The crystal structure of glycerol kinase from the hyperthermophilic archaeon Thermococcus kodakaraensis (Tk‐GK) in a dimeric form was determined at a resolution of 2.4 Å. This is the first crystal structure of a hyperthermophilic glycerol kinase. The overall structure of the Tk‐GK dimer is very similar to that of the Escherichia coli glycerol kinase (Ec‐GK) dimer. However, two dimers of Ec‐GK can associate into a tetramer with a twofold axis, whereas those of Tk‐GK cannot. This may be the reason why Tk‐GK is not inhibited by fructose 1,6‐bisphosphate, because the fructose 1,6‐bisphosphate binding site is produced only when a tetrameric structure is formed. Differential scanning calorimetry analyses indicate that Tk‐GK is a highly thermostable protein with a melting temperature (Tm) of 105.4 °C for the major transition. This value is higher than that of Ec‐GK by 34.1 °C. Comparison of the crystal structures of Tk‐GK and Ec‐GK indicate that there is a marked difference in the number of ion pairs in the α16 helix. Four ion pairs, termed IP1–IP4, are formed in this helix in the Tk‐GK structure. To examine whether these ion pairs contribute to the stabilization of Tk‐GK, four Tk‐GK and four Ec‐GK derivatives with reciprocal mutations at the IP1–IP4 sites were constructed. The determination of their stabilities indicates that the removal of each ion pair does not affect the stability of Tk‐GK significantly, whereas the mutations designed to introduce one of these ion pairs stabilize or destabilize Ec‐GK considerably. These results suggest that the ion pairs in the α16 helix contribute to the stabilization of Tk‐GK in a cooperative manner.

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Glycerol kinase from the hyperthermophilic archaeon Thermococcus kodakaraensis was crystallized and preliminary crystallographic studies of the crystals were performed. Crystals were grown at 293 K by the sitting-drop vapourdiffusion method. Native X-ray diffraction data were collected to 2.4 Å resolution using synchrotron radiation at station BL44XU of SPring-8. The crystal belongs to the rhombohedral space group R3, with unit-cell parameters a = b = 217.48, c = 66.48 Å . Assuming the presence of two molecules in the asymmetric unit, the V M value was 2.7 Å 3 Da À1 and the solvent content was 54.1%. The protein was also cocrystallized with substrates and diffraction data were collected to 2.7 Å resolution.

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