Pasteur Effect

[dttom]

Pasteur effect states that the presence of oxygen would suppress the glycolytic rate. Modern interpretation is that oxygen allows procession of Krebs cycle and electron transport chain, generating an amount of ATP far exceeding that from glycolysis alone. ATP in turns suppresses a rate-controlling enzyme in glycolysis of phosphofructose kinase 1 allosterically.

 

I was given a graph showing periodic relationship between ATP and NADH, which is a product of glycolysis and serves as an indicator of the procession of glycolysis. The graph shows an exactly out-of-phase relationship. However, I think that, according to the mechanism suggested, the kinase has to be given a period of time to 'feel' the suppressing effect of ATP, so the NADH curve should be slightly differing from out-of phase relationship by lagging a little bit.

 

Could anyone comment on this?

[CharonY]

Are you sure you got your definitions right? IIRC the Pasteur effect was referring to the inhibition of fermentation during aeration. Glycolysis of course also works under aerobic condition. Also, the Krebs cycle is normally active under both conditions, as it provides critical precursors for other anabolic pathways. What is true, however, is that the ATP generated during ethanol fermentation is gained from glycolysis, rather than the actual fermentation process. This does not make glycolysis a fermentative process per se, though.

Also I am surprised that you imply that the phosphofructokinase is inhibited by ATP, as it requires to transfer the phosphate group (in my memory there is some feed-back inhibition by PEP, though).

I am not quite sure how the graphs you are talking about look like, but what I think may be the point is the following: glycolysis produces precursors for the TCA as well as ATP. TCA produces reduction equivalents (and precursors for other pathways that are of no interest here).

Reductions equivalents are used in oxidative phosphorylation if possible, if not they have to be regenerated via fermentation (some fermentative processes generate additional ATP, others do not).

[dttom]

The definition I got it from my professor, but when I re-check it you are true that the effect is on fermentation, but as you said the energy-generating process is glycolysis so other description would be the same. Again ATP deprived from TCA and oxidative phosphorylation is not available in the absence of O2, hence ATP level drops, negative effect on phosphofructose kinase 1 relieved and NADH level rises (an indicator of glycolysis); but still I think there should be a delay. (what I think does not fit the graph explanation which is suggested by the professor) About how the graph is actually look like, I've drawn a graph here, but the actual one is not as perfect as this one, with random irregularity, but the exactly out-of-phase relationship could still be observed:

Edited February 23, 2010 by dttom

[CharonY]

The definition I got it from my professor, but when I re-check it you are true that the effect is on fermentation, but as you said the energy-generating process is glycolysis so other description would be the same.

It is not quite the same as the fermentation products are neglected when just focusing on glycolysis.

 

 

Respiration is often conveniently described as starting from glucose and ending with oxidative phosphorylation in some textbooks but that is an incorrect description.

 

For instance NADH does not necessarily mean that glycolysis is happening as it will also be produced during the TCA cycle. And the TCA cycle itself can be fueled by substrates other than sugars. In other words the glycolysis to energy image is correct, but only part of the picture (and especially for microorganism a lot of variations exist).

What is happening is slightly more complicated as the inhibitory effect of ATP is not limited to the PFK (which occurs at very high concentrations of ATP, I confused enzymes earlier, my bad), but the TCA cycle itself is also inhibited (e.g. the citrate synthase). Also note that this is not an on/off switch, but a gradual shift of the equilibria so that the gradual decline of one will result at the same time in the gradual increase of the other. So it is not that at some point the inhibitory effects suddenly ceases and only after that an increase of NADH occurs but is is a simultaneous shift in equilibrium reactions for each involved protein. Imagine the ratio of inhibited to non-inhibited enzymes shifting under each condition and you will get the picture.

[dttom]

I can understand it is a gradual process so when one increases another decreases, but what I doubt of is the timing, I think the quality which follows another quality should show some lagging behavior but not in simultaneous manner...