title: Characterisation of hexokinase in Toxoplasma gondii tachyzoites.
abstract: We have cloned the hexokinase [E.C. 2.7.1.1] gene of Toxoplasma gondii tachyzoite and obtained an active recombinant enzyme with a calculated molecular mass of 51,465Da and an isoelectric point of 5.82. Southern blot analysis indicated that the hexokinase gene existed as a single copy in the tachyzoites of T. gondii. The sequence of T. gondii hexokinase exhibited the highest identity (44%) to that of Plasmodium falciparum hexokinase and lower identity of less than 35% to those of hexokinases from other organisms. The specific activity of the homogeneously purified recombinant enzyme was 4.04 micromol/mg protein/min at 37 degrees C under optimal conditions. The enzyme could use glucose, fructose, and mannose as substrates, though it preferred glucose. Adenosine triphosphate was exclusively the most effective phosphorus donor, and pyrophosphate did not serve as a substrate. K(m) values for glucose and adenosine triphosphate were 8.0+/-0.8 microM and 1.05+/-0.25mM, respectively. No allosteric effect of substrates was observed, and the products, glucose 6-phosphate and adenosine diphosphate, had no inhibitory effect on T. gondii hexokinase activity. Other phosphorylated hexoses, fructose 6-phosphate, trehalose 6-phosphate which is an inhibitor of yeast hexokinase, and pyrophosphate, also did not affect T. gondii hexokinase activity. Native hexokinase activity was recovered in both the cytosol and membrane fractions of the whole lysate of T. gondii tachyzoites. This result suggests that T. gondii hexokinase weakly associates with the membrane or particulate fraction of the tachyzoite cell.
authors.affiliation: Department of Tropical Medicine and Parasitology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. asait@sc.itc.keio.ac.jp
authors.forename: Tomoya
authors.initials: T
authors.lastname: Saito
authors.forename: Takuya
authors.initials: T
authors.lastname: Maeda
authors.forename: Miki
authors.initials: M
authors.lastname: Nakazawa
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Takeuchi
authors.forename: Tomoyoshi
authors.initials: T
authors.lastname: Nozaki
authors.forename: Takashi
authors.initials: T
authors.lastname: Asai
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015139:Blotting, Southern; D005786:Gene Expression Regulation; D006593:Hexokinase / Q000737:chemistry / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D013379:Substrate Specificity; D014122:Toxoplasma / Q000201:enzymology* / Q000235:genetics
chemical_list: D006593:Hexokinase