Glyceraldehyde-3-Phosphate Is Converted to Pyruvate

In the second phase of glycolysis, glyceraldehyde-3-phosphate is converted to pyruvate. 

These reactions yield four molecules of ATP, two for each molecule of pyruvate produced. 

What reactions convert glyceraldehyde-3-phosphate to pyruvate  

This reaction, the characteristic reaction of glycolysis, should he looked at more closely. It involves the addition of a phosphate group to glyceraldehyde-3-phosphate as well as an electron-transfer reaction, from glyceraldehyde-3-phosphate to NAD+. We will simplify the discussion by considering the two parts separately. 

The half reaction of oxidation is that of an aldehyde to a carboxylic acid group, in which water can be considered to take part in the reaction. 

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Figure 11.2 Pathway for conversion of fructose to acetyl-CoA. The enzyme fructokinase phosphorylates fructose to form fructose 1-phosphate. (The enzyme is present only in the liver.) Fructose 1-phosphate is cleaved by aldolase to form glyceraldehyde and dihydroxyacetone phosphate. Glyceraldehyde is phos-phorylated to form glyceraldehyde 3-phosphate, catalysed by the enzyme triokinase. Dihydroxyacetone phosphate is converted to glyceraldehyde 3-phosphate, catalysed by the isomerase. Glyceraldehyde 3-phosphate is converted to pyruvate by the glycolytic reactions (Chapter 6).

Glyceraldehyde-3-phosphate is converted to pyruvate. Four ATP molecules and two NADH are produced. Because two ATP were consumed in stage 1, the net production of ATP per glucose molecule is 2. 

All can yield four molecules of ATP. Sucrose is converted into one molecule each of glucose and fructose each of these sugars requires two molecules of ATP to reach the stage of two molecules of glyceraldehyde 3-phosphate. From here to pyruvate, each glyceraldehyde 3-phosphate molecule yields two molecules of ATP by substrate-level phosphorylation of ADP. Thus, one molecule of glucose or of fructose generates two molecules of ATP. 

A. Dihydroxyacetone phosphate (compound I) is isomerized to glyceraldehyde 3-phosphate and converted in a series of steps to pyruvate (compound II). One of the reactions requires glyceraldehyde 3-phosphate dehydrogenase, which utilizes inorganic phosphate and produces one NADH. In the conversion of one 1,3-bisphosphoglycerate to one pyruvate, two ATP are produced. A mutase is not required. 

In the cytosol of eukaryotic cells, glucose is converted to pyruvate via the glycolytic pathway, with the net formation of two ATPs and the net reduction of two NAD molecules to NADH (see Figure 8-4). ATP is formed by two substrate-level phosphorylation reactions in the conversion of glyceraldehyde 3-phosphate to pyruvate. 

In the next step, FDP is broken into two three-carbon units, dihydroxyacetone phosphate (1,3-dihydroxypropanone phosphate, CH20HC0CH20P03, 5) and glyceraldehyde-3-phosphate (6), which exist in mutual equilibrium. Only the glyceraldehyde-3-phosphate is oxidized by NAD to pyruvate ion, with formation of two ATP molecules. As glycolysis proceeds, all the dihydroxyacetone phosphate is converted to glyceraldehyde-3-phosphate, so the result is the consumption of two NAD molecules and the formation of four ATP molecules per molecule of glucose. 

Fructose bisphosphate is cleaved by action of an aldolase (reaction 4) to give glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. These two triose phosphates are then equilibrated by triose phosphate isomerase (reaction 5 see also Chapter 13). As a result, both halves of the hexose can be metabolized further via glyceraldehyde 3-P to pyruvate. The oxidation of glyceraldehyde 3-P to the corresponding carboxylic acid, 3-phosphoglyceric acid (Fig. 17-7, reactions 6 and 7), is coupled to synthesis of a molecule of ATP from ADP and P . This means that two molecules of ATP are formed per hexose cleaved, and that two molecules of NAD+ are converted to NADH in the process. 

In a certain group of bacteria, still another pathway (Entner-Doudomff pathway) for the utilization of glucose has been studied. Here glucosc-6-phosphate is oxidized to 6-phosphogluconic acid which is dehydrated to 2-keto-3-deoxy-6-phusphogluconic acid. This substance is then split to pyruvic acid and glyceraldehyde-3-phosphatc l which alsu can be converted to pyruvic acid). 

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