Comparative characterization of hexose transporters of Plasmodium knowlesi, Plasmodium yoelii and Toxoplasma gondii highlights functional differences within the apicomplexan family

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Abstract
Chemotherapy of apicomplexan parasites is limited by emerging drug resistance or lack of novel targets. PfHT1, the Plasmodium falciparum hexose transporter 1, is a promising new drug target because asexual-stage malarial parasites depend wholly on glucose for energy. We have performed a comparative functional characterization of PfHT1 and hexose transporters of the simian malarial parasite P. knowlesi (PkHT1), the rodent parasite P. yoelii (PyHT1) and the human apicomplexan parasite Toxoplasma gondii (T. gondii glucose transporter 1, TgGT1). PkHT1 and PyHT1 share >70% amino acid identity with PfHT1, while TgGT1 is more divergent (37.2% identity). All transporters mediate uptake of d-glucose and d-fructose. PyHT1 has an affinity for glucose (Km0.12mM) that is higher than that for PkHT1 (Km0.67mM) or PfHT1 (Km1mM). TgGT1 is highly temperature dependent (the Q10 value, the fold change in activity for a 10°C change in temperature, was >7) compared with Plasmodium transporters (Q10, 1.5—2.5), and overall has the highest affinity for glucose (Km30μM). Using active analogues in competition for glucose uptake, experiments show that hydroxyl groups at the C-3, C-4 and C-6 positions are important in interacting with PkHT1, PyHT1 and TgGT1. This study defines models useful to study the biology of apicomplexan hexose permeation pathways, as well as contributing to drug development.