Phosphoglucose isomerase , sugar

We describe here the enzymological characteristics and application of isomerases, especially D-xylose (glucose) isomerase, phosphoglucose isomerase, triose phosphate isomerase, L-rhamnose isomerase, L-fucose isomerase, maleate cis-trans isomerase, and unsaturated fatty acid cis-trans isomerase. N-Acetyl-D-glucosamine 2-epimerase is not an isomerase, but for convenience we will also describe the characteristics and use of the enzyme because this section deals with sugar-metabolizing enzymes. 

The glucose-6-phosphate formed in the first reaction is rearranged to produce the structural isomer fructose-6-phosphate. The enzyme phosphoglucose isomerase catalyzes this isomerization. The result is that the C-1 carbon of the six-carbon sugar is exposed it is no longer part of the ring structure. Examination of the open-chain structures reveals that this isomerization converts an aldose into a ketose. 

No. The reaction catalyzed by phosphoglucose isomerase is a simple isomerization between an aldose and a ketose and involves the open-chain structures of both sugars. Since glucose 6-phosphate and fructose 6-phosphate are both reducing sugars, their Haworth ring structures are in equilibrium with their open-chain forms. This equilibration is very rapid and does not require an additional enzyme. Note that this isomerization reaction is of the same type as that catalyzed by triosephosphate isomerase. 

Sorbitol-6-phosphate is a competitive inhibitor of phosphoglucose isomerase. In contrast, 1,5-sorbitan-6-phosphate has no effect on the enzyme activity, which suggests that the phosphohexose isomerase substrate must be present in the open form. Thus, before a sugar such as glucose-6-phosphate can be acted upon by phosphoglucose isomerase, an opening of the ring is imperative. The mechanism by which such a molecular ransformation occurs, however, remains obscure [43]. 

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