Storage Lipids Are Converted into Carbohydrates in Germinating Seeds

After germinating, oil-containing seeds metabolize stored triacylglycerols by converting lipids to sucrose. Plants are not able to transport fats from the endosperm to the root and shoot tissues of the germinating seedling, so they must convert stored lipids to a more mobile form of carbon, generally sucrose. This process involves several steps that are located in different cellular compartments: oleosomes, gly-oxysomes, mitochondria, and cytosol.

Overview: Lipids to sucrose. The conversion of lipids to sucrose in oilseeds is triggered by germination and begins with the hydrolysis of triacylglycerols stored in the oil bodies to free fatty acids, followed by oxidation of the fatty acids to produce acetyl-CoA (Figure 11.18). The fatty

Metabolization Seed Storage Cycle

Citrate

^Aconitase Isocitrate

Every two molecules of acetyl-CoA produced are metabolized by the glyoxylate cycle to generate one succinate.

Succinate moves into the mitochondrion and is converted to malate.

Citrate

^Aconitase Isocitrate

Every two molecules of acetyl-CoA produced are metabolized by the glyoxylate cycle to generate one succinate.

Succinate moves into the mitochondrion and is converted to malate.

Malate is transported into the cytosol and oxidized to oxaloacetate, which is converted to phosphoenolpyruvate by the enzyme PEP carboxykinase. The resulting PEP is then metabolized to produce sucrose via the gluconeogenic pathway.

FIGURE 11.18 The conversion of fats to sugars during germination in oil-storing seeds. (A) Carbon flow during fatty acid breakdown and gluconeogenesis (refer to Figures 11.2, 11.3, and 11.6 for structures). (B) Electron micrograph of a cell from the oil-storing cotyledon of a cucumber seedling, showing glyoxysomes, mitochondria, and oleosomes. (Photo courtesy of R. N. Trelease.)

Glyoxysomes

Glyoxysomes

Glyoxysome Single Membrane

Mitochondria

Oleosomes

Mitochondria

Oleosomes acids are oxidized in a type of peroxisome called a gly-oxysome, an organelle enclosed by a single bilayer membrane that is found in the oil-rich storage tissues of seeds. Acetyl-CoA is metabolized in the glyoxysome (see Figure 11.18A) to produce succinate, which is transported from the glyoxysome to the mitochondrion, where it is converted first to oxaloacetate and then to malate. The process ends in the cytosol with the conversion of malate to glucose via gluconeogenesis, and then to sucrose.

Although some of this fatty acid-derived carbon is diverted to other metabolic reactions in certain oilseeds, in castor bean (Ricinus communis) the process is so efficient that each gram of lipid metabolized results in the formation of 1 g of carbohydrate, which is equivalent to a 40% recovery of free energy in the form of carbon bonds ([15.9 kJ/40 kJ] x 100 = 40%).

Lipase hydrolysis. The initial step in the conversion of lipids to carbohydrate is the breakdown of triglycerides stored in the oil bodies by the enzyme lipase, which, at least in castor bean endosperm, is located on the half-membrane that serves as the outer boundary of the oil body. The lipase hydrolyzes triacylglycerols to three molecules of fatty acid and glycerol. Corn and cotton also contain a lipase activity in the oil body, but peanut, soybean, and cucumber show lipase activity in the glyoxysome instead. During the breakdown of lipids, oil bodies and gly-oxysomes are generally in close physical association (see Figure 11.18B).

P-Oxidation of fatty acids. After hydrolysis of the tria-cylglycerols, the resulting fatty acids enter the glyoxysome, where they are activated by conversion to fatty-acyl-CoA by the enzyme fatty-acyl-CoA synthase. Fatty-acyl-CoA is the initial substrate for the P-oxidation series of reactions, in which Cn fatty acids (fatty acids composed of n number of carbons) are sequentially broken down to n/2 molecules of acetyl-CoA (see Figure 11.18A). This reaction sequence involves the reduction of V2 O2 to H2O and the formation of 1 NADH and 1 FADH2 for each acetyl-CoA produced.

In mammalian tissues, the four enzymes associated with P-oxidation are present in the mitochondrion; in plant seed storage tissues, they are localized exclusively in the gly-oxysome. Interestingly, in plant vegetative tissues (e.g., mung bean hypocotyl and potato tuber), the P-oxidation reactions are localized in a related organelle, the peroxi-some (see Chapter 1).

The glyoxylate cycle. The function of the glyoxylate cycle is to convert two molecules of acetyl-CoA to succi-nate. The acetyl-CoA produced by P-oxidation is further metabolized in the glyoxysome through a series of reactions that make up the glyoxylate cycle (see Figure 11.18A). Initially, the acetyl-CoA reacts with oxaloacetate to give citrate, which is then transferred to the cytoplasm for iso-

merization to isocitrate by aconitase. Isocitrate is reimported into the peroxisome and converted to malate by two reactions that are unique to the glyoxylate pathway.

1. First isocitrate (C6) is cleaved by the enzyme isocitrate lyase to give succinate (C4) and glyoxylate (C2). This succinate is exported to the motochondria.

2. Next malate synthase combines a second molecule of acetyl-CoA with glyoxylate to produce malate.

Malate is then oxidized by malate dehydrogenase to oxaloacetate, which can combine with another acetyl-CoA to continue the cycle (see Figure 11.18A). The glyoxylate produced keeps the cycle operating in the glyoxysome, but the succinate is exported to the mitochondria for further processing.

The mitochondrial role. Moving from the glyoxysomes to the mitochondria, the succinate is converted to malate by the normal citric acid cycle reactions. The resulting malate can be exported from the mitochondria in exchange for succinate via the dicarboxylate transporter located in the inner mitochondrial membrane. Malate is then oxidized to oxaloacetate by malate dehydrogenase in the cytosol, and the resulting oxaloacetate is converted to carbohydrate.

This conversion requires circumventing the irreversibil-ity of the pyruvate kinase reaction (see Figure 11.3) and is facilitated by the enzyme PEP carboxykinase, which utilizes the phosphorylating ability of ATP to convert oxaloac-etate to PEP and CO2 (see Figure 11.18A). From PEP, glu-coneogenesis can proceed to the production of glucose, as described earlier. Sucrose is the final product of this process, and the primary form of reduced carbon translocated from the cotyledons to the growing seedling tissues. Not all seeds quantitatively convert fat to sugar (see Web Topic 11.9).

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