Characterisation of the novel TPI-GAPDH fusion enzyme in Blastocystis hominis

A thesis submitted to the University of Bedfordshire, in partial fulfilment of the requirements for the degree of Master of Science by research.Protist parasites such as the human pathogens Trypanosoma brucei, Blastocystis hominis and the plant pathogen Phytophthora infestans cause devastating diseases worldwide and in the case of P. infestans, huge financial losses in potato crops. Treating these diseases is fraught with toxic side effects due to the similar functioning of all eukaryotes and emerging resistance problems. This highlights a requirement for new and innovative antiparasitic treatments that are specific against the target organism without damaging the host. Studies haves demonstrated protist metabolism as a potential drug target. A novel mitochondrial targeted glycolytically active TPI-GAPDH fusion enzyme has been discovered in a selection of stramenopiles (including B. hominis and P. infestans). This is interesting in terms of a potential selective drug target and when considering the mitochondrial targeting to the enzyme, suggesting a retention of glycolysis originating from the mitochondrial precursor before the endosymbiosis event (i.e. from alpha proteobacteria). This study characterises TPI-GAPDH from B. hominis by performing enzymatic assays on wild type TPI-GAPDH, mutated TPI-GAPDH and by splitting the protein into separate TPI and GAPDH enzymes. The enzymatic assays showed that GAPDH does not function as effectively when separated from TPI, having 2-fold decrease in enzyme activity compared against wtTPI-GAPDH. This suggests that TPI has a structural effect on GAPDH function. The enzymatic assays also demonstrated a competition for GAP between TPI and GAPDH, this goes against the canonical understanding of TPI. TPI functions to equilibrate DHAP to GAP in a 22:1 ratio and is generally considered to be a ‘perfect’ enzyme which is not rate limited in its function. However, when attached to GAPDH, TPI appears to be in competition with GAPDH for substrate. The apparent loss of function of B. hominis GAPDH when separated from TPI makes the linkage between the two enzymes a potential novel drug target within glycolysis in these protists