Phosphotriose

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

This enzyme [EC 5.3.1.1], also known as triosephosphate mutase (TIM) and phosphotriose isomerase, catalyzes the interconversion of D-glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (fUPAC glycerone phosphate). As pointed out by Rose, this enzyme is chiefly responsible for the largely symmetrical conversion of the two three-carbon segments of glucose to lactate and for the nearly uniform distribution of from pyruvate in the glucosyl units of hver glycogen. The... 

Phosphotriose Isomerase. Early work on aldolase was made difficult by the presence of phosphotriose isomerase. This enzyme is usually present in much larger effective concentrations than aldolase, and catalyzes the interconversion of phosphoglyceraldehyde and dihydroxyacetone phosphate. Since the equilibrium of this reaction lies to the... 

The formation of phosphotriose, the substrate for glycolysis, takes place by the splitting, in the presence of aldolase, of F—1,6—PP into a mixture of two triosephosphates. Equilibrium between the hexose-phosphate and the mixture of triosephosphates is established in the cell. 

One more point the phosphotriose dehydrogenase supplies two NADH + H+ per molecule of glucose. The fate of this reduced NAD+ is varied. One possibility is for it to be utilized in the respiratory chain for the formation of more ATP. More about that later. The NADH + H+furnished by glycolysis can, however, also be used in fermentation processes. 

In 1934 it was shown that yeast and mammalian muscle contain an enz3rme aldolase) which splits this fructose-l-6-diphosphate between carbon atoms 3 and 4 to give triose phosphates (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate). The same enzyme was, by 1949, shown to be universally distributed in higher plants. Further, the two isomeric triosephosphates formed are interconvertible by means of a second enzyme, phosphotriose isomerase. This is important because it is only the glyceraldehyde-3-phosphate which is normally further metabolised in respiration. 

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