Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Citric Acid Cycle
The glyoxylate cycle is a modification of the citric acid cycle

In plants and certain microorganisms, such as E. coli, acetyl groups frequently serve not only as a high-energy "fuel" but also as a source of metabolites used to build the carbon skeletons of CARBOHYDRATES. Two variants of The Citric Acid Cycle operate in such Cells: 1) the standard sequence of reactions that oxidizes acetyl-CoA to CO₂, characteristic of most Tissues, and 2) a specialized modification known as the Glyoxylate cycle (Fig. 16-18). The overall equation for the glyoxylate cycle, which can also be viewed as an anaplerotic pathway, is

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Fig. 16-18. The glyoxylate cycle. Reactions Catalyzed by isocitrate lyase (also called isocitratase) and malate synthase are highlighted in red. All other reactions are identical to those of The Citric Acid cycle.

In the glyoxylate cycle, acetyl-CoA condenses with oxaloacetate to yield citrate (Fig. 16-18). However, unlike in the citric acid cycle, isocitrate is not cleaved via the standard isocitrate dehydrogenase reaction; instead, it is broken down by the enzyme isocitrate lyase to form succinate and glyoxylate. The resulting glyoxylate then condenses with another molecule of acetyl-CoA to produce malate, a reaction catalyzed by malate synthase (Fig. 16-19). Malate is subsequently oxidized to oxaloacetate, which can condense with a new molecule of acetyl-CoA to initiate another turn of the cycle. With each turn of the glyoxylate cycle, two molecules of acetyl-CoA enter and one molecule of succinate is produced, which is then utilized in biosynthetic processes. Succinate can be converted via fumarate and malate into oxaloacetate, from which phosphoenolpyruvate is formed by the Reversal of the phosphoenolpyruvate carboxykinase reaction described above. Phosphoenolpyruvate serves as a precursor in glucose Biosynthesis (Ch. 20). Animals lack the glyoxylate cycle entirely, possessing neither isocitrate synthase nor malate lyase in their cells; instead, animal organisms utilize alternative pathways for synthesizing carbohydrates from simple precursors (Ch. 20). In contrast, the glyoxylate cycle is highly active in germinating seeds, where it converts acetyl groups (derived from the Fatty acids of stored triacylglycerols) into glucose. The Enzymes isocitrate lyase and malate synthase are localized within specialized cytoplasmic Organelles in plant cells known as glyoxysomes.