Atp

Figure 6.16 The Transition Step and the Tricarboxylic Acid Cycle The transition step links glycolysis and the TCA cycle, converting pyruvate to acetyl-CoA; it generates reducing power and 1 precursor metabolite.The TCA cycle incorporates the acetyl group of acetyl-CoA and using a series of steps, releases CO2; it generates ATP, reducing power in the form of both NADH and FADH2, and 2 different precursor metabolites.

■ Step 8: Malate is oxidized to form oxaloacetate; note that oxaloacetate is the starting compound to which acetyl-CoA is added to initiate the cycle. During the oxidation, NAD+ is reduced to form NADH + H+.

ated for each glucose molecule that enters glycolysis, the breakdown of one molecule of glucose causes the cycle to "turn" twice. Assuming no precursors leave the cycle, these two "turns'' generate:

Yield of the TCA Cycle

The tricarboxylic acid cycle "turns" once for each acetyl-CoA that enters. Because two molecules of acetyl-CoA are gener-

■ Reducing power—6 NADH +6 H+ and 2 FADH2. NAD+ is reduced to NADH + H+ during the oxidations

146 Chapter 6 Metabolism: Fueling Cell Growth

Table 6.7 Comparison of the Central Metabolic Pathways

Pathway

Characteristics

Glycolysis

Pentose phosphate cycle

Transition step

TCA cycle

Generally used in aerobic respiration, anaerobic respiration, and fermentation to obtain energy and precursor metabolites. Reducing power is also generated. In respiration, this reducing power can be used to drive the synthesis of ATP; in fermentation, it is consumed to recycle the electron carrier without an additional gain in energy.

Glycolysis generates:

• 2 ATP (net) by substrate-level phosphorylation

• 6 different precursor metabolites

May be used in aerobic respiration, anaerobic respiration, and fermentation to obtain precursor metabolites and reducing power in the form of NADPH.

The pentose phosphate cycle generates:

• 2 different precursor metabolites

Used in aerobic respiration and anaerobic respiration.The reducing power produced in this step can be used to drive the synthesis of ATP by oxidative phosphorylation.

The transition step, repeated twice to oxidize two molecules of pyruvate to acetyl-CoA, generates:

• 1 precursor metabolite

Used in aerobic respiration and anaerobic respiration.The reducing power produced in this cycle can be used to drive the synthesis of ATP by oxidative phosphorylation.

The TCA cycle, repeated twice to incorporate two acetyl groups, generates:

• 2 ATP by substrate-level phosphorylation (may involve conversion of GTP)

• 2 different precursor metabolites in the TCA cycle at steps 3, 4, and 8. FAD is reduced to FADH2 during the oxidation at step 6.

■ Precursor metabolites—Two precursor metabolites used by E. coli are formed as a result of steps 3 and 8.

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