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If a cell begins to produce more isoleucine than is needed for protein synthesis, the unused isoleucine accumulates and the increased concentration inhibits the catalytic activity of the first enzyme in the pathway, immediately slowing the production of isoleucine. Such feedback inhibition keeps the production and utilization of each metabolic intermediate in balance.

Although the concept of discrete pathways is an important tool for organizing our understanding of metabolism, it is an oversimplification. There are thousands of metabolic intermediates in a cell, many of which are part of more than one pathway. Metabolism would be better represented as a meshwork of interconnected and interdependent pathways. A change in the concentration of any one metabolite would have an impact on other pathways, starting a ripple effect that would influence the flow of materials through other sectors of the cellular economy. The task of understanding these complex interactions among intermediates and pathways in quantitative terms is daunting, but new approaches, discussed in Chapter 15, have begun to offer important insights into the overall regulation of metabolism.

Cells also regulate the synthesis of their own catalysts, the enzymes, in response to increased or diminished need for a metabolic product; this is the substance of Chapter 28. The expression of genes (the translation of information in DNA to active protein in the cell) and synthesis of enzymes are other layers of metabolic control in the cell. All layers must be taken into account when describing the overall control of cellular metabolism. Assembling the complete picture of how the cell regulates itself is a huge job that has only just begun.

SUMMARY 1.3 Physical Foundations

■ Living cells are open systems, exchanging matter and energy with their surroundings, extracting and channeling energy to maintain themselves in a dynamic steady state distant from equilibrium. Energy is obtained from sunlight or fuels by converting the energy from electron flow into the chemical bonds of ATP.

■ The tendency for a chemical reaction to proceed toward equilibrium can be expressed as the free-energy change, AG, which has two components: enthalpy change, AH, and entropy change, AS. These variables are related by the equation AG = AH - T AS.

■ When AG of a reaction is negative, the reaction is exergonic and tends to go toward completion; when AG is positive, the reaction is endergonic and tends to go in the reverse direction. When two reactions can be summed to yield a third reaction, the AG for this overall reaction is the sum of the AGs of the two separate reactions. This provides a way to couple reactions.

■ The conversion of ATP to Pj and ADP is highly exergonic (large negative AG), and many endergonic cellular reactions are driven by coupling them, through a common intermediate, to this reaction.

■ The standard free-energy change for a reaction, AG°, is a physical constant that is related to the equilibrium constant by the equation

■ Most exergonic cellular reactions proceed at useful rates only because enzymes are present to catalyze them. Enzymes act in part by stabilizing the transition state, reducing the activation energy, AG*, and increasing the reaction rate by many orders of magnitude. The catalytic activity of enzymes in cells is regulated.

■ Metabolism is the sum of many interconnected reaction sequences that interconvert cellular metabolites. Each sequence is regulated so as to provide what the cell needs at a given time and to expend energy only when necessary.

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