1,3-Bisphosphoglycerate oxidation

Glyceraldehyde 3-phosphate is oxidized to a carboxylic acid and then phosphorylated to yield 1,3-bisphosphoglycerate. 

Like all anhydrides (Section 21.5), the mixed carboxylic-phosphoric anhydride is a reactive substrate in nucleophilic acyl (or phosphoryl) substitution reactions. Reaction of 1,3-bisphosphoglycerate with ADR occurs in step 7 by substitution on phosphorus, resulting in transfer of a phosphate group to ADP and giving ATP plus 3-phosphoglycerate. The process is catalyzed by phospho-gjvcerate kinase and requires Mg2+ as cofactor. Together, steps 6 and 7 accomplish the oxidation of an aldehyde to a carboxylic acid. 

Figure 29.10 Mechanism of Step 6 in Figure 29.7, the oxidation and phosphorylation of glyceraldehyde 3-phosphate to give 1,3-bisphosphoglycerate.

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. 

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). 

Arsenate will replace phosphate in all phospho-rolytic reactions, e.g., in the cleavage of glycogen by glycogen phosphorylase, of sucrose by sucrose phosphorylase, and in the action of purine nucleoside phosphorylase.b Glucose 1-arsenate or ribose-1-arsenate is presumably a transient intermediate which is hydrolyzed to glucose. The overall process is called arsenolysis. Another reaction in which arsenate can replace phosphate is the oxidation of glyceraldehyde 3-phosphate in the presence of P to form 1,3-bisphosphoglycerate ... 

This requires the generation of six separate molecules of ribulose bisphosphate from ribulose phosphate, at the cost of one ATP each. Furthermore, two molecules of 1,3-bisphosphoglycerate must be made from the two 3-phosphoglycerates that are the initial product of each C02 fixation reaction. Conversion of each 1,3-bisphospho-glycerate requires an NADPH as well therefore, two NADPH equivalents are consumed for each C02 fixed. Another way of saying this is that carbon is reduced from an oxidation number of -4 in C02 to an oxidation number of zero in carbohydrate (CH20). Therefore, synthesis of one mole of glucose requires the input of 18 ATPs and 12 NADPHs. 

Step 6 involves the oxidation of D-glyceraldehyde 3-phosphate, accompanied by phosphorylation of the intermediate carboxylic acid, to produce D-l,3-bisphosphoglycerate. The enzyme is glyceral-dehyde-3-phosphate dehydrogenase. 

In glycolysis, ADP is phosphorylated to ATP during the oxidation of glyceraldehyde 3-phosphate to 3-phosphoglycerate. The phosphorylated intermediate that receives the energy of the oxidation is 1,3-bisphosphoglycerate. 

Step 6 begins with a nucleophilic addition reaction to the aldehyde group of glyceraldehyde 3-phosphate by a thiol group of an enzyme to form a hemithioacetal, which is oxidized by NAD+ to an acyl thioester. Nucleophilic acyl substitution by phosphate yields the product 1,3-bisphosphoglycerate. 

However, the oxidation does not take place directly. Instead, the carbon oxidation generates an acyl phosphate, 1,3-bisphosphoglycerate. The electrons released are captured by NAD+, which we will consider shortly. 

Glycolysis proceeds in the presence of arsenate, but the ATP normally formed in the conversion of 1,3-bisphosphoglycerate into 3-phosphoglycerate is lost. Thus, arsenate uncouples oxidation and phosphorylation by forming a highly labile acyl arsenate. 

A separate enzyme then transfers the phospho group from the 1 position of 1,3-bisphosphoglycerate to ADP to form ATP and 3-phosphoglycerate (step d). The overall sequence of Fig. 15-6 is the synthesis of one mole of ATP coupled to the oxidation of an aldehyde... 

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