Glucose-6-phosphate in glycolysis

L-Rhamnose is an important component of bacterial cell walls, and is metabolized in coli through a pathway similar to that of glucose 6-phosphate in glycolysis. Rhamnose isomerase catalyzes the first reaction in the pathway to produce l-rhamnulose from L-rhamnose (Fig. 17-30). The enzyme gene was cloned from coli and overexpressed12521, and the enzyme was purified and characterized12521. 

Although the impaired use of glucose 6-phosphate in glycolysis will lead to the storage of extra glycogen, it will not become excessive, because the increased concentration of glucose... 

The NADPH is produced from glucose 6-phosphate in the first three reactions in the pentose phosphate pathway (see below). Hence the pentose phosphate pathway is essential in the erythrocyte and glycolysis provides the substrate glucose 6-phosphate. Individuals with a reduced amount of glucose 6-phosphate dehydrogenase can suffer from oxidative damage to their cells and hence haemolysis. 

Some mammalian cells have the ability to metabolize glucose 6-phosphate in a pathway that involves the production of C3, C4, C5, C6, and C7 sugars. This process also yields the reduced coenzyme, NADPH, which is oxidized in the biosynthesis of fatty acids and steroids (Chap. 13). Consequently, this metabolic pathway is of major importance in those cells involved in fatty acid and steroid production, such as the liver, lactating mammary gland, adrenal cortex, and adipose tissue. The pentose phosphate pathway, which does not require oxygen and which occurs in the cytoplasm of these cells, has two other names the phosphogluconate pathway (after the first product in the pathway) and the hexose monophosphate shunt (since the end products of the pathway can reenter glycolysis). 

We can grasp the intricate interplay between glycolysis and the pentose phosphate pathway by examining the metabolism of glucose 6-phosphate in four different metabolic situations (Figure 20.24). 

The biosynthesis of glycogen occurs from glucose-6-phosphate. As indicated earlier, this is one of several alternative fates of glucose-6-phosphate in addition to glycolysis. Glucose-6-phosphate via phosphoglucomutase and glucose-1,6-bisphosphate can be reversibly... 

Substrate level control - In this control mechanism, high levels of the product of a reaction inhibit the ability of the small amounts of substrate present to react. An example is the first step in glycolysis, catalyzed by hexokinase. It is inhibited by the product of its action, glucose-6-phosphate. If glycolysis is blocked for any reason, glucose-6-phosphate accumulates. 

Recall How does the role of glucose-6-phosphate in gluconeogenesis differ from that in glycolysis ... 

Figure 29.8 Mechanism of step 2 in glycolysis, the isomerization of glucose 6-phosphate to fructose 6-phosphate.

Following hydrolysis, keto-enol tautomerization of the carbonyl group from C2 to Cl gives glucose 6-phosphate. The isomerization is the reverse of step 2 in glycolysis. 

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