Glycoaldehyde, active

In the well known "transketolase reaction" [9] for instance, the transfer of the fragment H0CH2-C=0 from a hexose to a triose takes place via the "active glycoaldehyde" (Scheme 5.8) ... 

Figure 20.21. Transketolase Mechanism. The carbanion of thiamine pyrophosphate (TPP) attacks the ketose substrate. Cleavage of a carbon-carbon bond frees the aldose product and leaves a two-carbon fragment joined to TPP. This activated glycoaldehyde intermediate attacks the aldose substrate to form a new carbon-carbon bond. The ketose product is released, freeing the TPP for the next reaction cycle.

This enzyme catalyzes the reversible transfer of the hydroxyketo group of a ketose phosphate to an aldose phosphate. The cofactor thiamine pyrophosphate (TPP) is associated with the enzyme and activates the ketose (Scheme 7). Most known donor ketoses (xylulose 5-phosphate, sedoheptulose 7-phosphate, fructose 6-phosphate, L-erythrose) have a trans arrangement of hydroxy groups at C-3 and C-4 hydroxypyruvate is an exception. A range of aldehydes (such as o-glyceraldehyde 3-phosphate, D-ribose 5-phosphate, o-erythrose 4-phosphate, glycoaldehyde) are acceptors. Transketolase has been... 

This activated glycoaldehyde intermediate attacks the aldose substrate to form a new carbon-carbon bond. The ketose product is released, freeing the TPP for the next reaction cycle. 

The analogy between the formation of active acetate from pyruvate and active glycoaldehyde from hydroxypyruvate is striking. In both instances diphosphothiamine plays an important role and the active intermediary is linked to the enzyme. The possibility that lipoic acid plays some part in the formation of active glycolaldehyde has yet to be considered experimentally. 

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