Fermentation stops short of the TCA cycle, using pyruvate or a derivative of it as a terminal electron acceptor. Many end products of fermentation are commercially valuable.
■ How do the Voges-Proskauer and methyl-red tests differentiate between members of the Enterobacteriaceae?
■ Compare and contrast the fermentation pathways that generate lactic acid and propionic acid.
■ Fermentation is used as a means of preserving foods. Why would it slow spoilage?
6.6 Catabolism of Organic
Cells can use a variety of organic compounds other than glucose as energy sources, including macromolecules such as polysac-charides, lipids, and proteins. To break these down into their respective sugar, amino acid, and lipid subunits, cells synthesize hydrolytic enzymes, which break bonds by adding water. To use a macromolecule in the surrounding medium, a cell must secrete the appropriate hydrolytic enzyme and then transport the resulting subunits into the cell. Inside the cell, the subunits are further degraded to form appropriate precursor metabolites (figure 6.24). Recall that precursor metabolites can be either oxidized in one of the central metabolic pathways or used in biosynthesis. ■ hydrolysis, p. 25
Starch and cellulose are both polymers of glucose, but different types of chemical bonds join their subunits. The nature of this difference profoundly affects the mechanisms by which they are degraded. Enzymes called amylases are produced by a wide
variety of organisms to digest starches. In contrast, cellulose is digested by enzymes called cellulases, which are produced by relatively few organisms. Among the organisms that can degrade cellulose are bacteria that reside in the rumen of animals, and many types of fungi. Considering that cellulose is the most abundant organic compound on earth, it is not surprising that fungi are important decomposers in terrestrial habitats. The glucose subunits released when polysaccharides are hydrolyzed can then enter glycolysis to be oxidized to pyruvate. ■ polysaccharides, p. 29 ■ cellulose, p. 31 ■ rumen, p. 780
Disaccharides including lactose, maltose, and sucrose are hydrolyzed by specific disaccharidases. For example, the enzyme ^-galactosidase breaks down lactose, forming glucose and galactose. Glucose can enter glycolysis directly, but the other monosaccharides must first be modified. ■ disaccharides, p. 30
The most common simple lipids are fats, which are a combination of fatty acids joined to glycerol. Fats are hydrolyzed by enzymes called lipases. The glycerol component is then converted to the precursor metabolite dihydroxyacetone phosphate; this enters the glycolytic pathway. The fatty acids are degraded using a series of reactions collectively called ^-oxidation. Each sequential reaction transfers a 2-carbon unit from the end of the fatty acid to coenzyme A, forming acetyl-CoA; this can enter the TCA cycle. Each reaction is an oxidation, generating one NADH + H+ and one FADH2. ■ simple lipids, p. 34
Proteins are hydrolyzed by enzymes called proteases, which break peptide bonds that join amino acid subunits. The amino group of the resulting amino acids is removed by a reaction called a deamination. The remaining carbon skeletons are then converted into the appropriate precursor molecules. ■ protein, p. 25
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