LEHNINGER PRINCIPLES OF BIOCHEMISTRY - VOL. 2. BIOENERGETICS AND METABOLISM - 2014

PART II. BIOENERGETICS AND METABOLISM

20. BIOSYNTHESIS OF CARBOHYDRATES IN PLANTS AND BACTERIA

20.5 Integration of Carbohydrate Metabolism in the Plant Cell

Carbohydrate METABOLISM in a typical plant Cell is considerably more complex than in a typical animal cell. Plant Cells carry out the same energy-yielding processes found in animal cells (Glycolysis, The Tricarboxylic Acid Cycle, and Oxidative Phosphorylation); they synthesize hexoses from three- or four-carbon compounds via Gluconeogenesis; they oxidize hexose phosphates to generate NADPH (the oxidative Pentose Phosphate Pathway); and they can assemble starch polymer chains from glucose residues linked by α(1 -> 4) bonds, as well as break them down into hexoses. Beyond these carbohydrate reactions common to animal cells, however, the photosynthesizing plant cell can fix CO₂, incorporating carbon into Organic compounds via the enzymatic action of rubisco; use photosynthetic products to form trioses, hexoses, and pentoses via The Calvin Cycle; and convert acetyl-CoA—derived from

The breakdown of Fatty acids—into four-carbon compounds, and subsequently into hexoses (via The Glyoxylate cycle and gluconeogenesis, respectively). These unique plant processes, absent in animal cells, are compartmentalized across multiple Organelles: the glyoxylate cycle occurs in glyoxysomes, the Calvin cycle in Chloroplasts, starch synthesis in amyloplasts, and organic acid storage in vacuoles. Integrating cellular events across these distinct compartments requires specific transporters in the membrane of each organelle to shuttle products from one organelle to another or into the Cytosol.

In Germinating Seeds, Fats and Proteins Are Converted to Glucose via Gluconeogenesis

Many plants store Lipids and proteins in their seeds to serve as essential sources of energy and biosynthetic precursors during germination, prior to The Development of the photosynthetic apparatus. Driven by active gluconeogenesis in germinating seeds, glucose is utilized to synthesize sucrose, Polysaccharides, and numerous metabolites derived from hexoses. In plant seeds, the bulk of the chemical energy required for initial growth is generated through the breakdown of sucrose.

As noted earlier (Chapter 14), animal cells can perform gluconeogenesis from three- and four-carbon precursors, but not from acetyl-CoA. Because the Pyruvate dehydrogenase reaction is irreversible (pp. 182–183), animal cells cannot convert acetyl-CoA into pyruvate or oxaloacetate. Unlike animals, plants and certain microorganisms can convert acetyl-CoA derived from Fatty acid oxidation into glucose (Fig. 20-33). Several Enzymes required for this conversion are sequestered in glyoxysomes, where a glyoxysome-specific β-oxidation isozyme breaks down fatty acids to acetyl-CoA (see Fig. 16-22). This physical segregation of the glyoxylate cycle and β-oxidation enzymes from mitochondrial tricarboxylic acid cycle enzymes prevents the further oxidation of acetyl-CoA to CO₂. Instead, acetyl-CoA is converted to succinate via the glyoxylate cycle (see Fig. 16-20). Succinate then enters the mitochondrial matrix, where tricarboxylic acid cycle enzymes convert it into oxaloacetate, which is subsequently transported to the cytosol. In the cytosol, oxaloacetate is converted via gluconeogenesis into fructose-6-phosphate, a precursor of sucrose. Thus, the Formation of fructose-6-phosphate or sucrose from stored lipids requires the integration of sequential reactions. Because only three of the four carbon atoms are retained when cytosolic oxaloacetate is converted into hexose, roughly 75% of the fatty acid carbon stored in seed lipids is converted into CARBOHYDRATES through these combined pathways (Fig. 20-33). The remaining 25% is lost as CO₂ during The conversion of oxaloacetate to phosphoenolpyruvate. Hydrolysis of stored triacylglycerols yields glycerol 3-phosphate, which enters the gluconeogenic pathway after being oxidized to dihydroxyacetone phosphate (Fig. 20-34).

Class="center">Fig. 20-33 Conversion of stored fatty acids to sucrose in germinating seeds. This pathway begins in glyoxysomes, producing succinate, which is then exported to Mitochondria, where tricarboxylic acid cycle enzymes convert it to oxaloacetate. Oxaloacetate is exported to the cytosol, serving as the Starting Material for gluconeogenesis and the synthesis of sucrose—the transport form of carbon in plants.

Fig. 20-34 Conversion of the glycerol moieties of triacylglycerols to sucrose in germinating seeds. The glycerol from triacylglycerols is oxidized to dihydroxyacetone phosphate, which enters gluconeogenesis via the triose phosphate isomerase reaction.

Glucogenic Amino Acids (see Table 14-4) derived from the breakdown of stored seed proteins also yield precursor molecules for gluconeogenesis through Transamination and oxidation reactions that produce succinyl-CoA, pyruvate, oxaloacetate, fumarate, and α-ketoglutarate (Chapter 18)—all of which serve as excellent starting Materials for gluconeogenesis.

Metabolic Pathways in Different Organelles Share Pools of Common Intermediates

Thus far, we have described individual reaction sequences of Metabolic Transformations in plant cells. However, these reactions are deeply interconnected; rather than viewing them as linear pathways, we must consider pools of metabolic intermediates shared across pathways and linked by readily reversible reactions (Fig. 20-35). One such metabolite pool comprises the hexose phosphates: glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate; a second pool consists of pentose 5-phosphates: ribose, ribulose, and xylulose; and a third comprises triose phosphates: dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. The magnitude and direction of metabolic flux through these pools shift in response to changing environmental conditions and vary among different tissue types. Transmembrane transporters in each organelle shuttle specific compounds in and out; The regulation of these transporters likely influences exchange between the pools.

Fig. 20-35 The hexose phosphate, pentose phosphate, and triose phosphate pools. The compounds within each pool are readily interconverted, with the corresponding reactions exhibiting very small changes in Standard Free energy. When one component of a pool is temporarily depleted, a new equilibrium is rapidly established to replenish it. The movement of sugar phosphates between cellular compartments is restricted, requiring specialized transporters in the organelle membranes.

During daylight hours, triose phosphates produced in leaf Tissues via the Calvin cycle exit the chloroplast and enter the cytosolic hexose phosphate pool, where they are converted into sucrose for transport to nonphotosynthetic tissues. In these tissues, sucrose is stored as starch or utilized as an energy source via glycolysis. In growing plants, hexose phosphates from the pool are also consumed in Cell wall synthesis. At night, starch is broken down and used to generate energy through glycolysis. This process Functions essentially as it does in nonphotosynthetic organisms, producing NADPH and ribose 5-phosphate via the oxidative pentose phosphate pathway.

Summary of Section 20.5 Integration of Carbohydrate Metabolism in The plant cell

■ Plants can synthesize sugars from acetyl-CoA, a breakdown product of fatty acids, through the combined action of the glyoxylate cycle and gluconeogenesis.

■ The various pathways of carbohydrate metabolism in plant cells overlap extensively, utilizing pools of common intermediates including hexose phosphates, pentose phosphates, and triose phosphates. Transporters in the membranes of chloroplasts, mitochondria, amyloplasts, and Peroxisomes regulate the movement of sugar phosphates among organelles. The directions of metabolic flux through these pools differ between day and night.