The reaction catalyzed by GK is the Mg*ATP2_ mediated phosphorylation of glucose to produce G-6-P and adenosine diphosphate (ADP). GK is a 52-kDa protein, and is smaller compared with other hexokinase family members, which are typically around 100 kDa. GK is also unusual in that it is not inhibited by its product, G-6-P. This ensures that the activity of GK is regulated only by glucose concentration. Another key feature of GK is that it demonstrates slight positive cooperativity with respect to glucose. Its Hill slope coefficient is 1.5-1.7, and it has a slightly sigmoidal glucose saturation curve with a substrate concentration at half maximal velocity (S0.5) of approximately 7.5 mM. The S0.5 of GK is significantly higher than that of other hexokinases. The catalytic activity of GK is most sensitive to changes when the glucose concentration nears 4 mM, which represents the inflection point in the glucose saturation curve. This concentration is centered in the normal human physiological range of blood glucose concentrations and is optimally situated to produce maximum enzyme velocities when blood glucose concentrations rise above normal levels.
With respect to Mg*ATP2_, GK kinetics are standard Michaelis-Menton type, with a Km (Michaelis constant) of ATP around 0.4mM, well below the intracellular physiologic concentration of ATP. This ensures that the enzyme rate is dictated by glucose concentration and not ATP concentration [1-4].
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