Glucose 6-phosphate isomerization

In glycolysis, why must D-glucose-6-phosphate isomerize to D-fmctose-6-phosphate before the cleavage reaction with aldolase occurs (See p. 1026.)... 

The reaction is irreversibly shifted to the right. Fructose 6-phosphate is isomerized to glucose 6-phosphate by glucose-phosphate isomerase. 

Isomerases Catalyze isomerization and interconversion within a molecule Glucose phosphate isomerase... 

In a second reaction, glucose-l-phosphate isomerizes to give glucose-6-phosphate. 

Efficient utilization of the fructose requires phosphorylation of the glyceraldehyde. Tracer experiments show that the carbon 1 of fructose appears as both carbons 1 and 6 of glucose. This is the result of triose phosphate isomerization followed by (conventional) aldolase condensation to hexose diphosphate. The conversion of fructose diphosphate to glucose-6-phosphate requires a phosphatase and an isomerase, as discussed in the pentose phosphate pathway. 

The six-carbon product of the transaldolase reaction enters the cytosolic pool of fructose 6-phosphate and may be utilized in energy production through the glycolytic pathway. Alternatively, it may be isomerized into glucose phosphate and reprocessed through the oxidative phase of the pathway. 

Reaction 2 Phosphoglucoisomerase Catalyzes the Isomerization of Glucose-6-Phosphate... 

One of the later steps in glucose biosynthesis is the isomerization of fructose 6-phosphate to glucose 6-phosphate. Propose a mechanism, using acid or base catalysis as needed. 

Glucose 6-phosphate is isomerized to fructose 6-phosphate by ring opening followed by a keto-enol tautomerization. 

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. 

Glucose 6-phosphate is an important compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis, the pentose phosphate pathway, glycogenosis, and glycogenolysis). In glycolysis, it is converted to fructose 6-phosphate by phosphohexose-isomerase, which involves an aldose-ketose isomerization. 

Glucose 6-phosphate is then isomerized to fructose 6-phosphate. This conversion of an aldose sugar to a ketose sugar is easy to rationalize in terms of keto-enol tautomerism (see Box 10.1). 

This is achieved by two keto-enol tautomerism reactions and a common enol (see Box 10.1). Mechanistically, it is identical to the isomerization of glucose 6-phosphate to fructose 6-phosphate seen earlier in the sequence, so we can move on to the next step of the pathway. 

In the next step, glucose 6-phosphate is isomerized into fructose 6-phosphate. 

---