Dihydroxyacetone phosphate tautomerism

Reduction of the acyl phosphate gives glyceraldehyde 3-phosphate, which Q undergoes keto-enol tautomerization to yield dihydroxyacetone phosphate. 

Tautomerism occurs elsewhere in the glycolytic pathway (see Section 15.2). The transformation of glyceraldehyde 3-phosphate into dihydroxyacetone phosphate involves two such keto-enol tautomerisms, and... 

Both the aldol and reverse aldol reactions are encountered in carbohydrate metabolic pathways in biochemistry (see Chapter 15). In fact, one reversible transformation can be utilized in either carbohydrate biosynthesis or carbohydrate degradation, according to a cell s particular requirement. o-Fructose 1,6-diphosphate is produced during carbohydrate biosynthesis by an aldol reaction between dihydroxyacetone phosphate, which acts as the enolate anion nucleophile, and o-glyceraldehyde 3-phosphate, which acts as the carbonyl electrophile these two starting materials are also interconvertible through keto-enol tautomerism, as seen earlier (see Section 10.1). The biosynthetic reaction may be simplihed mechanistically as a standard mixed aldol reaction, where the nature of the substrates and their mode of coupling are dictated by the enzyme. The enzyme is actually called aldolase. 

Step 5 is an isomerization of dihydroxyacetone phosphate to glyceraldehyde 3-phosphate that occurs by keto-enol tautomerization. 

Phosphorylation, reduction, and tautomerization. Reaction of 3-phos-phoglycerate with ATP generates the corresponding acyl phosphate, which is reduced by NADH/H to an aldehyde, Keto-eno] tautomerization of the aldehyde gives dihydroxyacetone phosphate, the same reaction as step 5 of glycolysis (Figure 29.4). 

STEPS 6-7 Reduction yields glyceratdchyde S-phosphate, which undergoes keto-enol tautomerization to give dihydroxyacetone phosphate. 

Tautomerism is important in the chemistry and metabohsm of carbohydrates (Chapter 26). For example, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are two intermediates in glycolysis, a metabohc pathway in almost all organisms. 

Scheme 11.5. A cartoon representation of the catalyzed (fructose-bisphosphate aldolase, EC 4.1.2.13) aldol-type condensation between glyceraldehyde 3-phosphate and dihydroxyacetone monophosphate to produce the six-carbon ketosugar fructose-1,6-bisphosphate. An active site lysine Lys-NH2 [" H3NCH2CH2CH2CH2CH(NH3 )C02 ] apparently serves as the catalyst through addition at the carbonyl followed by proton tautomerization.

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