Aerobic respiration

We shall now examine the fate of the pyruvate produced as the end-product of glycolysis. As we have seen, this depends on whether the organism in question is aerobic or anaerobic.

You will recall that during glycolysis, NAD+ was reduced to NADH. In order for glucose metabolism to continue, this supply of NAD+ must be replenished; this is achieved either by respiration or fermentation. Respiration is the term used to describe those ATP-generating processes, aerobic or anaerobic, by which oxidation of a substrate occurs, with an inorganic substance acting as the final electron acceptor. In aerobic respiration, that substance is oxygen; in anaerobic respiration, a substance such as nitrate or sulphate can fulfil the role.

In most aerobic organisms, the pyruvate is completely oxidised to CO2 and water by entering the tricarboxylic acid (TCA) cycle, also known as the Krebs cycle or simply the citric acid cycle (Figure 6.20). During this cycle, a series of redox reactions result in the gradual transfer of the energy contained in the pyruvate to coenzymes (mostly NADH). This energy is finally conserved in the form of ATP by a process of oxidative phosphorylation. We shall turn our attention to these important reactions in due course, but first let us examine the role of the TCA cycle in a little more detail.

Pyruvate does not itself directly participate in the TCA cycle, but must first be converted into the two-carbon compound acetyl-Coenzyme A:

The TCA cycle is a series of reactions that oxidize acetate to CO2, generating reducing power in the form of NADH and FADH2 for use in the electron transport chain.

Pyruvate (3C)

Coenzyme A

NAD+

NADH

This is an important intermediate, as lipids and amino acids can also be metabolised into this form, and thereby feed into the TCA cycle. The main features of the cycle are as follows:

• each reaction is catalysed by a separate enzyme

• four of the reactions involve substrate oxidation, with energy, in the form of electrons, passing to form NADH (mainly) and FADH2

• the two carbons present in acetyl-CoA are removed as CO2

• one reaction involves the generation of ATP by substrate-level phosphorylation.

For each 'turn' of the citric acid cycle, one molecule of ATP, three molecules of NADH and one molecule of FADH2 are produced (FADH2 is the reduced form of another coenzyme, FAD). Since these derive from oxidation of a single acetyl-CoA molecule,

Figure 6.20 The TCA cycle. Acetyl-CoA may derive from the pyruvate of glycolysis or from lipid or amino acid metabolism. It joins with the four-carbon oxaloacetate to form the six-carbon citric acid. Two decarboxylation steps reduce the carbon number back to four and oxaloacetate re-enters the cycle once more. Although no ATP results directly from the cycle, the third phosphate on GTP can be easily transferred to ADP (GTP + ADP = GDP + ATP), thus generating one molecule of ATP per cycle. In addition, substantial reducing power is generated in the form of NADH and FADH2. These carry electrons to the electron transport chain, where further ATPs are generated

Figure 6.20 The TCA cycle. Acetyl-CoA may derive from the pyruvate of glycolysis or from lipid or amino acid metabolism. It joins with the four-carbon oxaloacetate to form the six-carbon citric acid. Two decarboxylation steps reduce the carbon number back to four and oxaloacetate re-enters the cycle once more. Although no ATP results directly from the cycle, the third phosphate on GTP can be easily transferred to ADP (GTP + ADP = GDP + ATP), thus generating one molecule of ATP per cycle. In addition, substantial reducing power is generated in the form of NADH and FADH2. These carry electrons to the electron transport chain, where further ATPs are generated we need to double these values per molecule of glucose originally entering glycolysis. Several of the intermediate molecules in the TCA cycle also act as precursors in other pathways, such as the synthesis of amino acids, fatty acids or purines and pyrimidines (see Anabolic metabolism, below). Other pathways regenerate such intermediates for continued use in the TCA cycle (see Box 6.3).

So far, we are a long way short of the 38 molecules of ATP per molecule of glucose mentioned earlier; we have only managed two ATPs from glycolysis and a further two

Box 6.3 The glyoxylate cycle

The components of the TCA cycle may act as precursors for the biosynthesis of other molecules (e.g., both a-ketoglutarate and oxaloacetate can be used for the synthesis of amino acids). For the TCA cycle to continue, it must replace these compounds. Many microorganisms are able to do this by converting pyruvate to oxaloacetate via a carboxylation reaction. A pathway that replenishes intermediate compounds of another in this way is termed anaplerotic. Organisms that use acetate (or molecules that give rise to it e.g. fatty acids) as sole carbon source regenerate TCA intermediates by means of the glyoxylate cycle (sometimes known as the glyoxylate shunt or bypass). This resembles the TCA cycle, but the two decarboxylation reactions (i.e. those where CO2 is removed) are missed out (compare with Figure 6.20).

Fatty Acids

AcetylCoA CoA

AcetylCoA CoA

Thus, isocitrate is converted directly to succinate and glyoxylate, and in another unique reaction, the glyoxylate is joined by acetyl-coA to form malate. The result of this is that succinate can be removed to participate in a biosynthetic pathway, but oxaloacetate is still renewed via glyoxylate and malate.

Figure 6.21 The electron transport chain. Electrons from NADH and FADH2 pass from one electron carrier to another, with a gradual release of energy as ATP by chemiosmosis (see Figure 6.22). The electron carriers are arranged in order of their reduction potential (tendency to gain electrons) and oscillate between the oxidised and the reduced state. FMN = flavin mononucleotide, Q = coenzyne Q.

Figure 6.21 The electron transport chain. Electrons from NADH and FADH2 pass from one electron carrier to another, with a gradual release of energy as ATP by chemiosmosis (see Figure 6.22). The electron carriers are arranged in order of their reduction potential (tendency to gain electrons) and oscillate between the oxidised and the reduced state. FMN = flavin mononucleotide, Q = coenzyne Q.

from the TCA cycle. Where do all the rest come from? Most of the energy originally stored in the glucose molecule is now held in the form of the reduced coenzymes (NADH and FADH2) produced during glycolysis and the TCA cycle. This is now converted to no less than 34 molecules of ATP per glucose molecule by oxidative phosphorylation in the remaining steps in aerobic respiration (three from each molecule of NADH and two from each of FADH2).

In the final phase of aerobic respiration, electrons are transferred from NADH and FADH2, via a series of carrier molecules known collectively as the electron transport (or respiratory) chain to oxygen, the terminal electron acceptor (Figure 6.21). This in turn is reduced to the molecules of water you will remember from our overall equation on page 122. In procaryotes, this electron transfer occurs at the plasma membrane, while in eucaryotes it takes place on the inner membrane of mitochondria. Table 6.2 summarises the locations of the reactions in the different phases of carbohydrate metabolism.

The electron transport chain is a series of donor/ acceptor molecules that transfer electrons from donors (e.g. NADH) to a terminal electron acceptor (e.g. O2).

MICROBIAL METABOLISM Table 6.2 Location of respiratory enzymes

Reaction

Glycolysis TCA cycle Electron Transport

Procaryotes

Cytoplasm Cytoplasm Plasma membrane

Eucaryotes Cytoplasm

Mitochondrial matrix Mitochondrial inner membrane

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