Glyoxylate cycle and Entner-doudoroff pathway

Glyoxylate cycle:

The conversion of phosphoenolpyruvate to pyruvate and of pyruvate to acetyl-CoA are so exergonic as to be essentially irreversible. If the cell cannot convert acetate into phosphoenolpyruvate, acetate cannot serve as the starting material for the gluconeogenic pathway that leads from phosphoenolpyruvate to glucose without this capacity a cell or organism is unable to convert fuels that are degraded to acetate into carbohydrate phosphoenolpyruvate can be synthesized from oxaloacetate in the reversible reaction catalyzed by PEP carboxykinase.

Oxaloacetate + ATP ↔ phosphoenolpyruvate + CO₂+ ADP

Because carbon atoms from acetate molecules that enter the citric acid cycle appear 8 steps later in the oxaloacetate. It might appear that operation of TCA cycle could generate oxaloacetate from acetate and thus generate phosphoenolpyruvate for gluconeogenesis. However, examination of the stoichiometry of the cycle reveals that there is no net conversion of acetate into oxaloacetate through the cycle; for every 2carbons that enters the cycle as acetyl CoA, two leaves as CO₂.

In plants, in certain invertebrates and in some microorganism such as E. Coli and yeast, acetate can serve both as energy-rich fuel and as a source for phosphoenol pyruvate for carbohydrate synthesis. These organisms have a pathway glyoxylate cycle that allows the net conversion of acetate to oxaloacetate.

The glyoxylate pathway is essentially a bypass of the CO₂ evolving steps of the tricarboxylic acid cycle. In fact, the pathway is often called as “glyoxylate bypass”. The enzymes are localized in organelles called glyoxysomes. Two moles of acetate are required for each turn of the pathway rather than one mole of acetate required in the TCA cycle. The first mole of acetate condenses with oxaloacetate to form citrate which is then converted to isocitrate. The inducible enzyme isocitrate to succinate and glyoxylate. The glyoxylate reacts with the 2^nd mole of acetate (acetyl) CoA to produce malate, which is then converted back to oxaloacetate.

This bypass set of reaction enables plant and microorganism to transform acetyl-CoA derived from the breakdown of fatty acids into carbohydrates. The excess succinate provided by the glyoxylate pathway can be channeled back up the glycolytic pathway for the formation of sugar and subsequently polysaccharides.

Entner doudoroff pathway:

• Only in prokaryotes

Eg:pseudomonas, acetobacter,etc.

• Alternative pathway for glucose metabolism
• Micheal Doudoroff and Nathan either.

Most bacteria have the glycolytic and pentose phosphate pathways but some substitute at Entner-Doudoroff pathway for glycolysis. This pathway was first reported by Michael Doudoroff and Nathan enter in pseudomonas saccharopine. This pathway is only found in prokaryotes. The Entner Doudoroff, rhizobium, Acetobacter, agrobacterium, escherichia Coli, Zymomonas Moviles, xanthomonads, completes and a few other gram –ve genera. Very few gram +ve bacteria have this pathway with enterococcus fecal being a rare exception. The EntnerDoudroff pathway begins with the same reaction as the pentose phosphate pathway. The formation of glucose 6 phosphate and 6phosphogluconate. The reactions occurring in the Entner Doudoroff pathway are:

1. Glucose is phosphorylated by enzyme hexokinase and is the irreversible reaction.
2. Glucose 6 phosphates are oxidized by glucose 6 phosphate dehydrogenase to 6phosphogluro acetone (producing NADPH) and this is then hydrolysed by lactonase to 6-phosphogluconate.

Instead of being further oxidized, 6-phosphogluconate is dehydrated to form 2 keto3-D oxy-6-phosphogluconate or KDPG. The key intermediate in this pathway in the presence of 6-phosphogluconate dehydratase.

KDPG is then cleaved by aldolase enzyme to pyruvate and glyceraldehyde 3-phosphate. The glyceraldehyde 3-phosphateis converted to pyruvate similar to the reactions of the glycolytic pathway.

[Note: include glycolytic pathway starting from glyceraldehyde 3-phosphate]

If the Entner-Doudoroff pathway degrades glucose to pyruvate in this way, it yields 1ATP, 1NADPH, and 1NADH per glucose metabolites.

The NADH and FADH₂ form in the glycolysis, fatty acid oxidation, and the citric acid cycle are energy rich molecules because they contain a pure of electrons that have a high transfer potential. When these electrons are transferred to molecular oxygen, a large amount of energy is liberated. This released energy is used for various cellular function. Oxidative phosphorylation is the process in which ATP is formed s electrons are transferred from NADH, FADH₂ by a series of electron carriers. This is the major source of ATP in aerobic organisms. For eg., oxidative phosphorylation generates 32 of the 36 ATP that is formed when glucose is completely oxidized to CO₂ and water.

The electrons are transferred using various proteins and components namely:

Complex I – NADH dehydrogenase

Complex II– Succinate dehydrogenase

Complex III- cytochrome reductase

Complex IV- cytochrome oxidase

C- cytochrome C complex

The energy is synthesized by ATP synthetase.

Chemiosmotic coupling hypothesis of ATP synthetase:

This is a simpler radically different and novel mechanism which was positively by peter Mitchell a British biochemist in 1961. He purposed that electron transport and ATP synthesis are coupled by a proton gradient rather than other mechanism postulated at that time. According to this model, the transfer of electrons through the respiratory chain result in the pumping of the protons (H⁺) from the matrix site to the cytoplasmic side of an inner mitochondrial membrane. The concentration of H⁺ becomes higher on the cytoplasmic site. Thus, creating an electrochemical potential difference. This consist of a chemical potential (difference in pH) which becomes positive on the cytoplasmic site. Due to this uneven distribution of pH, H⁺ flow from ATP synthetase in which free energy is released. The free energy released is used to phosphorylate ADP into ATP using ATP synthetase enzyme.

References:

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.