Chapter18SUMMARY - @va Nearly every living cell carries out...

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Unformatted text preview: @va Nearly every living cell carries out a catabolic process known as glycolysis— the stepwise degradation of glucose (and other simple sugars). Glycolysis is a paradigm of metabolic pathways. Carried out in the cytosol of cells, it is basically an anaerobic process; its principal steps occur with no require- ment for oxygen. 18.1 What Are the Essential Features of Glycolysis? Glycoly- sis consists of two phases. In the first phase, a series of five reactions, glu- cose is broken down to two molecules of glyceraldehyde—3-phosphate. In the second phase, five subsequent reactions convert these two mole- cules of glyceraldehyde-B-phosphate into two molecules of pyruvate. Phase 1 consumes two molecules of ATP (Figure 18.2). The later stages of glycolysis result in the production of four molecules of ATP. The net is 4 — 2 = 2 molecules of ATP produced per molecule of glucose. 18.2 Why Are Coupled Reactions Important in Glycolysis? Coupled reactions permit the energy of glycolysis to be used for gener- ation of ATP. Conversion of one molecule of glucose to pyruvate in gly- colysis drives the production of two molecules of ATP. 18.3 What Are the Chemical Principles and Features of the First Phase of Glycolysis? In the first phase of glycolysis, glucose is converted into two molecules of glyceraldehyde—3-phosphate. Glu- cose is phosphorylated to glucose-6.1), which is isomerized to fructose- 6-P. Another phosphorylation and then cleavage yields two 3-carbon intermediates. One of these is glyceraldehyde-3—P, and the other, dihy- droxyacetone-P, is converted to glyceraldehyde—3—P. Energy released from this high—energy molecule in the second phase of glycolysis is then used to synthesize ATP. 18.4 What Are the Chemical Principles and Features of the Second Phase of Glycolysis? The second half of the glycolytic path- way involves the reactions that convert the metabolic energy in the glucose molecule into ATP. Phase 2 starts with the oxidation of glyceraldehyde- 3-phosphate, a reaction with a large enough energy “kick” to produce a high-energy phosphate, namely, 1,3-bisphosphoglycerate. Phosphoryl transfer from 1,?»BPG to ADP to make ATP is highly favorable. The prod- uct, 3-phosphoglycerate, is converted via several steps to phosphoenol— pyruvate (PEP), another high-energy phosphate. PEP readily transfers its phosphoryl group to ADP in the pyruvate kinase reaction to make another ATP. . ”(mm-u UV 18.5 What Are the Metabolic Fates of NADH and Pyruvatt Produced in Glycolysis? In addition to ATP, the products of gly colysis are NADH and pyruvate. Their processing depends upon other cellular pathways. NADH must be recycled to NAD+, lest NAD+ become limiting in glycolysis. NADH can be recycled by both aerobic and anaer- obic paths, either of which results in further metabolism of pyruvate. What a given cell does with the pyruvate produced in glycolysis depends in part on the availability of oxygen. Under aerobic conditions, pyruvate can be sent into the citric acid cycle, where it is oxidized to C02 with the production of additional NADH (and FADHQ). Under aerobic condi- tions, the NADH produced in glycolysis and the citric acid cycle is reoxidized to NAD+ in the mitochondrial electron transport chain. Under anaerobic conditions, the pyruvate produced in glycolysis is not sent to the citric acid cycle. Instead, it is reduced to ethanol in yeast; in other microorganisms and in animals, it is reduced to lactate. These processes are examples of fermentation—the production of ATP energy by reaction pathways in which organic molecules function as donors and acceptors of electrons. In either case, reduction of pyruvate provides a means of reoxidizing the NADH produced in the glyceraldehyde-3— phosphate dehydrogenase reaction of glycolysis. 18.6 How Do Cells Regulate Glycolysis? The standard-state free energy changes for the ten reactions of glycolysis are variously pos~ itive and negative and, taken together, offer little insight into the cou- pling that occurs in the cellular milieu. On the other hand, the values of AG under cellular conditions fall into two distinct classes. For reac— tions 2 and 4 through 9, AG is very close to zero, meaning these reac- tions operate essentially at equilibrium. Small changes in the concen« trations of reactants and products could “push” any of these reactions either forward or backward. By contrast, the hexokinase, phosphofruc- tokinase, and pyruvate kinase reactions all exhibit large negative A Gval— ues under cellular conditions. These reactions are thus the sites of gly- colytic regulation. 18.7 Are Substrates Other Than Glucose Used in Glycolysis? Fructose enters glycolysis by either of two routes. Mannose, galactose, and glycerol enter via reactions that are linked to the glycolytic pathway, as shown in Figures 18.33 through 18.36. ...
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