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Catabolic pathways gradually oxidize an energy source in a controlled manner so that the energy released can be harvested. A specific enzyme catalyzes each step. Substrate-level phosphorylation uses chemical energy to synthesize ATP; oxidative phosphorylation employs proton motive force to synthesize ATP. Reducing power in the form of NADH and FADH2 is used to generate the proton motive force; the reducing power of NADPH is utilized in biosynthesis. Precursor metabolites are metabolic intermediates that can be used in biosynthesis. The central metabolic pathways generate ATP, reducing power, and precursor metabolites.

■ How does the fate of electrons carried by NADPH differ from those carried by NADH?

■ Why are the central metabolic pathways called amphibolic pathways?

■ Why does fermentation release less energy than respiration?

6.2 Enzymes

Recall that enzymes are proteins that act as biological catalysts, facilitating the conversion of a substrate into a product (see figure 6.6). They do this with extraordinary specificity and speed, usually acting on only one, or a very limited number of, substrates. They are neither consumed nor permanently changed during a reaction, allowing a single enzyme molecule to be rapidly used over and over again. In only one second, the fastest enzymes can transform more than 104 substrate molecules to products. More than a thousand different enzymes exist in a cell; most are given a common name that reflects their function and ends with the suffix -ase. For example, those that degrade proteins are collectively called proteases. ■ enzymes, p. 131

Mechanisms and Consequences of Enzyme Action

An enzyme has on its surface an active or catalytic site, typically a relatively small crevice (figure 6.10). This is the critical site to which a substrate binds by weak forces. The binding of the

Substrate

Products are released

Substrate

Enzyme

Enzyme-substrate complex formed

Enzyme

Enzyme Substrate

unchanged

Enzyme Substrate

Figure 6.10 Mechanism of Enzyme Action (a)The substrate binds to the active site, forming an enzymesubstrate complex.The products are then released, leaving the enzyme unchanged and free to combine with new substrate molecules. (b) A model showing an enzyme and its substrate. (c)The binding of the substrate to the active site causes the shape of the flexible enzyme to change slightly.

Figure 6.10 Mechanism of Enzyme Action (a)The substrate binds to the active site, forming an enzymesubstrate complex.The products are then released, leaving the enzyme unchanged and free to combine with new substrate molecules. (b) A model showing an enzyme and its substrate. (c)The binding of the substrate to the active site causes the shape of the flexible enzyme to change slightly.

substrate to the active site causes the shape of the flexible enzyme to change slightly. This mutual interaction, or induced fit, results in a temporary intermediate called the enzymesubstrate complex. The substrate is held within this complex in a specific orientation so that the activation energy for a given reaction is lowered, allowing the products to be formed. The products are then released, leaving the enzyme unchanged and free to combine with new substrate molecules. Note that enzymes may also catalyze reactions in which two substrates are joined to create one product. Theoretically, all enyzme-catalyzed reactions are reversible. The free energy change of certain reactions, however, makes them effectively non-reversible.

The interaction of the enzyme with its substrate is very specific. The substrate fitting into the active site may be likened to a hand fitting into a glove. Not only must it fit spatially, but appropriate chemical interactions such as hydrogen and ionic bonding need to occur to induce the fit. This requirement for a precise fit and interaction explains why, with minor exceptions, a different enzyme is required to catalyze every reaction in a cell. Very few molecules of any particular enzyme are needed, however, as each is swiftly reused again and again. ■ hydrogen bonds, p. 21 ■ Ionic bonds, p. 20

Cofactors and Coenzymes

Enzymes sometimes act with the assistance of a non-protein component called a cofactor (figure 6.11). Coenzymes are organic cofactors that act as loosely bound carriers of molecules or electrons (table 6.5). They include the electron carriers FAD, NAD+, and NADP+ and coenzyme A (CoA). Recall that the transition step between glycolysis and the TCA cycle generates acetyl-CoA; this compound is actually coenzyme A carrying an acetyl group. Other cofactors attach tightly to enzymes. For example, magnesium, zinc, copper, and other trace elements required for growth often function as cofactors. ■ trace elements, p. 90

All coenzymes transfer substances from one compound to another, but they function in different ways. Some remain

Cofactor

Substrate

Figure 6.11 Enzymes May Act in Conjunction with a Cofactor Cofactors are non-protein components and may be either coenzymes or trace elements.

bound to the enzyme during the transfer process, whereas others separate from the enzyme, carrying the substance being transferred along with them. The same coenzyme can assist different enzymes. Because of this, far fewer different coenzymes are required than enzymes. Like enzymes, coenzymes are recycled as they function and, consequently, are needed only in minute quantities.

Most coenzymes are synthesized from vitamins (see table 6.5). Some bacteria, such as E. coli, can synthesize all their required vitamins and convert them to the necessary coen-zymes. In contrast, humans and other animals must be provided with vitamins from external sources. Most often they must be supplied in the diet, but in some cases vitamins synthesized by bacteria residing in the intestine can be absorbed. If an animal lacks a vitamin, the functions of all the different enzymes whose

Table 6.5 Some Coenzymes and Their Function

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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