AcetylCoA

1 CoA-SH

nadh

Oxaloacetate

Citrate

Malate

I so citrate

Tricarboxylic Acid Cycle if-xrnad*

nadh

Fumarate a-katogl utarate fadh2 ' ®

LFAD JJJ,

Succinate^

CoA-SH

CoA-SH

(Complex II)

CoA-SH

CoA-SH

Figure 5.8 Tricarboxylic acid cycle (TCA). The TCA cycle is an 8-step reaction that oxidizes acetate to CO2. In step 1, the 2-carbon acetate is complexed with the 4-carbon oxaloacetate to form the 6-carbon citrate. In subsequent steps, decarboxylations yield two molecules of CO2, pairs of electrons are transferred to three NAD+ and one FAD, and one GTP (ATP) is produced. Succinate dehydrogenase (SDH), which catalyzes reaction 6, is the only enzyme of the cycle associated with the inner mitochondrial membrane. SDH is part of Complex II in electron transport.

Figure 5.8 Tricarboxylic acid cycle (TCA). The TCA cycle is an 8-step reaction that oxidizes acetate to CO2. In step 1, the 2-carbon acetate is complexed with the 4-carbon oxaloacetate to form the 6-carbon citrate. In subsequent steps, decarboxylations yield two molecules of CO2, pairs of electrons are transferred to three NAD+ and one FAD, and one GTP (ATP) is produced. Succinate dehydrogenase (SDH), which catalyzes reaction 6, is the only enzyme of the cycle associated with the inner mitochondrial membrane. SDH is part of Complex II in electron transport.

TABLE 5.1 TCA Cycle Enzymes

Step

Enzyme

AG°'

1

Citrate synthase

-7.5

2

Aconitase

+ 1.5

3

Isocitrate dehydrogenase

-2.0

4

a-ketoglutarate dehydrogenase

-7.2

5

Succinyl-CoA synthase

-0.8

6

Succinate dehydrogenase

~0

7

Fumarate hydratase

-0.9

8

Malate dehydrogenase

+7.1

Thus, complete oxidation of one molecule of glucose generates thus far 10 NADH, 2 FADH2, and 4 ATP.

The energy in these stored electrons can be converted to ATP "dollars" in the electron transport system (see Sec. 5.3.3).

The discussion of the various metabolic pathways has, thus far, focused on oxidation of glucose, but it should be remembered that other sugars, lipids, proteins, and nucleic acids can enter these pathways. Triglycerides (lipid) can be disassembled into glycerol and fatty acids. The glycerol can enter glycolysis, and b-oxidation of fatty acids yields acetyl-CoAthat can enter the TCA cycle. Amino acids (protein) can be deaminated to produce carbon skeletons that can enter as pyruvate (alanine) or as various intermediates in the TCA cycle. Therefore, virtually all biological molecules can be considered to be energy substrates.

5.3.3 Electron transport

Electron transport is the first system required to "cash in" the energy associated with electrons transferred to the coenzymes NAD+ and FAD during the TCA cycle. In the system, electrons from the coenzymes are passed through a series of carriers to oxygen. Most of the carriers are associated with four (I, II, III, and IV) large protein complexes or respiratory complexes in the inner mitochondrial membrane. The transfers of electrons from NADH [Eq. (5.5)] and FADH2 [Eq. (5.6)] to oxygen are highly exergonic processes (Fig. 5.9 and Application Box 5.1) that release enough energy to synthesize multiple molecules of ATP from ADP and Pi. The exact number of ATP molecules produced is unclear, but accepted average values are three ATP from NADH and two ATP from FADH2.

Multiple carriers that undergo reversible reduction-oxidation (redox) reactions allow for a stepwise release of energy from the electrons stored in NADH and FADH2. With each transfer, the electrons lose free energy and as will be discussed in Sec. 5.3.4, the "packets" of released energy are stored in the form of a proton electrochemical gradient. The carriers

Intermembrane space

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