Theoretical identification on the role of Lys15 for Sulfolobus tokodaii hexokinase

Abstract

Phosphorylation mechanisms of glucose catalyzed by complexes of glucose–ATP–Mg²⁺–StHK, glucose–ATP–Mg²⁺–K15A mutant and glucose–ATPγS–Mg²⁺–StHK have been extensively studied using the quantum mechanical/molecular mechanical (QM/MM) method. Structural analyses show that Mg²⁺ ion plays a key role in the stabilization of the β-phosphate in the whole catalytic reaction and contributes much to the departure of γ-phosphoryl (or γ-thiophosphoryl) group. Besides, the existence of K15 could also facilitate the stabilization of the β-phosphate directly and influence the binding of γ-phosphate (or γ-thiophosphate) with C6-hydroxyl group indirectly. For each complex, two catalytic processes (the phosphate transfer and proton transfer steps) are studied. The phosphate transfer process is calculated to be the rate-determining step in all three complexes, where the energy barrier of the phosphate transfer is 4.0, 11.7 and 10.9 kcal mol⁻¹ for glucose–ATP–Mg²⁺–StHK, glucose–ATP–Mg²⁺–K15A mutant and glucose–ATPγS–Mg²⁺–StHK complexes, respectively. Both the ATP and ATPγS bound StHK are exothermic, where the catalytic reaction is endothermic for K15A mutant. Calculations suggest that the influence of K15A mutation to the reactive activity is larger than that of the exchange of ATP to ATPγS. We suppose that K15 might play the similar role with those conserved arginine residue in human hexokinase I–IV.