Pyruvate glyceraldehyde-3-phosphate converted

The reduction of pyruvate to lactate converts NADH to NAD+, which is required in the glyceraldehyde 3-phosphate dehydrogenase reaction. This prevents glycolysis from stopping owing to too low a concentration of NAD+ and allows continued production of ATP. 

This molecular complexity is intriguing and must have a physiological meaning. Kaplan and his associates have proposed the following explanation. In skeletal muscle the main function of LDH is to convert pyruvate to lactate for the purpose of oxidizing the NADH generated by glyceraldehyde phosphate oxida-... 

IPP, a Cs-compound, is the source of carotenoids, isoprenoids, terpenes, quinones, and phytol of BChls and Chls. There are two known independent pathways of IPP synthesis the classical mevalonate (MVA) pathway and the alternative, non-mevalonate, l-deoxy-D-xylulose-5-phosphate (DOXP) pathway [19, 20]. In the MVA pathway, acetyl-coenzyme A is converted to IPP through mevalonate. The enzymes and genes involved in this pathway are well studied [21]. The DOXP pathway was found in the 1990s, and in this pathway, pyruvate and glyceraldehyde are converted to IPP. 

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).

E. In the second phase of glycolysis, two glyceraldehyde 3-phosphate molecules from glucose are converted to pyruvate in conjunction with several important energy-generating reactions (Figure 6—1). 

Outline in detail, using structural formulas, the enzyme-catalyzed reactions by which cells in the human body convert glyceraldehyde 3-phosphate into pyruvate. 

Describe the reaction steps in gluconeogenesis by which pyruvate is converted into glyceraldehyde 3-phosphate. 

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. 

The oxidative pentose phosphate cycle is often presented as a means for complete oxidation of hexoses to C02. For this to happen the C3 unit indicated as the product in Fig. 17-8A must be converted (through the action of aldolase, a phosphatase, and hexose phosphate isomerase) back to one-half of a molecule of glucose-6-P which can enter the cycle at the beginning. On the other hand, alternative ways of degrading the C3 product glyceraldehyde-P are available. For example, using glycolytic enzymes, it can be oxidized to pyruvate and to C02 via the citric acid cycle. 

Starting with the simple compounds acetyl-CoA, glyceraldehyde-3-phosphate, and pyruvate, which arise via the central pathways of metabolism, the key intermediate isopentenyl diphosphate is formed by two independent mutes. It is then converted by bacteria, fungi, plants, and animals into thousands of different naturally occurring products. These include high polymers, such as rubber, as well as vitamins, sterols, carotenoids, and over 30,000 different terpenes and related compounds. Many of the latter are found only in specific plants where they may function as defensive compounds or pheromones. 

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). 

Fig. 8.2 Glycolysis and related pathways. Glycolysis is a central metabolic machinery in which one mole of glucose is catabolized to two moles of pyruvate, NADH, and ATP. Under aerobic conditions, pyruvate is further oxidized by mitochondrial system. In erythrocytes DHAP is a dead-end product however, in brain it can be converted into direction of lipid synthesis. Glycolysis and the pentose phosphate pathway (pentosePP) are interconnected via fructose-6-P and glyceral-dehyde-3-P. A high level of NADPH favors lipid synthesis via pentose phosphate shunt (pentosePP). At TPI inhibition (TPI deficiency), glyceraldehyde-3-Pcan be produced via G6PDH as well, to contribute to the glycolytic flux. a-GDH catalyzes the...

There is no overall change in the oxidation state when glucose is converted to lactate, because glyceraldehyde-3-phosphate dehydrogenase oxidizes glyceraldehyde 3-phosphate to 1,3-bisphos-phoglycerate, but lactate dehydrogenase reduces pyruvate to lactate. These two reactions also reduce NAD+ to NADH, then reoxidize the NADH to NAD+. 

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. 

Mode 4. Both NADPH and ATP are required. Alternatively, ribose 5-phosphate formed by the oxidative phase of the pentose phosphate pathway can be converted into pyruvate. Fructose 6-phosphate and glyceraldehyde 3-phosphate derived from ribose 5-phosphate enter the glycolytic pathway rather than reverting to glucose 6-phosphate. In this mode, ATP and NADPH are concomitantly generated, and five of the six carbons of glucose 6-phosphate emerge in pyruvate. 

As shown in Figure 4-23, G-6-P can materialize within a tissue from two other sources. The G-l-P released during breakdown of glycogen, can be converted to G-6-P It can be produced via the gluconeogenic pathway involving conversion of pyruvate to OAA, to FEF to glyceraldehyde-3-phosphate, and finally to G-6-P. 

---