Liver glucose-6-phosphate

Figure 6.19 Regulation of the synthesis of glycogen from glucose in liver and muscle. Insulin is the major factor stimulating glycogen synthesis in muscle it increases glucose transport into the muscle and the activity of glycogen synthase, activity which is also activated by glucose 6-phosphate but inhibited by glycogen. The latter represents a feedback mechanism and the former a feedforward. The mechanism by which glycogen inhibits the activity is not known. The mechanism for the insulin effect is discussed in Chapter 12.

Fig. 3.—Enzymic Interoonversion of D-Glucose and Liver Glycogen. In this reaction, 5-10% of D-glucose is produced, in addition to a-D-glucopyranosyl phosphate.

Galactokinase. Galactose is phosphorylated by a specific galactokinase, formed in liver and yeast. The product is galactose-l-phosphate (VIII). Galactose phosphate is converted to glucose phosphate by reactions to be discussed later (p. 242). 

The liver utilizes glucose-6-phosphatase to remove the phosphate from glucose 6-phosphate. Non-phosphorylated glucose from liver glycogen can traverse the plasma membrane and be transported via the blood circulation to other tissues. Glucose-6-phosphatase is absent from skeletal muscle and brain and so glucose is retained by these tissues as glucose 6-phosphate which cannot permeate the plasma membrane. 

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