12.4 Aerobic Respiration

For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria. The crucial events in aerobic respiration are:

• The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of \(\mathrm{CO}_2\).
• The passing on of the electrons removed as part of the hydrogen atoms to molecular \(\mathrm{O}_2\) with simultaneous synthesis of ATP.

What is interesting to note is that the first process takes place in the matrix of the mitochondria while the second process is located on the inner membrane of the mitochondria.

Pyruvate, which is formed by the glycolytic catabolism of carbohydrates in the cytosol, after it enters mitochondrial matrix undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvic dehydrogenase. The reactions catalysed by pyruvic dehydrogenase require the participation of several coenzymes, including \(\mathrm{NAD}^{+}\)and Coenzyme A.

\(\text { Pyruvic acid }+\mathrm{CoA}+\mathrm{NAD}^{+} \underset{\text { Pyruvate dehydrogenase }}{\stackrel{\mathrm{Mg}^{2+}}{\longrightarrow}}\text { Acetyl } \mathrm{CoA}+\mathrm{CO}_2+\mathrm{NADH}+\mathrm{H}^{+}\)

During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis).

The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle, more commonly called as Krebs’ cycle after the scientist Hans Krebs who first elucidated it.

Tricarboxylic Acid Cycle or TCA (Krebs’ cycle)

The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid (Figure 12.3). The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released. Citrate is then isomerised to isocitrate. It is followed by two successive steps of decarboxylation, leading to the formation of \(\alpha\)-ketoglutaric acid and then succinyl-CoA. In the remaining steps of citric acid cycle, succinyl-CoA is oxidised to OAA allowing the cycle to continue. During the conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised. This is a substrate level phosphorylation. In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP. Also there are three points in the cycle where \(\mathrm{NAD}^{+}\)is reduced to \(\mathrm{NADH}+\mathrm{H}^{+}\)and one point where \(\mathrm{FAD}^{+}\)is reduced to \(\mathrm{FADH}_2\). The continued oxidation of acetyl CoA via the TCA cycle requires the continued replenishment of oxaloacetic acid, the first member of the cycle. In addition it also requires regeneration of \(\mathrm{NAD}^{+}\)and \(\mathrm{FAD}^{+}\)from NADH and \(\mathrm{FADH}_2\) respectively. The summary equation for this phase of respiration may be written as follows:

\(
\begin{aligned}
\text { Pyruvic acid }+4 \mathrm{NAD}^{+}+\mathrm{FAD}^{+}+2 \mathrm{H}_2 \mathrm{O}+\mathrm{ADP}+\mathrm{Pi} \stackrel{\text { Mitochondrial Matrix }}{\longrightarrow} 3 \mathrm{CO}_2+4 \mathrm{NADH}+4 \mathrm{H}^{+} \\
+\mathrm{FADH}_2+\mathrm{ATP}
\end{aligned}
\)

We have till now seen that glucose has been broken down to release \(\mathrm{CO}_2\) and eight molecules of \(\mathrm{NADH}+\mathrm{H}^{+}\); two of \(\mathrm{FADH}_2\) have been synthesised besides just two molecules of ATP in TCA cycle. You may be wondering why we have been discussing respiration at all – neither \(\mathrm{O}_2\) has come into the picture nor the promised large number of ATP has yet been synthesised. Also what is the role of the NADH \(+\mathrm{H}^{+}\)and \(\mathrm{FADH}_2\) that is synthesised? Let us now understand the role of \(\mathrm{O}_2\) in respiration and how ATP is synthesised.

Electron Transport System (ETS) and Oxidative Phosphorylation

The following steps in the respiratory process are to release and utilise the energy stored in \(\mathrm{NADH}+\mathrm{H}^{+}\)and \(\mathrm{FADH}_2\). This is accomplished when they are oxidised through the electron|transport system and the electrons are passed on to \(\mathrm{O}_2\) resulting in the formation of \(\mathrm{H}_2 \mathrm{O}\). The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS) (Figure 12.4) and it is present in the inner mitochondrial membrane. Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents via \(\mathrm{FADH}_2\) (complex II) that is generated during oxidation of succinate in the citric acid cycle. The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome \(c\) via cytochrome \(b c_1\) complex (complex III). Cytochrome \(c\) is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome \(c\) oxidase complex containing cytochromes \(a\) and \(a_3\), and two copper centres.

When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate. The number of ATP molecules synthesised depends on the nature of the electron donor. Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of \(\mathrm{FADH}_2\) produces 2 molecules of ATP. Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor. Unlike photophosphorylation where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason that the process is called oxidative phosphorylation.

You have already studied about the mechanism of membrane-linked ATP synthesis as explained by chemiosmotic hypothesis in the earlier chapter. As mentioned earlier, the energy released during the electron transport system is utilised in synthesising ATP with the help of ATP synthase (complex V). This complex consists of two major components, \(\mathrm{F}_1\) and \(\mathrm{F}_0\) (Figure 12.5). The \(\mathrm{F}_1\) headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. \(F_0\) is an integral membrane protein complex that forms the channel through which protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic site of the \(\mathrm{F}_1\) component for the production of ATP. For each ATP produced, \(4 \mathrm{H}^{+}\)passes through \(\mathrm{F}_0\) from the intermembrane space to the matrix down the electrochemical proton gradient.