Aldose-ketose interconversions

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

Aldose-ketose interconversions 692 Aldose reductase 774 Algae 20-22 Algin 21... 

In the first example the actual chemistry is identical an aldose-ketose interconversion. Note that the geometry of the alcohol on C-2 is identical. The second example has essentially the same chemistry. Logically, to do the same chemistry, nature could have stumbled upon the same mechanism twice. 

C. A. Collyer, K. Henrick, and D. M. Blow, Mechanism for aldose-ketose interconversion by D-xylose isomerase involving ring opening followed by a 1,2-hydride shift, J.Mol.Bid. 1990, 212, 211-235. 

Aldose-ketose interconversions, e.g. glucose 6-phosphate to fructose 6-phosphate (Figure 8.4), also proceed through a common enol intermediate. 

In addition to aldose-ketose interconversions by hydride and by proton transfer, a number of metal-catalysed reactions involving interconversion of... 

Figure 9.4 Aldose-ketose interconversion via an enediol intermediate. 

So a basic solution of either one rapidly becomes a mixture of the two, and the reactions in (a) can then occur. This aldose ketose interconversion is general. Glucose and fructose are interconverted by aqueous base, for example. We saw this same mechanism in Problem 41. 

Isomerization of carbohydrates in alkaline media, considered to embrace both epimerization of aldoses and ketoses and aldose-ketose interconversion ... 

Many of the mechanistic aspects of glucose isomerase catalysed aldose-ketose interconversion have been under discussion for some time and are still not fully understood. By comparison with triose phosphate isomerase (TIM, EC 5.3.1.1) and glucose 6-phosphate isomerase (EC 5.3.1.9), the base-catalysed formation of an 1,2-enediol was invoked as the key step of the epimerisation based on the work of Rose and co-workers with tritium-labelled substrates [26]. An unexplained featme of the epimerisation process was that in contrast to isomerisations with triose phosphate isomerase no proton exchange with the medium could be observed with D-xylose isomerase, a fact that was attributed to the phosphate group of the former as a mediator for the exchange process [26]. Subsequently, additional important differences between triose phosphate isomerase and xylose isomerase were recognised. For example, D-xylose isomerase is appar-endy a very slow enzyme catalysing about five molecules per second per active site with an absolute requirement for divalent cations, while TIM does not need co-factors and operates at nearly 1000-fold the speed of D-xylose isomerase at... 

Figure 6 Schematic reaction mechanism of the ring-opening step in the aldose-ketose interconversion catalyzed by D-xylose isomerase ...

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