Glucokinase, inhibition

Liver contains an enzyme called glucokinase, which also carries out the reaction in Figure 19.4 but is highly specific for D-glucose, has a much higher for glucose (approximately 10.0 mAf), and is not product-inhibited. With such... 

Specific activation or inhibition Transport of glucose can be increased or decreased by specihc compounds insulin increases the transport whereas phloridzin, a plant glycoside, inhibits glucose transport by muscle. Insulin increases glucokinase activity in liver, whereas a plant sugar, mannoheptulose, inhibits glucokinase activity. Hexokinase is inhibited by its product, glucose 6-phosphate. 

The glucose concentration is the major factor regulating glycogen synthesis in liver. Glucose activates glucokinase directly as a substrate and indirectly via an increase in the concentration of fructose 6-phosphate. It also activates glycogen synthase but it inhibits glycogen phosphorylase (see text). 

Promotes glucose storage as glycogen (induces glucokinase and glycogen synthase, inhibits phosphorylase)... 

Different responses of isozymes to allosteric modulators. This difference is useful in fine-tuning metabolic rates. Hexokinase IV (glucokinase) of liver and the hexokinase isozymes of other tissues differ in their sensitivity to inhibition by glucose 6-phosphate. 

Glucokinase (GK) has a much lower affinity for glucose than hexokinase therefore it is not fully active at the physiological ranges of blood glucose. Additionally, GK is not inhibited by its product G-6-P, whereas hexokinase is. 

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