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Chapter 13

Carbohydrate Metabolism

13.1 The hexose phosphate pool
13.2 Biosynthetic pathways that consume hexose phosphates: synthesis of sucrose and starch
13.3 Catabolic pathways that generate hexose phosphates: sucrose and starch degradation
13.4 The triose phosphate/pentose phosphate metabolite pool
13.5 Interactions between the hexose phosphate and pentose phosphate/triose phosphate pools
13.6 Starch used as an overflow when the synthesis of sucrose exceeds the capacity of the leaves to export it: an example of the integrated control of metabolism in two cell compartments
13.7 Modulation of gene expression by carbohydrates
13.8 Energy-conserving reactions of glycolysis
13.9 Supply of energy and reducing power for biosynthetic reactions

David T. Dennis
Stephen D. Blakely



Carbohydrate metabolism has been studied extensively in animals and microorganisms but has received far less attention in plants. Researchers, as well as the authors of most general biochemistry texts, have assumed that metabolic data from any model system could be directly applied to plants because the individual reactions involved in carbohydrate metabolism appear to be very similar in all organisms. As our knowledge of metabolism has increased, however, we have discovered many aspects of plant carbohydrate metabolism that are unique.
      Several important attributes distinguish plants from other organisms and help explain the singular nature of plant carbohydrate metabolism. These factors result in a unique carbohydrate metabolism that only superficially resembles carbohydrate metabolism in other organisms.

• Plants are autotrophic.
• Plants are photosynthetic.
• Plants are sessile.
• Plants contain plastids.
• Plants have cell walls.

     Plants are autotrophic. Almost all plants convert simple nutrients such as carbon dioxide, water, and inorganic ions into all the intermediates required for the biosynthesis of nucleic acids, proteins, lipids, and polysaccharides as well as coenzymes and numerous secondary metabolites. Thus, in addition to fulfilling the energy requirements of the cell, plant carbohydrate metabolism must feed numerous anabolic pathways (Fig. 13.1). This biosynthetic investment affects the regulation of carbohydrate metabolism.

Figure 13.1
Central role of carbohydrate metabolism in the supply of carbon skeletons for biosynthetic reactions. G6PDH, glucose-6-phosphate dehydrogenase; 6PGDH, 6-phosphogluconate dehydrogenase; PFK, ATP-dependent phosphofructokinase; FBPase, fructose-1,6-bisphosphatase; PFP, pyrophosphate- dependent phosphofructokinase.





Figure 13.2
Photosynthetic electron transfer generates ATP and energy-rich reductants used to assimilate carbon, nitrogen, and sulfur. Fdx, ferredoxin.




     Plants are photosynthetic. Apart from a very few parasitic species, plants harness light and use its energy to fix and reduce carbon dioxide (see Chapter 12). The resulting triose phosphates produced can supply carbon to leaf cells or be converted to sucrose for export to other parts of the plant. Other activities associated with the capture of light energy include nitrite and sulfate reduction and ammonia assimilation (see Fig. 13.2 and Chapters 8 and 16). In addition, photosynthesis supplies many biosynthetic pathways with reducing equivalents and ATP. Because the processes associated with light absorption are diurnal, plants must cope with major variations in the supply of nutrients during the light and dark periods. This imposes the need for a flexibility in metabolism that is not seen in other organisms.
      Plants are sessile. Terrestrial plants are unable to relocate when faced with physical or chemical stress and must tolerate changes in temperature; fluctuations in the availability of light, water, and nutrients; and herbivory. To ensure survival in unforgiving circumstances, plant metabolism is highly flexible (see Box 13.4 for discussion). In addition, plants expend considerable resources in the production of defense compounds (see Chapter 24).
      Plants contain plastids. These organ-elles, found only in plants, perform most of the biosynthetic reactions that occur in plant cells. Duplication of some pathways in both cytosol and plastid, and the requirement for coordination of metabolism in all parts of the cell, have resulted in the evolution of several plastid envelope transporters (Fig. 13.3). The properties and functions of these transporters differ among various tissues and plastid types. Besides linking the cytosol and plastid compartments, these transporters also play important roles in metabolic regulation. The segregation of synthesis reactions into plastids presents another problem for the plant cell. Syntheses require an abundant supply of ATP and reducing power in the form of NAD(P)H, both of which must be generated within the plastid. Various mechanisms have evolved to satisfy this need (Fig. 13.4).
      Plants have cell walls. Cell walls are composed of cellulose, hemicellulose, and lignin (see Chapters 2 and 24). Synthesis of these wall components represents a major drain on carbohydrates, because the pathways for the synthesis of wall components can account for 30% or more of cellular carbohydrate metabolism.
      The topic of carbohydrate metabolism is typically discussed in terms of discrete anabolic and catabolic pathways. However, these pathways can be envisioned as a series of “pools” of metabolic intermediates linked by reversible enzyme reactions (Fig. 13.5). Metabolites can be added to or withdrawn from these pools to serve the needs of the various pathways.
      Two pools feed plant carbohydrate metabolism. One is composed of hexose phosphates, the other of pentose phosphate pathway intermediates and the triose phosphates glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. The direction of flow through these pools depends on the requirements of the cell (Fig. 13.6). In photosynthetic tissues, carbon flux through pathways also is governed by whether cells are in light or darkness. Metabolite pools in the cytosolic and plastid compartments communicate by way of highly specific carrier proteins (see Fig. 13.3 and Chapter 3).
      To highlight the function of metabolite pools, in this chapter we will frame some pathways differently from most biochemistry texts. For example, glycolysis in animals is typically described as the catabolism of either glucose or the storage polysaccharide glycogen to pyruvate. Starch metabolism in plants, however, is compartmentalized in plastids, and the complements of glycolytic enzymes present are different in the cytosol, chloroplasts, and heterotrophic plastids. When viewed as only one of many pathways linking the hexose phosphate and triose phosphate/pentose phosphate pools, glycolysis is revealed as a set of reactions contributing to an integrated whole plant metabolism rather than a discrete pathway to be memorized and quickly forgotten.

Figure 13.3
Transporters located in the inner envelope membrane of plastids exchange inorganic phosphate (P i ) for carbon metabolites. In the transport processes illustrated here, photosynthetic chloroplasts export C 3 intermediates, whereas the non-photosynthetic plastids import a more diverse group of sugars and organic acids. All the transport reactions are reversible, and the direction of transport is dependent on the concentrations of P i and metabolites on each side of the envelope. 3-PGA, 3-phosphoglycerate; 2-PGA, 2-phosphoglycerate.

Figure 13.4
Mechanism for supplying cofactors to plastids. Cofactors such as NAD(P)H or ATP are required for biosynthetic pathways. (A) Cofactors can be generated within the plastid by catabolic pathways located in the organelle, or by light energy in the case of the chloroplast. (B) A simple shuttle system for the transfer of reducing equivalents into a plastid. Cofactors may be imported from the cytosol either by direct transport (e.g., exchange of ADP for ATP) or via shuttles, as shown here.

Figure 13.5
A metabolic pool, showing four metabolites at equilibrium. Flow through this pool will be dictated by the addition of specific metabolites (A), or withdrawal of intermediates (B and C). In many cases, the metabolic status of the cell will determine whether a particular compound (D) enters or leaves the pool.


Figure 13.6
The use and synthesis of cofactors (NADH, NADPH, and ATP) in photosynthetic and nonphotosynthetic plastids. The contribution of photosynthetic electron transfer and photophosphorylation to the supply of cofactors in chloroplasts has not been included. 1,3-BisPGA, 1,3-bisphosphoglycerate.

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