Glyoxysomes are found to occur in the cells of yeast, Neurospora, and oil-rich seeds of many higher plants. They resemble peroxisomes in morphological details, except that, their crystalloid core consists of dense rods of 6.0 μm diameter.
Functions of Glyoxysomes
Glyoxysomes perform following biochemical activities of plants cells:
1. Fatty acid metabolism.
During germination of oily seeds, the stored lipid molecules of spherosomes are hydrolyzed by the enzyme lipase (glycerol ester hydrolase) to glycerol and fatty acids. The phospholipid molecules are hydrolyzed by the enzyme phospholipase.
The long chain fatty acids which are released by the hydrolysis are then broken down by the successive removal of two carbon or C2 fragments in the process of β-oxidation.
During β-oxidation process, the fatty acid is first activated by enzyme fatty acid thiokinase to a fatty acyl-CoA which is oxidized by a FAD-linked enzyme fatty acyl-CoA dehydrogenase into trans-2-enoyl-CoA.
Trans-2-enoyl-CoA is hydrated by an enzyme enoyl hydratase or crotonase to produce the L-3- hydroxyacyl-CoA, which is oxidized by a NAD linked L-3-hydroxyacyl- CoA dehydrogenase to produce 3-Keto acyl-CoA.
The 3-keto acyl-CoA loses a two-carbon fragment under the action of the enzyme thiolase or β-keto thiolase to generate an acetyl-CoA and a new fatty acyl-CoA with two less carbon atoms than the original.
This new fatty acyl-CoA is then recycled through the same series of reactions until the final two molecules of acetyl-CoA are produced. The complete β-oxidation chain can be represented as follows:
In plant seeds β-oxidation occurs in glyoxysomes. But in other plant cells, β-oxidation occurs in glyoxysomes and mitochondria. The glyoxysomal β-oxidation requires oxygen for oxidation of reduced flavoprotein produced as a result of the fatty-acyl-CoA dehydrogenase activity.
In animal cells β-oxidation occurs in mitochondria. In-plant cells, the acetyl-CoA, the product of the β-oxidation chain is not oxidized by the Krebs cycle, because it remains spatially separated from the enzymes of the Krebs cycle, instead of it, acetyl-CoA undergoes the glyoxylate cycle to be converted into succinate.
2. Glyoxylate cycle.
The glyoxylate pathway occurs in Glyoxysomes and it involves some of the reactions of the Krebs cycle in which citrate is formed from oxaloacetate and acetyl-CoA under the action of citrate synthetase enzyme.
The citrate is subsequently converted into isocitrate by aconitase enzyme. The cycle then involves the enzymatic conversion of isocitrate to glyoxylate and succinate by isocitratase enzyme:
The glyoxylate and another mole of acetyl-CoA form a mole of malate by malate synthetase:
This malate is converted to oxaloacetate by malate dehydrogenase for the cycle to be completed. Thus, overall, the glyoxylate pathway involves:
2 Acetyl-CoA + NAD+ → Succinate + NADH + H+
Succinate is the end product of the glyoxysomal metabolism of fatty acid and is not further metabolized within this organelle. The synthesis of hexose or gluconeogenesis involves the conversion of succinate to oxaloacetate, which presumably takes place in the mitochondria since the glyoxysomes do not contain the enzymes fumarase and succinic dehydrogenase.
Two molecules of oxaloacetate are formed from four molecules of acetyl-CoA without carbon loss. This oxaloacetate is converted to phosphoenolpyruvate in the phosphoenolpyruvate carboxykinase reaction with the loss of two molecules of CO2:
2 Oxaloacetate + 2ATP ⇌ 2 Phosphoenol pyruvate + 2CO2 + 2ADP
The phosphoenol pyruvate is converted into monosaccharides (e.g., glucose, fructose), disaccharide (sucrose) and polysaccharide (starch) by following reaction: