Glucose glycolytic cycle

Glucose is not the only hexose used for glycolysis— fructose, mannose, and galactose can also enter the glycolytic cycle after phosphorylation. Like glucose, fructose can be used only after phosphorylation in one of three ways [33] (1) phosphorylation to fructose-6-phosphate by hexokinase, (2) phosphorylation to fructose-6-phosphate by a specific fructokinase, and (3) phosphorylation to fructose-1-phosphate by fructokinase (Fig. 1-7). It is well established that the glu-cokinase of liver and muscle can also phosphorylate fructose. Fructose can enter muscle metabolism only in the form of fructose-6-phosphate. This is strikingly different from liver metabolism in which fructose is converted to fructose-1-phosphate by a specific fructokinase. 

ATP Synthesis. Inasmuch as the first steps in glucose metabolism start with the phosphorylation of hexoses in the presence of ATP, a direct effect of insulin on the formation of the high-energy phosphates could explain the effect of the hormone on glucose utilization, and an impairment of the rate of ATP synthesis could be responsible for the biochemical lesions of diabetes. Without ATP, glucose cannot enter the glycolytic cycle. Insulin could affect phosphorus metab-... 

There exists in animal and plant tissues, as well as in some bacteria, a system of enzymes which oxidatively converts glucose to tetrosephos-phate, triosephosphate, and C02 by reactions other than those involved in the Embden-Meyerhof glycolytic cycle. Warburg, Dickens, Horecker, S. S. Cohen,Dische, and Racker have made important contributions to our knowledge of this reaction sequence, which appears to provide the starting materials for the synthesis of ribose, deoxyribose, and their corresponding nucleotides. For ease of presentation the reactions may be subdivided as follows ... 

Two and twelve moles of ATP are produced, respectively, per mole of glucose consumed in the glycolytic pathway and each turn of the Krebs (citrate) cycle. In fat metaboHsm, many high energy bonds are produced per mole of fatty ester oxidized. Eor example, 129 high energy phosphate bonds are produced per mole of palmitate. Oxidative phosphorylation has a remarkable 75% efficiency. Three moles of ATP are utilized per transfer of two electrons, compared to the theoretical four. The process occurs via a series of reactions involving flavoproteins, quinones such as coenzyme Q, and cytochromes. 

The glycolytic pathway described in this chapter begins with the breakdown of glucose, but other sugars, both simple and complex, can enter the cycle if they can be converted by appropriate enzymes to one of the intermediates of glycolysis. Figure 19.32 shows the mechanisms by which several simple metabolites can enter the glycolytic pathway. Fructose, for example, which is pro-... 

By combining the glycolytic pathway, the Krebs cycle, and oxidative phosphorylation, the energy yield from the aerobic degradation of glucose will be... 

C]Glucose Catabolism An actively respiring bacterial culture is briefly incubated with [1-14C] glucose, and the glycolytic and citric acid cycle intermediates are isolated. Where is the 14C in each of the intermediates listed below Consider only the initial incorporation of 14C, in the first pass of labeled glucose through the pathways. 

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. 

The glycolytic pathway from glucose to pyruvate, indicating two anaerobic options (ethanol or lactate) and one aerobic option (TCA cycle). The red arrow indicates how NADH formed in glycolysis... 

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