3- Phospho-D-glyceraldehyde

Scheme 113. A cartoon representation of the rednction of 1,3-bisphospho-D-glycerate to 3-phospho-D-glyceraldehyde. The rednction is accompanied by the oxidation of NADPH to NADPh...

With the formation of glyceraldehyde 3-phosphate (3-phospho-D-glyceraldehyde), the rebuilding of ribulose-1,5-diphosphate to continue turning the cycle (the Calvin or RPP cycle) by combining fragments continues in earnest. But it is important to note that the formation of glyceraldehyde 3-phosphate cannot be underestimated. 

Phospho-D-glyceraldehyde is isomerized by triose phosphate isomerase to dihydroxyacetone phosphate (reaction 6). The two trioses, 3-phospho-D-glycer-aldehyde and dihydroxyacetone phosphate, are then condensed together in a aldol-type condensation, catalyzed by aldolase to give D-fructose-l,6-bisphos-phate (reaction 7). This is a key intermediate in the photosynthetic process for the formation of many other carbohydrates, including D-glucose (see Fig. 10.5). 

Isomerases catalyze the isomerization of one compound into another. There are many important isomerization reactions in the metabolism of carbohydrates. D-Glucose-6-phosphate is converted into D-fructose-6-phosphate by phosphoglu-coisomerase. Dihydroxyacetone phosphate is converted into 3-phospho-D-glyceraldehyde by the enzyme triose phosphate isomerase. In the Calvin cycle of photosynthesis, this same enzyme converts 3-phospho-D-glyceraldehyde into dihydroxyacetone phosphate. 

Glyceraldehyde-3-phosphate dehydrogenase (NADP+) (nonphosphorylating) [EC 1.2.1.9], also referred to as triose-phosphate dehydrogenase, catalyzes the reaction of D-glyceraldehyde 3-phosphate with NADP+ and water to produce 3-phospho-D-glycerate and NADPH. 

D-glyceraldehyde 3-phosphate + phosphate + NAD(+) = 3-phospho-D-glyceroyl phosphate + NADH. 

Phospho-2-keto-3-deoxygluconate aldolase, an enzyme in the Entner-Doudoroff pathway, that catalyzes the cleavage 6-phospho-2-keto-3-deoxy-D-gluconate to form pyruvate and D-glyceraldehyde 3-phosphate. 

This enzyme [EC 4.1.2.4], also referred to as phospho-deoxyriboaldolase and deoxyriboaldolase, catalyzes the reversible conversion of 2-deoxy-D-ribose 5-phosphate to D-glyceraldehyde 3-phosphate and acetaldehyde. 

The lower-carbon, phosphorylated sugars, so important in biochemical processes, also came in for the further attention of Fischer and Ballou. New methods of synthesis were worked out for 2-0-phospho-o-glyceric acid, D-glyceraldehyde 3-phosphate, dihydroxyacetone phosphate, and (with Dr. MacDonald) the enantiomorphous erythrose 4-phosphates. The suspected biochemical importance of the n-erythrose 4-phosphate was then quickly established when Srinivasan, Katagiri, and Sprinson of Columbia University demonstrated its condensation with 0-phospho-enolpyruvic acid to 5-dehydroshikimic acid by Escherichia coli. 

Triose-phcfiphate isomerase (TIM) (EC 5.3.1.1) and phosphoglycerate kinase (PGK) (EC 2.7.2.3) are two ubiquitous enzymes in the major pathways of carbohydrate metabolism (glycolysis, gluconeogenesis, and pentose phosphate pathway), catalyzing the interconversion of dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate, and the phospho-group transfer between... 

Many of the simple trioses, tetroses, and pentoses do not occur naturally in the free state but are commonly found as phosphate-ester derivatives. The phospho-esters are important intermediates in the breakdown and synthesis of carbohydrates by living organisms. D-Glucose is converted into D-fructose-l,6-bisphos-phate that is then cleaved in half to give D-glyceraldehyde-3-phosphate and dihydroxy acetone phosphate (see Chapter 11). D-Erythrose is found as the 4-phosphate in the pentose-phosphate pathway of carbohydrate degradation and in the photosynthetic process. D-Ribose-5-phosphate, D-ribulose-5-phosphate, D-xy-lose-5-phosphate, and D-xylulose-5-phosphate are found in the pentose phosphate pathway as well as in the photosynthetic pathway (see Chapter 10). D-Ribulose-1,5-bisphosphate is the direct intermediate to which CO2 is added in the photosynthetic pathway. D-Ribose-5-phosphate also is the precursor of RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). See Fig. 1.7 for the structures of these common sugar phosphates. 

CH2O D-glycemldehyde iH20 3-phospho- dehydrogenase D-glyceraldehyde... 

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