Carbohydrate Metabolism: Citric Acid Cycleand Electron Transport

The respiratorypathways in the plant mitochondriainclude the citricacid cycle, the electron transportchain and ADP phosphorylation.

Pyruvate and malate from glycolysis are imported into themitochondrion.

Herzig et al. “Identification and Functional Expression of the MitochondrialPyruvate Carrier”, Science 337:93-96 (6 July 2012)

The protein responsible for import of pyruvate into mitochondria has only recently been discovered.

Major products of theCAC are NADH, FADH2, and ATP.

Per each pyruvate,4 NADH, 1 FADH2,1 ATP

NADH and FADH2 are oxidized in the electron transport chain.

Notice the 10 H+ exported across the membrane for each NADH oxidized.

Notice the “uncouplingprotein”: what would be the effect of activating this protein?

Alternative oxidase (AOX) Pathway: Cyanide Resistant Respiration

Physiological roles:

Thermogenesis--oxidation releases energy without coupling it to formationof ATP. The energy appears as heat. Occurs in certain family membersof the Araceae, where tissues of the spadix heat and release odors.

Prevention of over-oxidation--reduces the probability of superoxide formationby reduced complex IV when ATP consumption is slowed by low temperature or other stresses.

Amorophophallistitanum

Here is a true mystery: one fall, a silo filled with wheatexploded and was completely destroyed. The company thathandled the silo’s insurance looked for accelerants and other signs of sabotage, but without success. The weather was cool,and it had rained for two days before. What was the cause of the explosion?

Here is a true mystery: one fall, a silo filled with wheatexploded and was completely destroyed. The company thathandled the silo’s insurance looked for accelerants and other signs of sabotage, but without success. The weather was cool,and it had rained for two days before. What was the cause of the explosion?

An investigation by the claims adjuster found that a roof leakwas reported two weeks before. Is this significant?

Here is a true mystery: one fall, a silo filled with wheatexploded and was completely destroyed. The company thathandled the silo’s insurance looked for accelerants and other signs of sabotage, but without success. The weather was cool,and it had rained for two days before. What was the cause of the explosion?

An investigation by the claims adjuster found that a roof leakwas reported two weeks before. Is this significant?

A plant physiologist explained that the moisture had mostlikely stimulated germination of the wheat. Germinating seedsrespire, and the respiration releases heat: ΔGo’ = -2840 kJ/mol. The heat from the complete respiration of just 1 kg of starch is(1000/168)(2840) = 17,000 kJ. The heat build-up was sufficient to explode the wheat and the structure.

The reactions of the citric acid cycle are under allosteric control.NADH inhibits the three dehydrogenasesin the cycle. ATP inhibits the electrontransport chain. ADPand Pi stimulate the ETC and pyruvateoxidation.

The citric acid cycle, like glycolysis and the PPP,provides intermediatesfor many essentialcell components.

Summary

Pyruvate from glycolysis is imported into mitochondria, oxidized by pyruvate oxidase and the citric acid cycle.

The NADH produced is oxidized by the electron transport chain,forming an e- gradient (like that across thylakoid membranes); the e- gradient is used to power ATP synthesis.

The maximum yield of ATP/sucrose is about 60.

Uncoupling protein and the alternative oxidase reduce the yield of ATP/NADH and release heat.

Reactions in the citric acid cycle, as in glycolysis, are underallosteric control.

Glycolysis and the CAC produce intermediates for severalbiochemical pathways.