Glucose-6-phosphate/phosphatase

ATP"-----> Glucose-6-phosphate) phosphatase layer (heat source equivalent glucose-6-phosphate- ... 

Glucose-6-phosphatase is another enzyme competitively inhibited by galactose-l-phosphate (15). The inhibition of hepatic glucose-6-phos-phatase is probably important in relation to hypoglycemia. 

W17. Willmer, J. S., Changes in hepatic enzyme levels after adrenalectomy. II. Glucose-6-phosphatase, glucose-6-phosphate dehydrogenase, and 6-phospho-gluconate dehydrogenase. Can. J. Biochem. and Physiol. 38, 1449-1456 (1960). 

The reverse of this reaction does occur in some cells (e.g. hepatocytes), catalysed by the enzyme glucose-6-phosphatase, but the direction is always towards glucose formation, i.e. glucose 6-phosphate is never produced by this reaction in vivo. The question is, why not ... 

The conversion of glucose 6-phosphate to glucose occurs in a reaction catalysed by the enzyme glucose-6-phosphatase ... 

The enzyme glucose 6-phosphatase, which catalyses the hydrolysis of glucose 6-phosphate to form glucose, is also present in the liver cell. 

Both glucose 6-phosphatase and glucokinase are simultaneously active, the result of which is a substrate cycle, the glucose/glucose 6-phosphate cycle (Figure 6.29(a)). 

The glucose/glucose 6-phosphate cycle, which involves the enzymes glncokinase and glucose 6-phosphatase. 

Transporters in the plasma membrane of hepatocytes allow insulin-independent transport of glucose and other sugars in both directions. In contrast to muscle, the liver possesses the enzyme glucose-6-phosphatase, which can release glucose from glucose-6-phosphate. 

Glucose 6-phosphate can then be metabolized by glycolysis in the liver or muscle, or it can be dephosphorylated by the action of glucose 6-phosphatase mainly in the liver and released into the bloodstream for use by other tissues of the body. 

The most common glycogen storage disease. Type I or von Gierke disease, is a deficiency in glucose 6-phosphatase in which glycogen structures are normal however, the liver is unable to dephosphory-late glucose 6-phosphate, and it remains trapped In the cell. 

Isoproterenol is the most potent stimulant of skeletal muscle glycogenolysis, followed by epinephrine and norepinephrine. (3z-Adrenoceptors mediate muscle glycogenolysis. Stimulation of skeletal muscle glycogenolysis will raise blood lactic acid levels rather than blood glucose levels because skeletal muscle lacks the enzyme glucose-6-phosphatase, which catalyzes the conversion of glucose-6-phosphate to glucose. 

FIGURE 15-6 Hydrolysis of glucose 6-phosphate by glucose 6-phosphatase of the ER. The catalytic site of glucose 6-phosphatase faces the lumen of the ER. A glucose 6-phosphate (G6P) transporter (T1) carries the substrate from the cytosol to the lumen, and the prod-... 

Because muscle and adipose tissue lack glucose 6-phosphatase, they cannot convert the glucose 6-phosphate formed by glycogen breakdown to glucose, and these tissues therefore do not contribute glucose to the blood. 

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