Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy Conservation Processes
Tricarboxylic acid cycle
Other pathways for the oxidation of one- and two-carbon substrates

The Tricarboxylic Acid Cycle occurs in the Cells of Bacteria, Fungi, Protozoa, plants, and animals, and is one of the most important cycles in aerobic organisms. However, some bacteria and plants possess alternative pathways for The oxidation of one- and two-carbon molecules. Typically, the utilization of alternative pathways is triggered by The Use of a non-carbohydrate substrate as the sole source of carbon and energy. In such cases, cells must find an alternative pathway for substrate oxidation that yields intermediates capable of being converted into essential cellular metabolites.

An example of such a situation is the functioning of The Glyoxylate cycle in the seed cells of higher plants. When triacylglycerols serve as the primary storage substance in seeds, and their fatty acid residues are converted to acetyl-CoA during oxidation, the cells utilize the glyoxylate cycle (Figs. 11.6, 11.7). A key feature of this cycle is that isocitrate is not decarboxylated as in the TCA cycle, but is instead cleaved by isocitrate lyase into succinate (which returns to the β-oxidation pathway) and glyoxylate (Fig. 11.6).

Glyoxylate then condenses with another molecule of acetyl-CoA (catalyzed by malate synthase) to form L-malate. This four-carbon compound can either be oxidized to oxaloacetate or decarboxylated to form Pyruvate. Both oxaloacetate and Pyruvate can be used by The Cell as precursors for The Biosynthesis of many essential substances, including glucose (Fig. 11.7).

This process is also utilized by bacteria, such as E. coli and Pseudomonas, as well as some protozoa, when acetate serves as the sole carbon source. In bacteria, the TCA and glyoxylate cycles are not spatially separated, whereas in eukaryotes, some of the Enzymes required for the glyoxylate pathway are localized in specialized Organelles called glyoxysomes. Structurally, glyoxysomes resemble Peroxisomes. Since the glyoxylate pathway can be viewed as a set of anaplerotic reactions (replenishing the pools of succinate, malate, and oxaloacetate), it is often considered part of the TCA cycle (Fig. 11.6).

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Fig. 11.7. Reactions of the glyoxylate cycle

Some bacteria exist under conditions where the sole carbon source consists of two-carbon compounds such as glycolate, Glycine, and oxalate. All of these substances can be converted into glyoxylate (Fig. 11.8), which, in turn, is oxidized to CO₂ and H₂O in the Dicarboxylic Acid Cycle. In this cycle, acetyl-CoA serves as the glyoxylate acceptor and is regenerated through a series of reactions. Each turn of the cycle is accompanied by The formation of two carbon dioxide molecules and the release of two pairs of electrons, which enter the Respiratory Chain as part of two NADH molecules.

Fig. 11.8. Oxidation of two-carbon substrates. Cells of many microorganisms, as well as some plants and animals, are capable of oxidizing other unusual substrates, including one-carbon compounds (such as formate), to extract energy. In each case, a unique metabolic pathway is utilized, allowing the electrons removed from the substrate to be used in the respiratory chain for ATP synthesis.