point in the glycolytic pathway, the net energy gain is 0, because this step simply replenishes the ATP that was expended in steps 1 and 2.

■ Step 8: This chemically rearranges 3PG to form 2-phosphoglycerate (2PG).

■ Step 9: A water molecule is removed from 2PG, creating phosphoenolpyruvate (PEP), a molecule that contains an unstable (and therefore) high-energy phosphate bond.

■ Step 10: Substrate-level phosphorylation occurs once more as the energy of PEP is used to generate ATP, for a net gain of ATP. Removal of the phosphate group from PEP forms pyruvate, the end product of glycolysis.

144 Chapter 6 Metabolism: Fueling Cell Growth

Yield of Glycolysis

For every glucose molecule degraded, the steps of glycolysis produce:

■ ATP—The maximum possible energy gain as ATP in glycolysis is:

Energy expended 2 ATP molecules (steps 1 and 3) Energy harvested 4 ATP molecules (steps 7 and 10) Net gain 2 ATP molecules

■ Reducing power—An oxidation takes place at step 6, which occurs twice in glycolysis, converting 2 NAD+ to 2 NADH + 2H+.

■ Precursor metabolites—Five intermediates of glycolysis as well as the end product, pyruvate, are precursor metabolites used by E. coli. The siphoning off of any of these for biosynthesis decreases the amount of ATP and reducing power generated during glycolysis.

Pentose Phosphate Pathway

The other pathway used by cells to break down glucose is the pentose phosphate pathway. This complex pathway generates 5- and 7-carbon sugars, in addition to glyceraldehyde 3-phosphate (G3P) that can be directed to step 4 in glycolysis for further breakdown. The greatest importance of the pentose phosphate pathway is its contribution to biosynthesis. The reducing power it generates is in the form of NADPH, which is used in biosynthetic reactions when a reduction is required. In addition, two of its intermediates, ribose 5-phosphate and erythrose 4-phosphate, are important precursor metabolites. The pentose phosphate pathway, like glycolysis, can operate in the presence or absence of molecular oxygen.

Yield of the Pentose Phosphate Pathway

The yield of the pentose phosphate pathway varies, depending on which of several possible alternatives are taken. It can produce:

■ Reducing power—A variable amount of reducing power in the form of NADPH is produced.

■ Precursor metabolites—Two intermediates of the pentose phosphate pathway are precursor metabolites.

Transition Step

The transition step links glycolysis to the TCA cycle (see figure 6.16). In prokaryotic cells, the entire oxidation process takes place in the cytoplasm. In eukaryotic cells, however, pyruvate must first enter the mitochondria since the enzymes of the gly-colytic pathway are located in the cytoplasm of the cell, whereas those of the TCA cycle are found only within the matrix of the mitochondria. ■ mitochondria, p. 75

The transition step involves several integrated reactions catalyzed by a group of enzymes that form a large multi-enzyme complex. In the concerted series of reactions, carbon dioxide is first removed from the pyruvate, a process called decarboxyla tion. Then, an oxidation occurs, reducing NAD+ to form NADH + H+. Finally, the remaining 2-carbon acetyl group is joined to the coenzyme A to form acetyl-CoA.

Yield of the Transition Step

■ Reducing power—The transition step, which occurs twice for every molecule of glucose that enters glycolysis, is an oxidation. This reduces 2 NAD+ to form 2 NADH + 2 H+.

■ Precursor metabolites—The end product of the transition step, acetyl-CoA, is a precursor metabolite.

Tricarboxylic Acid Cycle

The eight steps of the tricarboxylic acid cycle complete the oxidation of glucose (figure 6.16). The pathway itself does not directly use O2. A large amount of reducing power is generated, however, and in aerobic respiration those electrons are ultimately passed to molecular oxygen. Allosteric enzymes regulate the cycle at several steps, but these will not be discussed.

The TCA cycle incorporates the acetyl groups that result from the transition step, releasing CO2 in this net reaction:

2 acetyl groups (2 C) + 6 NAD+ + 2 FAD + 2 ADP + 2 Pi d 4 CO2 + 6 NADH +6 H+ + 2 FADH2 + 2 ATP

In addition to generating ATP and reducing power, the steps of the TCA cycle form 2 more precursor metabolites used by E. coli (see table 6.3).

■ Step 1: The cycle begins when CoA transfers its acetyl group to the 4-carbon compound oxaloacetate, thereby forming the 6-carbon compound citrate.

■ Step 2: Citrate is chemically rearranged to form an isomer, isocitrate. ■ isomer, p. 30

■ Step 3: Isocitrate is oxidized and a molecule of CO2 is removed, forming the 5-carbon compound a-ketoglutarate. During the oxidation step, NAD+ is reduced to form NADH + H+.

■ Step 4: Like the transition step that converts pyruvate to acetyl-CoA, this reaction involves a group of reactions catalyzed by a complex of enzymes. In this step, a-ketoglutarate is oxidized, CO2 is removed, and CoA is added, producing the 4-carbon compound succinyl-CoA. During the oxidation step, NAD+ is reduced to form NADH + H+.

■ Step 5: This removes CoA from succinyl-CoA, harvesting the energy to make ATP. The reaction forms succinate. Note that some types of cells make guanosine triphosphate (GTP) rather than ATP at this step. This compound, however, can be converted to ATP.

■ Step 6: Succinate is oxidized to form fumarate. During the oxidation, FAD is reduced to form FADH2.

■ Step 7: A molecule of water is added to fumarate to form malate.

Nester-Anderson-Roberts: I. Life and Death of 6. Metabolism: Fueling Cell © The McGraw-Hill

Microbiology, A Human Microorganisms Growth Companies, 2003

Perspective, Fourth Edition

6.3 The Central Metabolic Pathways 145



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