Question

Question 1

a

Why in the absence of oxygen does oxaloacetate alone cause an accumulation of citrate? Would any of the other intermediates in the cycle cause an accumulation of citrate in the presence of oxygen? (5 pts)

b

Toward the end of the paper Krebs states, "While the citric acid cycle thus seems to occur generally in animal tissues, it does not exist in yeast or in E. coli, for yeast and E. coli do not oxidize citric acid at an appreciable rate." Why do you suppose Krebs got this point wrong? (5 pts)

Answer 1

Answer-1a- Absence of oxygen results in the accumulation of citrate, from oxaloacetate because TCA cycle stops in this case as the reducing equivalents such as NAD and FAD become shorter in supply.

TCA cycle in general doesn't require oxygen for its operation but the reducing equivalents like NADH and FADH₂ generated by it, pass on to the electron transport chain (ETC) in order to generate energy in the form of ATP. Oxygen is the final electron acceptor of the electron transport chain and its essential for its operation.

That is why, when oxygen is absent, Oxidative phosphorylation by the ETC doesn't take place and hence stopping the recycling of NADH and FADH₂ to NAD⁺ and FAD. Due to this feedback regulation, TCA cycle doesn't oxidise citrate and and hence, causing its accumulation.

In the presence of the Oxygen, Acetyl-CoA can result in the accumulation of citrate as it is another substrate that combines with the oxaloacetate to form citrate. The citrate formed by this pathway is shuttled to the formation of fatty-acids and glucose further by the enzyme 'citrate-lyase'.

Hence, citrate levels are involved in the maintenance of energy balance based on the availability of the oxygen.

Answer-1b- This is because in E.coli and Yeast, Citrate is obtained from the external environment to be fluxed into the TCA cycle. And the transport of citrate from the outside is regulated by the Citrate transporter (CitT).

Expression of this citrate transporter is regulated by the availability of oxygen:

• In the presence of oxygen, E.coli can't express citrate transporter due to which citrate can't enter the cell and hence, TCA cycle doesn't occur.
• While only in the absence of oxygen, TCA cycle occurs. Hence, E.Coli and Yeast grow anaerobically by fermenting citrate.

Hence, the variation in the rate of the citrate oxidation stated by krebs is not at all a convincing point.