Gluconeogenesis glyoxylate cycle

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

In plants, these reactions take place in organelles called glyoxysomes. Succinate, released midcycle, can be converted into carbohydrates by a combination of the citric acid cycle and gluconeogenesis. Thus, organisms with the glyoxylate cycle gain a metabolic versatility. 

Many aroma compounds in fruits and plant materials are derived from lipid metabolism. Fatty acid biosynthesis and degradation and their connections with glycolysis, gluconeogenesis, TCA cycle, glyoxylate cycle and terpene metabolism have been described by Lynen (2) and Stumpf ( ). During fatty acid biosynthesis in the cytoplasm acetyl-CoA is transformed into malonyl-CoA. The de novo synthesis of palmitic acid by palmitoyl-ACP synthetase involves the sequential addition of C2-units by a series of reactions which have been well characterized. Palmitoyl-ACP is transformed into stearoyl-ACP and oleoyl-CoA in chloroplasts and plastides. During B-oxi-dation in mitochondria and microsomes the fatty acids are bound to CoASH. The B-oxidation pathway shows a similar reaction sequence compared to that of de novo synthesis. B-Oxidation and de novo synthesis possess differences in activation, coenzymes, enzymes and the intermediates (SM+)-3-hydroxyacyl-S-CoA (B-oxidation) and (R)-(-)-3-hydroxyacyl-ACP (de novo synthesis). The key enzyme for de novo synthesis (acetyl-CoA carboxylase) is inhibited by palmitoyl-S-CoA and plays an important role in fatty acid metabolism. 

OAA is an intermediate in several important pathways, including gluconeogenesis, citric acid cycle, glyoxylate cycle, urea cycle, and amino acid metabolism (see here). 

Because the decarboxylation reactions are bypassed, the two carbons lost during each turn of the citric acid cycle are retained in the glyoxylate cycle. In fact, the glyoxylate cycle results in the net synthesis of oxaloacetate, a four-carbon molecule, because each turn of the cycle incorporates two molecules of acetyl-CoA. The oxaloacetate can then can be used for other purposes, such as gluconeogenesis. Animal cells can use oxaloacetate from the citric acid cycle for gluconeogenesis too, but there is no net synthesis of glucose from acetyl-CoA because for every carbon introduced via acetyl-CoA, one is lost via C02. 

There are several possible reasons why storage lipid is mobilized and the glyoxylate cycle stimulated following nitrate addition to N-starved cultures. These are listed as follows 1.) to replenish, via gluconeogenesis, carbohydrates exhausted during nitrate assimilation 2.) to replenish, anaplerotically, TCA cycle intermediates drawn off for protein synthesis during rapid nitrate assimilation 3.) to increase, via malate glycolysis, the amount of cytosolic NADH available for NR. These possibilities are detailed in Fig.3. 

The pathways in the central carbon metabolism involve TCA cycle and glyoxylate shunt, glycolysis, phosphotransferase system (PTS), gluconeogenesis, pentose phosphate pathway (PPP).The carbon flux partitions at different nodes in the central metabolism and major flux partitioning for the product of interest may occur at principal branch points. Engineering of the enzymes in these branch points of the biosynthetic pathways will direct the carbon flux toward the product of interest leading to maximal product yield. 

---