:2. Kinetic study values of the OPRT domain in the bifunctional OMPDC-OPRT enzyme were found to be relatively low at µM level and at the perfect catalytic efficiency (k_cat/K_m).

The recombinant fusion of OMPDC-OPRT exhibited a high expression level of E. coli BL21(DE3) at 18°C. The kinetic parameter is greater than 10⁸M^-1s^-1.