Isomerisation of Reducing Sugars by Hydride Shift

Non-enzymic aldose-ketose isomerisations that are acid catalysed appear to involve a 1,2-hydride shift. During acid-catalysed rearrangement of glucose to fructose, the label of [2- H]glucose substrate is retained in the [l- H]fructose product, distributed equally between the proR and proS positions. In the reverse sense retention of the label of tritiated fructose in the glucose and mannose products was not complete. Similar observations were made for the xylose-xylulose interconversion. With an appropriate sugar configuration (ribose), even the base-catalysed reaction proceeds partly with retention of label, presumably by the same mechanism as with trioses. 

The enzymes were originally xylose rather than glucose isomerases, the specificity being reflected more in (1-3 mM for xylose, 150-400 mM for 

Rhamnose isomerase of Escherichia coli appears from the structure of its ligand complexes to have a similar mechanism to glucose isomerase.  

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