Aerobic Respiration

Anaerobic fermentation (glucose to lactate) releases only a small portion (~47 kcal/mol) of the total energy (—686 kcal/mol) available in the glucose molecule. However, if oxygen is available as an electron acceptor, glucose and other substrates can be completely oxidized, and a much larger amount of energy can be extracted. Aerobic respiration utilizing oxygen as an electron acceptor takes place in the mitochondrion (for morphology, see Sec. 2.2.1).

It is believed that mitochondria evolved from a bacterium engulfed by an ancestral eukaryotic cell, a theory that is supported by mito-chondrial structure (Fig. 5.6). Consistent with a bacterial lineage, the mitochondrion has its own DNA and RNA, and the organelle synthesizes some of its proteins. The mitochondrion consists of two membranes surrounding an inner matrix. The outer membrane is relatively porous because like some bacteria, it contains large membrane channels called porins. In contrast, the inner membrane restricts the movement of solutes, and crossing the membrane usually requires a specific transport mechanism. Infoldings of the inner membrane form cristae that increase surface area and, to a certain degree, create an intermembrane

Figure 5.6 The mitochondrion, a double-membrane organelle critical for aerobic respiration. The inner membrane of the mitochondrion forms folds or cristae which act as scaffolds for proteins involved in electron transport. The matrix contains many enzymes involved in the tricarb-oxylic acid cycle (TCA).

Intermembrane space

Intermembrane space

Figure 5.6 The mitochondrion, a double-membrane organelle critical for aerobic respiration. The inner membrane of the mitochondrion forms folds or cristae which act as scaffolds for proteins involved in electron transport. The matrix contains many enzymes involved in the tricarb-oxylic acid cycle (TCA).

space that is more isolated from the porous outer membrane. The large surface area provided by the folded inner membrane serves as a scaffold for the proteins that participate in electron transport (see Sec. 5.3.3) and oxidative phosphorylation (see Sec. 5.3.4). The matrix of the mitochondrion contains most of the enzymes required for pyru-vate oxidation (see Sec. 5.3.1) and the tricarboxylic acid cycle (TCA; see Sec. 5.3.2).

5.3.1 Pyruvate oxidation

Pyruvate + CoA + NAD+ s acetyl-CoA + CO2 + NAD + H+

If oxygen is available, pyruvate produced during glycolysis will be transported into the matrix of the mitochondrion and undergo oxidative decarboxylation (Fig. 5.7) before entering the TCA cycle. This reaction is catalyzed by a large multienzyme complex called pyruvate dehy-drogenase (PDH). Pyruvate is decarboxylated to acetate, oxidized by losing a pair of electrons to NAD+, and coupled to coenzyme A (CoA) forming acetyl-CoA. Oxidation of pyruvate to acetyl-CoA is a highly exergonic process (AG°' = -7.5 kcal/mol) that essentially is irreversible, and there is no return pathway that can circumvent this reaction. Thus, acetyl-CoA is committed to entering the TCA cycle or to entering pathways such as fat synthesis or ketone formation. Accordingly, acetyl-CoA derived from b-oxidation (breakdown of fatty acids to acetyl-CoA) cannot be used as substrate for gluconeogenesis.

PDH activity is tightly regulated by a number of factors. Enzyme activity is allosterically decreased by NADH, ATP, and acetyl-CoA and increased by NAD+, AMP, and CoA. Hence, PDH activity is decreased by factors that reflect energy abundance and by feedback inhibition from the immediate product of the reaction it catalyzes. PDH activity is decreased by phosphorylation, a reaction catalyzed by a PDH kinase, and conversely is activated by a PDH phosphatase that dephosphorylates the

Pyruvate OOO C02

tPDH

NAD+ NADH

ADP ATP

Acetyl-CoA

Figure 5.7 Pyruvate oxidation, the reaction that links glycolysis to the TCA cycle. The reaction is catalyzed by pyruvate dehydroge-nase (PDH), a large multienzyme complex. Phosphorylation of PDH decreases its activity. In pyruvate oxidation, pyruvate is decarboxy-lated to acetate, oxidized by passing electrons to NAD+, and coupled to coenzyme A (CoA).

enzyme. Insulin activates PDH by stimulating the PDH phosphatase, and thus insulin promotes conversion of pyruvate to acetyl-CoA.

5.3.2 TCA cycle

A summary of TCA-cycle reactions is presented in Eq. (5.4). The TCA cycle is an eight-step cycle that starts with a condensation reaction where the 2-carbon acetate of acetyl-CoA is added to a 4-carbon oxaloac-etate to form a 6-carbon citrate (Fig. 5.8). The reaction, which is catalyzed by citrate synthase (Table 5.1), is highly exergonic (AG°' = —7.5 kcal/mol, Table 5.1). Steps 3, 4, and 8 of the cycle are oxidations where electrons are passed to NAD+ to form NADH. Step 6 also is an oxidation, but the electron acceptor is FAD. This oxidation is catalyzed by succinate dehydrogenase (SDH) (Table 5.1), which is associated with the inner membrane of the mitochondrion, unlike the other TCA enzymes that are in the matrix. SDH and bound FAD form a part of Complex II in the electron transport system, so electrons passed to FAD can be transferred directly to coenzyme Q (CoQ) in the system (see Sec. 5.3.3). Steps 3 and

4 involve decarboxylations with the release of carbon dioxide. The carbons released in these reactions are not the two carbons brought in by the acetate. Nevertheless, loss of the second carbon returns the cycle to a 4-carbon intermediate, which is succinate complexed to CoA. Hydrolysis of the bond between succinate and CoA provides sufficient energy for the generation of GTP from GDP and Pi that occurs in step

5 (Fig. 5.8). The terminal phosphate of GTP can be transferred to ADP to form ATP. Oxaloacetate regenerated in step 8 can undergo condensation with acetate and begin a new cycle.

As might be expected, the enzymes of the TCA cycle are regulated by allosteric factors. All the dehydrogenases that pass electrons to NAD+ to form NADH (steps 3, 4, and 8) are inhibited by high levels of NADH. Alpha-ketoglutarate dehydrogenase (step 4) also is inhibited by suc-cinyl-CoA, the product of the reaction it catalyzes (Fig. 5.8 and Table 5.1). Isocitrate dehydrogenase activity (step 3) is increased by ADP.

The only usable energy produced in the TCA cycle from two molecules of acetyl-CoA (derived from one glucose molecule) is that stored in two molecules of ATP. These two ATP are in addition to the two ATP produced by substrate phosphorylation in glycolysis. Most of the energy extracted from the acetyl-CoA is in the form of high-energy electrons in 6 NADH (3 per acetyl-CoA) and 2 FADH2. High-energy electrons also were stored during glycolysis (2 NADH) and pyruvate oxidation (2 NADH).

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