2.3- bisphosphoglycerate formation

FIGURE 16.10 Formation of a covalent intermediate in the glyceraldehyde-3-phos-phate dehydrogenase reaction. Nucleophilic attack by a cysteine —SH group forms a covalent acylcysteine intermediate. Following hydride transfer to NAD, nucleophilic attack by phosphate yields the product, 1,3-bisphosphoglycerate. 

In erythrocytes, the first site in glycolysis for generation of ATP may be bypassed, leading to the formation of 2,3-bisphosphoglycerate, which is important in decreasing the affinity of hemoglobin for Oj. 

The formation of 1,3-bisphosphoglycerate involves the synthesis of a high-energy phosphate bond as the aldehyde of glyceraldehyde 3-phosphate is oxidized to a carboxylic acid and then phosphorylated by reaction with inorganic phosphate. 

Formation of 1,3-bisphosphoglycerate is coupled to reduction of NAD by transfer of two electrons and a proton to form NADH + H. ... 

Hemoglobin also binds H+ and C02, resulting in the formation of ion pairs that stabilize the T state and lessen the protein s affinity for 02 (the Bohr effect). Oxygen binding to hemoglobin is also modulated by 2,3-bisphosphoglycerate, which binds to and stabilizes the T state. 

The outcome of these coupled reactions, both reversible under cellular conditions, is that the energy released on oxidation of an aldehyde to a carboxylate group is conserved by the coupled formation of ATP from ADP and Pj. The formation of ATP by phosphoryl group transfer from a substrate such as 1,3-bisphosphoglycerate is referred to as a substrate-level phosphorylation, to distinguish this mechanism from respiration-linked phosphorylation. Substrate-level phosphorylations involve soluble enzymes and chemical intermediates (1,3-bisphosphoglycerate in this case). Respiration-linked phosphorylations, on the other hand, involve membrane -bound enzymes and transmembrane gradients of protons (Chapter 19). 

A requirement for all fermentations is the existence of a mechanism for coupling ATP synthesis to the fermentation reactions. In the lactic acid and ethanol fermentations this coupling mechanism consists of the formation of the intermediate 1,3-bisphosphoglycerate by the glyceraldehyde 3-phosphate dehydrogenase (Fig. 10-3, step a). This intermediate contains parts of both the products ATP and lactate or ethanol. 

Carreras, J., F. Climent, R. Bartrons, and G. Pons. 1982. Effect of vanadate on the formation and stability of the phosphoenzyme forms of 2,3-bisphosphoglycerate-dependent phosphoglycerate mutase and of phosphoglucomutase. Biochim. Biophysic. Acta 705 238-242. 

Isomerization of 3-phosphoglycerate then gives 2-phosphoglycerate in a step catalyzed by the enzyme phosphoglycerate mutase. The process involves formation of a 2,3-bisphosphoglycerate intermediate, followed by selective transfer of the 3-phosphate. 

The pathway involved is glycolysis, the conversion of glucose to pyruvate. The initial step is phosphorylation of glucose by the enzyme hexokinase and the final step is formation of pyruvate from phosphoenolpyruvate by pyruvate kinase (Chapter 11). The BPG is made from an intermediate compound, 1,3-bisphosphoglycerate, by the enzyme bisphosphoglycerate mutase. 

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