Germinating seeds can convert stored lipids into glucose by a cyclic process known as the glyoxylate cycle. Animal-like vertebrate can’t use the acetate of acetyl CoA into gluconeogenesis. So they can’t convert lipid into glucose via gluconeogenesis, because stored lipid firstly breaks into acetate of acetyl CoA and then enter into gluconeogenesis they have no such process to enter the acetate into gluconeogenesis because their acetate is librated into CO2 via TCA cycle and oxaloacetate which receive acetate reform here no extra formation of succinate or oxaloacetate. However germinating seeds have a different type of cycle to take over this problem, they can convert acetate of acetyl CoA into succinate which will enter into gluconeogenesis via converting into oxaloacetate, the process takes place into glyoxysomes.
Figure: Overview of the Glyoxylate Cycle. Image Source: Wikipedia.
Process of Glyoxylate cycle
This process takes place into glyoxysomes and produced succinate, which enters into mitochondria and converted into fumarate and then malate now this malate enters into the cytoplasm and converted into oxaloacetate which is converted into PEP and enter into gluconeogenesis. The process of glyoxylate cycle has some steps reactions as such the TCA cycle, and also some are different. The detailed process of the glyoxylate cycle is are as follow:
Here in the first step as the citric acid cycle, claisen condensation takes, oxaloacetate receives acetate from acetyl CoA and converted into citrate by the action of citrate synthase enzyme.
This step is also similar to the aconitase step of TCA, here also citrate is converted into isocitrate by the action of the aconitase enzyme.
Here isocitrate dehydrogenase does not work, unlike TCA here a separate enzyme Isocitrate lyase work which breaks isocitrate into, 2 carbon compound glyoxylate, and 4 carbon compound succinate.
In this step entry of another acetate from Acetyl CoA take place, glyoxylate receives acetate from Acetyl CoA and converted into four-carbon compound malate.
In this step malate is converted into oxaloacetate by the action of malate dehydrogenase, hence the NAD+ reduced into NADH, and oxaloacetate is restored, which can enter into cyclic repeat.
It is the complete Glyoxylate cycle but here we have to talk about Succinate, which is produced in this cycle.
- Succinate enters into the glycolytic pathway via some step of the TCA cycle. Succinate is converted into fumarate by the action of succinate dehydrogenase and FAD reduced into FADH2, inside the mitochondria.
- Now fumarate is converted into malate by the action of the fumarase enzyme.
- Now this malate is converted into oxaloacetate in presence of malate dehydrogenase enzyme and NAD+ is reduced into NADH.
- Now this oxaloacetate can enter into gluconeogenesis.
And this plant can convert stored lipids into glucose without the extra cost of energy. And NADH FADH produced in this cycle can produce ATP via ETS and some of this ATP is utilized during gluconeogenesis. While vertebrate gluconeogenesis is a very costly process and needs extra energy, germinating seed takes over this problem by glyoxylate cycle and can utilize Acetyl CoA in gluconeogenesis.
The Citric acid cycle (TCA) and glyoxylate cycle are coordinately regulated because intermediate products of glyoxylate can enter into TCA. And succinate enters into gluconeogenesis via some step of TCA.
- Lehninger Principle of Biochemistry by David L. Nelson and Michael M. Cox, 6th edition
- 2% – https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Metabolism/Catabolism/Kreb’s_Cycle
- 1% – https://www.sciencedirect.com/topics/engineering/oxaloacetate
- 1% – https://www.news-medical.net/life-sciences/Citric-Acid-Cycle-Regulation.aspx
- 1% – https://www.britannica.com/science/glyoxylate-cycle
- 1% – https://en.wikipedia.org/wiki/Succinic_Acid