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ATP Is the Universal Carrier of Metabolic Energy, Linking Catabolism and Anabolism

Cells capture, store, and transport free energy in a chemical form. Adenosine triphosphate (ATP) (Fig. 1-12) functions as the major carrier of chemical energy in all cells. ATP carries energy between metabolic pathways by serving as the shared intermediate that couples endergonic reactions to exergonic ones. The terminal phosphate group of ATP is transferred to a variety of acceptor molecules, which are thereby activated for further chemical transformation. The adenosine diphosphate (ADP) that remains after the phosphate transfer is recycled to become ATP, at the expense of either chemical energy (during oxidative phosphorylation) or solar energy in photosynthetic cells (by the process of photophosphorylation). ATP is the major connecting link (the shared intermediate) between the catabolic and anabolic networks of enzyme-catalyzed reactions in the cell (Fig. 1-13).

Figure 1-12 (a) Structural formula and (b) balland-stick model for adenosine triphosphate (ATP). The removal of the terminal phosphate of ATP is highly exergonic, and this reaction is coupled to many endergonic reactions in the cell. These linked networks of enzyme-catalyzed reactions are virtually identical in all living organisms.

Figure 1-13 ATP is the chemical intermediate linking energy-releasing to energy-requiring cell processes. Its role in the cell is analogous to that of money in an economy: it is "earned/produced" in exergonic reactions and "spent/consumed" in endergonic ones.

Metabolism Is Regulated to Achieve Balance and Economy

Not only can living cells simultaneously synthesize thousands of different kinds of carbohydrate, fat, protein, and nucleic acid molecules and their simpler subunits, they can also do so in the precise proportions required by the cell. For example, when rapid cell growth occurs, the precursors of proteins and nucleic acids must be made in large quantities, whereas in nongrowing cells the requirement for these precursors is much reduced. Key enzymes in each metabolic pathway are regulated so that each type of precursor molecule is produced in a quantity appropriate to the current requirements of the cell. Consider the pathway shown in Figure 1-14 (see also Fig. 1-11), which leads to the synthesis of isoleucine (one of the amino acids, the monomeric subunits of proteins). If a cell begins to produce more isoleucine than is needed for protein synthesis, the unused isoleucine accumulates. High concentrations of isoleucine inhibit the catalytic activity of the first enzyme in the pathway, immediately slowing the production of the amino acid. Such negative feedback keeps the production and utilization of each metabolic intermediate in balance.

Living cells also regulate the synthesis of their own catalysts, the enzymes. Thus a cell can switch off the synthesis of an enzyme required to make a given product whenever that product is available ready-made in the environment. These self adjusting and self regulating .properties allow cells to maintain themselves in a dynamic steady state, despite fluctuations in the external environment.

Living cells are self regulating chemical engines, adjusted for maximum economy.

Figure 1-14 Regulation of a biosynthetic pathway by feedback inhibition. In the pathway by which isoleucine is formed in five steps from threonine (Fig. 1-11), the accumulation of the product isoleucine (F) causes inhibition of the first reaction in the pathway by binding to the enzyme catalyzing this reaction and reducing its activity. (The letters A to F represent the corresponding compounds shown in Fig. 1-11. )

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