Aerobic eukaryotes

Fig. 6.13 The development of aerobic eukaryotic organisms from the earliest anaerobic ancestors via aerobic bacteria, all single cells.

The second metabolic pathway which we have chosen to describe is the tricarboxylic acid cycle, often referred to as the Krebs cycle. This represents the biochemical hub of intermediary metabolism, not only in the oxidative catabolism of carbohydrates, lipids, and amino acids in aerobic eukaryotes and prokaryotes, but also as a source of numerous biosynthetic precursors. Pyruvate, formed in the cytosol by glycolysis, is transported into the matrix of the mitochondria where it is converted to acetyl CoA by the multi-enzyme complex, pyruvate dehydrogenase. Acetyl CoA is also produced by the mitochondrial S-oxidation of fatty acids and by the oxidative metabolism of a number of amino acids. The first reaction of the cycle (Figure 5.12) involves the condensation of acetyl Co and oxaloacetate to form citrate (1), a Claisen ester condensation. Citrate is then converted to the more easily oxidised secondary alcohol, isocitrate (2), by the iron-sulfur centre of the enzyme aconitase (described in Chapter 13). This reaction involves successive dehydration of citrate, producing enzyme-bound cis-aconitate, followed by rehydration, to give isocitrate. In this reaction, the enzyme distinguishes between the two external carboxyl groups... 

Mitochondria are present in the cytoplasm of aerobic eukaryotic cells. They are frequently found in close proximity to the fuel sources and to the structures that require ATP for maintenance and functional activity (e.g., the contractile mechanisms, energy-dependent transport systems, and secretory processes). The number of mitochondria in a single cell varies from one type of cell to another a rat liver cell contains about 1000, while one giant amoeba has about 10,000. In a given cell, the number of mitochondria may also depend on the cell s stage of development or functional activity. 

Aerobic biooxidation is most effective as O, as acceptor of electrons, provides the organisms with the greatest amoimt of energy. Participate in it aerobic eukaryotes and prokaryotes. Decomposition of organic compounds often goes to the formation of H O, CO and carbonate ions. 

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