CHAPTER 19 Carbohydrate Biosynthesis
We have now reached a turning point in the study of cellular metabolism. The preceding chapters of Part III have described how the major foodstuffs-carbohydrates, fatty acids, and amino acids-are degraded via converging catabolic pathways to enter the citric acid cycle and yield their electrons to the respiratory chain. The exergonic flow of electrons to oxygen is coupled to the endergonic synthesis of ATP. We now turn to anabolic pathways, which use chemical energy in the form of ATP and NADH or NADPH to synthesize cell components from simple precursor molecules. Anabolic pathways are generally reductive rather than oxidative. Catabolism and anabolism proceed simultaneously in a dynamic steady state, so that the energy-yielding degradation of cell components is counterbalanced by biosynthetic processes, which create and maintain the intricate orderliness of living cells.
Several organizing principles of biosynthesis deserve emphasis at the outset. The first principle is that the pathway taken in the synthesis of a biomolecule is usually different from the pathway taken in its degradation. Although the two opposing pathways may share many reversible reactions, there is always at least one enzymatic step that is unique to each pathway. If the reactions of catabolism and anabolism were catalyzed by the same set of enzymes acting reversibly, the flow of carbon through these pathways would be dictated exclusively by mass action (p. 371), not by the cell’s changing needs for energy, precursors, or macromolecules.
Second, corresponding anabolic and catabolic pathways are controlled by different regulatory enzymes. These opposing pathways are regulated in a coordinated, reciprocal manner, so that stimulation of the biosynthetic pathway is accompanied by inhibition of the corresponding degradative pathway, and vice versa. Biosynthetic pathways are usually regulated at their initial steps, so that the cell avoids wasting precursors to make unneeded intermediates; intrinsic economy prevails in the molecular logic of living cells.
Third, energy-requiring biosynthetic processes are coupled to the energy-yielding breakdown of ATP in such a way that the overall process is essentially irreversible in vivo. Thus the total amount of ATP (and NAD(P)H) energy used in a given biosynthetic pathway always exceeds the minimum amount of free energy required to convert the precursor into the biosynthetic product. The resulting large, negative, free-energy change for the overall process assures that it will occur even when the concentrations of precursors are relatively low.
This chapter provides many opportunities for elaboration of the three principles outlined above, as we describe pathways for carbohydrate biosynthesis. The chapter is divided into four parts. First we consider gluconeogenesis, the ubiquitous pathway for synthesis of glucose. We then describe how glucose is converted into a variety of polysaccharides: glycogen in animals and many microorganisms, starch and sucrose in plants. At this point the focus shifts entirely to plants. The third topic is the incorporation of CO2 into more complex molecules (CO2 fixation), a process that represents the ultimate source of reduced carbon compounds for all organisms. The chapter ends with a discussion of the regulation of carbohydrate metabolism in plants. The overall regulation of carbohydrate metabolism in mammals is covered separately in Chapter 22.
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