Isomerisation glyceraldehydes

The isomerisation of aldoses and ketoses by hydride transfer, although suggested for enzymic isomerisations of aldoses and ketoses (rather than their phosphates), and having precedent in the metal ion-catalysed isomerisations of ot-hydroxy ketones, was considered to be the less common route until the paradigmal aldose-ketose isomerisation, that of glyceraldehyde to dihydrox-yacetone, was examined by NMR spectroscopy. Identification and analysis of all products of the reaction in D2O permitted concentrations to be chosen which minimised side-reactions. A useful feature of the system was that it was possible to integrate the areas of proton resonances in the CHD and CH2... 

D-glyceraldehyde (Z)-l-ene-l,2-diol anion L-glyceraldehyde Figure 4.47 Isomerisation of trioses. 

Isomerisation of pentoses, tetroses, trioses as weU as isomerisation of disaccharides takes place analogously. For example, D-ribose produces ketose D-ribulose as the major product and a smaller amount of aldose D-arabinose, glyceraldehyde yields ketose 1,3-dihydroxyacetone (Figure 4.47), the major product of disaccharide lactose is lactulose, which is accompanied by smaller amount of epilactose. 

ISOMERISATION OF DIHYDROXYACETONE PHOSPHATE TO GLYCERALDEHYDE 3-PHOSPHATE CATALYSED BY TRIOSE-PHOSPHATE... 

A representative example of how enzymes stabilise reactive intermediates is the loss of a proton from the a-position of a carbonyl compound, a rate-limiting step in carbon-carbon bond formations and racemisations, which proceeds via an unstable enol or enolate intermediate. A particularly interesting illustrative case is that of triose-phosphate isomerase (TM) and how it catalyses the isomerisation between dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP) (Figure 14.11). The uphill direction from DHAP to GAP is essential for optimal throughput in the glycolytic pathway, since only GAP can proceed further along this pathway. Thus, the TIM reaction ensures efficient... 

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