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The Glyoxylate Cycle

Seeds are often rich in fats. At germination these fat reserves are used, to some extent, as energy sources, e.g. via oxidative degradation as outlined above. However, the fats can also be converted to carbohydrates. In this case they are first broken down by lipases into glycerol and fatty acids. It has already been mentioned that glycerol can be converted to carbohydrate via the triose phosphates. However, the fatty acids can also be [Pg.96]

The result, then, is an augmented supply of malate and, in turn, of ox-alacetate. Part of the oxalacetate is condensed with acetyl CoA to form citrate and so keep the glyoxylate cycle going. Another portion of the oxalacetate can also be phosphorylated and decarboxylated to give phosphoenolpyruvate. This in turn can give rise to hexoses via triose phosphate. [Pg.98]

We have said that the glyoxylate cycle is /orw Z/y a variant of the citric acid cycle. This statement needs further underpinning Beevers was able to show that both of the key enzymes, isocitratase and malate synthetase, are localized in special organelles, the glyoxysomes, which can be distinguished from the mitochondria. Here again we come across the phenomenon of compartmentalization —this time between such closely related processes as the citric acid cycle and the glyoxylate cycle. [Pg.98]

Place of the fats in total metabolism (Fig. 74). In conclusion let us summarize the position of the biosynthesis of fats in total metabolism their glycerol component originates from dihydroxyacetone phosphate and their fatty acids are formed from acetate and malonate. Thus, the biosynthesis of the fats involves the incorporation of certain intermediates of biological oxidation. Conversely, intermediates of biological oxidation are furnished when they are degraded—provided one ignores a-oxidation of fatty acids. [Pg.98]

But there are some criteria by which we re second rate, For our anaplerotic routes can t make glyoxylate And so we fall flat at turning of fat Into new carbohydrate. [Pg.36]

And PEP carboxykinase turns into C3 But oxaloacetic can t be made so easily (Acetyl CoA in the TCA Ends as CO2 set free). [Pg.36]

But humble Tetrahymena or leaf or oily seed Can manifest a pathway that is elegant indeed. [Pg.36]

Two new enzyme activities equip them to succeed At turning with style all spare acetyl To the sugars that they need. [Pg.36]

The pathway s compartmentalised twixt metabolic homes Involving mitochondria and the peroxisomes (The microbodies otherwise or else glyoxisomes) [Pg.36]

Some plants and bacteria that can use acetate as their sole source of carbon are able to oxidize acetyl-CoA via the citric acid cycle, or the acetate can be converted to carbohydrates via a pathway that is a modification of the citric acid cycle. This pathway is known as the glyoxylate cycle (Fig. 12-10) [Pg.355]

These reactions produce two important intermediate compounds, succinate and malate (which is converted into oxaloacetate). The two decarboxylation steps of the citric acid cycle are bypassed, and so there is no oxidation of acetyl-CoA to C02. Two molecules of acetyl-CoA are used, but all the carbon atoms are retained. [Pg.357]

Question What is the overall reaction of the glyoxylate cycle  [Pg.357]

What are the overall chemical changes that occur during one complete turn of the citric acid [Pg.357]

The overall reactions are the complete oxidation of one molecule of acetyl-CoA, the production of two molecules of C02, the reduction of three molecules of NAD+ and one of FAD, and the phosphorylation of one molecule of GDP. [Pg.357]


FIGURE 20.28 The glyoxylate cycle. The first two steps are identical to TCA cycle reactions. The third step bypasses the C09-evolving steps of the TCA cycle to produce snc-cinate and glyoxylate. The malate synthase reaction forms malate from glyoxylate and another acetyl-CoA. The result is that one torn of the cycle consumes one oxaloacetate and two acetyl-CoA molecnles bnt produces two molecnles of oxaloacetate. The net for this cycle is one oxaloacetate from two acetyl-CoA molecnles. [Pg.669]

The enzymes of the glyoxylate cycle in plants are contained in glyoxysomes, organelles devoted to this cycle. Yeast and algae carry out the glyoxylate cycle in the cytoplasm. The enzymes common to both the TCA and glyoxylate pathways exist as isozymes, with spatially and functionally distinct enzymes operating independently in the two cycles. [Pg.670]

The Glyoxylate Cycle Helps Plants Grow in the Dark... [Pg.670]

Glyoxysomes do not contain all the enzymes needed to run the glyoxylate cycle succinate dehydrogenase, fumarase, and malate dehydrogenase are absent. Consequently, glyoxysomes must cooperate with mitochondria to run their cycle (Figure 20.31). Succinate travels from the glyoxysomes to the mitochondria, where it is converted to oxaloacetate. Transamination to aspartate follows... [Pg.670]

Barrett, J., Ward, C.W. and Fairbairn, D. (1970) The glyoxylate cycle and the conversion of triglycerides to carbohydrates in developing eggs of Ascaris lumbricoides. Comparative Biochemistry and Physiology 35, 577-586. [Pg.287]

Rotte C, Stejskal F, Zhu G, Keithly JS, Martin W (2001) Pyruvate NADP+ oxidoreductase from the mitochondrion of Euglena gracilis and from the apicomplexan Cryptosporidium parvum a biochemical relic linking pyruvate metabolism in mitochondriate and amitochondriate protists. Mol Biol Evol 18 710-720 Schnarrenberger C, Martin W (2002) Evolution of the enzymes of the citric acid cycle and the glyoxylate cycle of higher plants. A case study of endosymbiotic gene transfer. Eur J Biochem 269 868-883... [Pg.178]

The Glyoxylate Cycle Produces Four-Carbon Compounds from Acetate... [Pg.623]

In plants, certain invertebrates, and some microorganisms (including E. coli and yeast) acetate can serve both as an energy-rich fuel and as a source of phosphoenolpyruvate for carbohydrate synthesis. In these organisms, enzymes of the glyoxylate cycle catalyze the net conversion of acetate to succinate or other four-carbon intermediates of the citric acid cycle ... [Pg.623]

FIGURE 16-22 Relationship between the glyoxylate and citric acid cycles. The reactions of the glyoxylate cycle (in glyoxysomes) proceed simultaneously with, and mesh with, those of the citric acid cycle (in mitochondria), as intermediates pass between these compartments. The conversion of succinate to oxaloacetate is catalyzed by citric acid cycle enzymes. The oxidation of fatty acids to acetyl-CoA is described in Chapter 17 the synthesis of hexoses from oxaloacetate is described in Chapter 20. [Pg.625]

The glyoxylate cycle is active in the germinating seeds of some plants and in certain microorganisms that can live on acetate as the sole carbon source. In plants, the pathway takes place in glyoxysomes in seedlings. It involves several citric acid cycle enzymes and two additional enzymes isocitrate lyase and malate synthase. [Pg.626]

In the glyoxylate cycle, the bypassing of the two decarboxylation steps of the citric acid... [Pg.626]

The partitioning of isocitrate between the citric acid cycle and the glyoxylate cycle is controlled at the level of isocitrate dehydrogenase, which is regulated by reversible phosphoiylation. [Pg.626]

Vertebrates lack the glyoxylate cycle and cannot synthesize glucose from acetate or the fatty acids that give rise to acetyl-CoA. [Pg.626]

Eastmond, P.J. Graham, I. A. (2001) Re-examining the role of the glyoxylate cycle in oilseeds. Trends Plant Sci. 6, 72-77. Intermediate-level review of studies of the glyoxylate cycle in Arabidopsis. [Pg.627]

Plants can synthesize sugars from acetyl-CoA, the product of fatty acid breakdown, by the combined actions of the glyoxylate cycle and gluconeogenesis. [Pg.782]

It is especially prominent in plants that store large amounts of fat in their seeds (oil seeds). In the germinating oil seed the glyoxylate cycle allows fat to be converted rapidly to sucrose, cellulose, and other carbohydrates needed for growth. [Pg.988]

A key enzyme in the glyoxylate cycle is isocitrate lyase, which cleaves isocitrate (Eq. 13-40) to succinate and glyoxylate. The latter is condensed with a second acetyl group by the action of malate synthase (Eq. 13-38). The L-malate formed in this reaction is dehydrogenated to the regenerating substrate oxalo-... [Pg.988]


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