Characterization of Plasmodium falciparum serine hydroxymethyltransferase-A potential antimalarial target

Serine hydroxymethyltransferase (SHMT) is a ubiquitous enzyme required for folate recycling and dTMP synthesis. A cDNA encoding Plasmodium falciparum (Pf) SHMT was expressed as a hexa-histidine tagged protein in Escherichia coli BL21-CodonPlus® (DE3)-RIL. The protein was purified and the process yielded 3.6 mg protein/l cell culture. Recombinant His6-tagged PfSHMT exhibits a visible spectrum characteristic of pyridoxal-5′-phosphate enzyme and catalyzes the reversible conversion of l-serine and tetrahydrofolate (H4folate) to glycine and 5,10-methylenetetrahydrofolate (CH2-H4folate). Steady-state kinetics study indicates that His6-tagged PfSHMT catalyzes the reaction by a ternary-complex mechanism. The sequence of substrate binding to the enzyme was also examined by glycine product inhibition. A striking property that is unique for His6-tagged PfSHMT is the ability to use d-serine as a substrate in the folate-dependent serine-glycine conversion. Kinetic data in combination with expression result support the proposal of SHMT reaction being a regulatory step for dTMP cycle. This finding suggests that PfSHMT can be a potential target for antimalarial chemotherapy.