The Glyoxysome

Enzymes (1) Lipases (2) Fatty acid thiokinase (3) Acyl CoA dehydrogenase (4) Crotonase (5) jS-Hydroxyacyl CoA (6) )5-Ketoacyl thiolase (7) Citrate synthetase (8) Aconitase (9) Isocitrate lyase (10) Malate synthetase (11) Malate dehydrogenase (12) Catalase (13) Succinate dehydrogenase (14) Fumarase (15) Malate 

Coenzymes and energy suppliers FADI(H) flavin adenine dinucleotide/(reduced) NADI(H) nicotinamide adenine dinucleotide/(reduced) GTP guanosine triphosphate ATP adenosine triphosphate UTP uridine triphosphate GDP guanosine diphosphate ADP adenosine diphosphate AMP adenosine monophosphate CoA coenzyme A. Based on Ching, 1972 [5] 

At the time that the glyoxysome particle was discovered, it was assumed that jS-oxidation of fatty acids was completed in the mitochondrion and the resultant acetyl CoA transported to the glyoxysome for conversion to succinate. But it was difficult to explain how this acetyl CoA avoided complete oxidation in the mitochondrion or even how it could be transferred from one organelle to another. The problem was resolved when it was shown that j5-oxidation also occurs in the glyoxysome and that the acetyl CoA is produced and consumed in the same organelle [44, 72]. 

Activity of phosphofructokinase and fructose 1,6-diphosphatase in mammals can be finely controlled within the cell to regulate glycolysis and gluconeogenesis, and there is some evidence to suggest a similar control mechanism in seedling tissues [53a]. 

8 Cotyledons expanded, 121 hypocotyl elongated, plant etiolated 1054 2.20 894 332 

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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... 

Microbodies (97-101) are spherical organelles (0.1-2.0 pm in diameter) bounded by a single membrane. They possess a granular interior and sometimes crystalline protein body. A specialized type of microbody is the glyoxysome (0.5-1.5 pm) containing enzymes ofthe glyoxy-late cycle. Glyoxysomes are found in the endosperm or cotyledons of oily or fatty seeds. 

Using the so-called glyoxylic acid cycle, plants and bacteria are able to convert acetyl-CoA into succinate, which then enters the tricarboxylic acid cycle. For these organisms, fat degradation therefore functions as an anaplerotic process. In plants, this pathway is located in special organelles, the glyoxysomes. 

Topic LI). Thus, animals cannot convert fatty acids into glucose. In contrast, plants have two additional enzymes, isocitrate lyase and malate synthase, that enable them to convert the carbon atoms of acetyl CoA into oxaloacetate. This is accomplished via the glyoxylate pathway, a route involving enzymes of both the mitochondrion and the glyoxysome, a specialized membranous plant organelle. 

One class of such organelles is the glyoxysome which is prominent in a wide range of storage tissues, e.g. cotyledons of castor bean, water melon, peanut and cucumber and in pine seeds (Galliard, 1980). The glyoxysome contains all the enzymes of the -oxidation complex, together with the fatty acid... 

The lipolytic process in germinating oil seeds is still unclear. How seed oils, present in oil bodies, are converted to products which can then be handled by the fatty acid oxidation system of glyoxysomes, is not clear. It appears that membrane-bound lipases are involved in the process. A further area for clarification is the intracellular transport of insoluble triacylglycerols from oil bodies to the site of complete (or partial) hydrolysis and thence to the glyoxysomal enzymes of gluconeogenesis. Recent electron microscope studies with several oil seeds have provided evidence for a close association of ribosome-containing membranes with spherosomes these may be lipolytic membranes involved in triacylglycerol breakdown (Wanner and Theimer, 1978). 

Thus, all the processes of fatty acid activation and oxidation are efficiently concentrated as components of the glyoxysomal system converting lipid to carbohydrate precursors, as described by Beevers (this volume. Chapter 4). 

Whereas mitochondrial oxidation utilizes NAD" and flavoproteins coupled to the oxidative phosphorylation system, the glyoxysomal system differs in two m jor respects (Fig. 5). First, the flavoprotein reduced in the acyl-CoA dehydrogenase step is oxidized directly by molecular oxygen with the formation of HjOj, which is then degraded by catalase second, NADHj produced in the oxidation of 2-hydroxyacyl-CoA is not reoxidized in the glyoxy-somes thus an external NAD -NADH redox system is required. 

Fig. 2. (A) The glyoxylate cycle as a bypass of the TCA cycle (after Komberg and Krebs, 1957). (B) The glyoxylate cycle as it functions in the glyoxysome, showing the production of succinate from 2 mol of acetyl-CoA. The five steps constituting the cycle are catalyzed by the following enzymes (1) citrate synthetase, (2) aconitase, (3) isocitrate lyase, (4) malate synthetase, (5) malate dehydrogenase.

Further investigation of the glyoxysome has revealed that it also contains all the cellular catalase, as well as glycolate oxidase and urate oxidase (Breidenbach et al., 1968 Theimer and Beevers, 1971). These are the distinctive enzymes of organelles first recognized in electron micrographs of mamma-... 

Thus we are now in a position not only to delineate the pathway of conversion of fat to sucrose with confidence but to know where within the cells the various stages of the sequence occur (Fig. 4). The overall pathway depends on the cooperation among the spherosomes, where the stored fat is hydrolyzed by an acid lipase in the membrane, the glyoxysomes, the mitochondria, and enzymes in the cytosol. This cooperation implies specific movement of metabolites among different cellular compartments, but these transport processes are not yet understood. 

There is a scarcity of data for the analyses of glyoxysomal membranes and plasma membranes of plant cells. The glyoxysomal membrane from castor bean (Ricinus communis L.) endosperm has PC as the major lipid together with PE, 80% of the phospholipid can be accounted for. Plasma membranes isolated from oat Avena saliva L.) roots showed relatively lai ge amounts of phosphatidic acid (PA), (1,2-diacyl-jn-glycerol 3-phosphate) and an unknown lipid (Keenan et al., 1973). It is possible that some lipid degradation took place in the preparation of this sample. Further analyses of the lipid composition would be most appropriate. The analyses of Keenan et al. (1973) indicated that only 28.6% of the plasma membrane lipid was phospho-... 

Isocitrate lyase has been purified from the glyoxysomes of cucumber (Cucumis sativus) cotyledons. The glycoproteinaceous nature of the enzyme was demonstrated by periodate oxidation studies, and by the in vivo incorporation of 2-amino-2-deoxy-D-[ H]glucose into a species which was precipitable by antibodies to isocitrate lyase. 

Since fats are stored in oil bodies, it is reasonable to expect that the enzyme responsible for their degradation should be closely associated with these structures. During the first 11 days after imbibition a peanut cotyledon may decrease in dry weight from 345 mg to 143 mg, with a concomitant decrease in fat content of 55%. This represents hydrolysis of 9.4 pmoles of triglyceride per cotyledon per day. Even so, less than 1% of the total lipase activity of a peanut cotyledon has been found associated with the oil body and 99% is associated as an acid lipase (pH 4.6) with a particulate fraction [76]. This fraction has been claimed to be mitochondrial, but that is unlikely. The precise location of the acid lipase is still undetermined but it could be associated with the glyoxysomes. 

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