Glucose in glycolysis

We saw in Chapter 14 that the energy yield from the production of two molecules of pyruvate from one molecule of glucose in glycolysis is 2 ATP and 2 NADH. In oxidative phosphorylation (Chapter 19), passage of two electrons from NADH to 02 drives the formation of about 2.5 ATP, and passage of two electrons from FADH2 to 02 yields about 1.5 ATP. This stoichiometry allows us to calculate the overall yield of ATP from the complete... 

Figure 13.12 Routes for utilizing substrates other than glucose in glycolysis. 

There were also two ATP produced per glucose in glycolysis and two NADH, which will give rise to another hve ATP (seven more ATP total). In the next chapter, we shall say more about the subject of ATP production from the complete oxidation of glucose. 

Figure 14.24 NAD, as the coenzyme in glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is used to oxidize glyceraldehyde-3-phosphate (GAP) to 1,3-bisphosphoglycerate during the degradation of glucose in glycolysis. One of the ways that NADH can be reoxidized to NAD is by the electron transport chain in mitochondria, where, under aerobic conditions, rearomatization of NADH helps to drive ATP synthesis.

FIGURE 14.2 The breakdown of glucose by glycolysis provides a prime example of a metabolic pathway. Ten enzymes mediate the reactions of glycolysis. Enzyme A, fructose 1,6, hiphos-phate aldolase, catalyzes the C—C bondbreaking reaction in this pathway. 

Because the 2 NADH formed in glycolysis are transported by the glycerol phosphate shuttle in this case, they each yield only 1.5 ATP, as already described. On the other hand, if these 2 NADH take part in the malate-aspartate shuttle, each yields 2.5 ATP, giving a total (in this case) of 32 ATP formed per glucose oxidized. Most of the ATP—26 out of 30 or 28 out of 32—is produced by oxidative phosphorylation only 4 ATP molecules result from direct synthesis during glycolysis and the TCA cycle. 

The next steps of glucose catabolism are called the citric acid cycle. The pyruvic acid formed in glycolysis is transported into the mitochondria, which arc subcellular organelles with double (inner and outer) membranes. They are referred to as the powerhous-... 

Figure 29.8 Mechanism of step 2 in glycolysis, the isomerization of glucose 6-phosphate to fructose 6-phosphate.

Following hydrolysis, keto-enol tautomerization of the carbonyl group from C2 to Cl gives glucose 6-phosphate. The isomerization is the reverse of step 2 in glycolysis. 

It may be identified as a nonequilibrium reaction in which the of the enzyme is considerably lower than the normal substrate concentration. The first reaction in glycolysis, catalyzed by hexokinase (Figure 17-2), is such a flux-generating step because its for glucose of 0.05 mmol/L is well below the normal blood glucose concentration of 5 mmol/L. 

Glucose 6-phosphate is an important compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis, the pentose phosphate pathway, glycogenosis, and glycogenolysis). In glycolysis, it is converted to fructose 6-phosphate by phosphohexose-isomerase, which involves an aldose-ketose isomerization. 

Although glucose 6-phosphate is common to both pathways, the pentose phosphate pathway is markedly different from glycolysis. Oxidation utilizes NADP rather than NAD, and CO2, which is not produced in glycolysis, is a characteristic product. No ATP is generated in the pentose phosphate pathway, whereas ATP is a major product of glycolysis. 

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