BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

6. METABOLIC PHYSIOLOGY

6.12. Mobilization of Reserve Lipids

During the germination of lipid-storing seeds, the neutral fats accumulated in oleosomes are broken down, and the carbon is used to synthesize CARBOHYDRATES. These carbohydrates subsequently cover the metabolic costs of both anabolism and Catabolism in the seedling while it relies on heterotrophic Nutrition. The Cytoplasm, glyoxysomes, and Cell/35.html">Mitochondria all participate in these metabolic reactions. Glyoxysomes, Organelles related to Peroxisomes (see 6.5.6), occur in vast numbers in storage Cells during the mobilization of reserve Lipids; they disappear in the light as Photosynthesis begins and are replaced by peroxisomes, which from that point onward are involved in Photorespiration, among other Functions (see 6.5.6). Recent studies have shown that peroxisomes evolve from glyoxysomes in the light As a result of changes in their enzyme composition.

The mobilization of reserve lipids begins with the hydrolytic release of Fatty acids from triglycerides, catalyzed by lipases. The resulting glycerol is first converted into glycerol-3-phosphate by glycerol-3-kinase utilizing ATP, which is then transformed into dihydroxyacetone phosphate by glycerol-3-phosphate dehydrogenase (see Fig. 6.101). This triose phosphate enters the cytoplasmic sugar METABOLISM.

Fatty acids are released into the cytoplasm and subsequently enter the glyoxysomes (presumably via diffusion through porin-formed

pores permeable to low-molecular-weight metabolites, see 6.5.6), where they are converted into acetyl-CoA via β-oxidation. Unlike animals, plants lack the capacity for mitochondrial β-oxidation; it takes place exclusively in glyoxysomes or peroxisomes. The reaction sequence of this oxidation (Fig. 6.103) closely resembles the mitochondrial pathway, with the sole exception that in mitochondria, the FAD associated with acyl-CoA dehydrogenase—which is reduced to FADH₂ during the reaction—is oxidized by molecular oxygen rather than by the Respiratory Chain (see 6.10.3.3, Fig. 6.96). The reaction product, H₂O₂, is chemically highly aggressive and undergoes disproportionation catalyzed by catalase, which is abundantly present in glyoxysomes and peroxisomes: H₂O + 1/2O₂.

Class="center">Fig. 6.103. The process of β-Oxidation of Saturated fatty acids in glyoxysomes. Complete β-Oxidation of Unsaturated fatty acids requires several additional enzymatic reactions, which will not be discussed here

The subsequent fate of acetyl-CoA is illustrated in Fig. 6.104. In the Glyoxylate cycle (Fig. 6.105), two acetate units from acetyl-CoA are formally condensed into succinate. This reaction sequence has been found, aside from green plants, in Fungi and Bacteria (which are thus able to grow on acetate by using it as a carbon source), but not in animals. Notably, glyoxysomes are present exclusively in Eukaryotic cells.

Fig. 6.104. Reaction scheme for The conversion of neutral fats (= triglycerides) into hexoses and the cellular compartments involved. The phosphoenolpyruvate carboxykinase (PEP carboxykinase) reaction is detailed in Fig. 6.79

Fig. 6.105. Sequence of reactions in The glyoxylate cycle. The major glyoxysomal Enzymes are highlighted against a gray Background

The characteristic Enzymes of the glyoxylate cycle are isocitrate lyase and malate synthase, while the remaining enzymes are familiar from the citrate cycle (see Fig. 6.94). Investigations using cucumber cell cultures have revealed that glucose deprivation activates the genes encoding glyoxylate cycle enzymes. Through this regulatory mechanism, the conversion of fats into carbohydrates is adjusted to meet the physiological demands of the plant.

The succinate produced in the glyoxylate cycle leaves the glyoxysomes via porins and, through a series of Citric Acid Cycle reactions in the mitochondria, is converted into oxaloacetate. In the cytoplasm, oxaloacetate is then transformed into phosphoenolpyruvate by phosphoenolpyruvate carboxykinase. Hexose is subsequently synthesized from this metabolite via Gluconeogenesis (see 6.10.3.4, Fig. 6.98). As a result of this described reaction sequence, theoretically 75% of the carbon from any given fatty acid (three out of four C-atoms) can be converted into hexoses, while the remainder (one out of four C-atoms) is lost as CO₂ during the phosphoenolpyruvate carboxykinase reaction.

Radioactive isotope labeling experiments using ¹⁴C-fatty acids demonstrated that the theoretical maximum yield of fatty acid-to-carbohydrate conversion is indeed achieved experimentally. Thanks to efficient compartmentalization, the plant almost entirely avoids carbon loss through Side Reactions.

In addition to the conversion of fats into carbohydrates observed during the germination of oil-storing seeds, analogous biochemical processes occur during leaf senescence (involving the conversion of Membrane Lipids into carbohydrates to facilitate their export into the trunk) and in tree trunks in the spring to supply the shoots with carbohydrates (regarding the conversion of lipids stored in autumn into carbohydrates and their loading into the xylem, see 6.3.5).