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I read the following about thermogenin:

"When thermogenin is inserted into the inner mitochondrial membrane, it accentuates mitochondrial proton leak and dissipates the proton motive force. Since oxidation is no longer coupled to phosphorylation, thermogenin is said to cause uncoupling. In the presence of thermogenin, oxidation and proton pumping continue at high rates but with low rates of ATP synthesis. So Thermogenin enhances mitochondrial proton leak so there would be heat."

Why does proton pumping without ATP synthesis generate heat?

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    $\begingroup$ please add a link to your source. Is it webpage or book? $\endgroup$ – Oct18 is day of silence on SE Oct 19 '18 at 19:59
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Uncoupling ATP synthesis from the ETC (electron transfer chain; this is where oxidation takes place) by thermogenin or any other method means that the energy used to generate or uphold the proton gradient gets 'lost'.

Since physics teaches us that energy is only transferred and never truly lost, the energy consumed by the ETC must most go somewhere - heat / thermal energy is the easiest and therefore most likely option.

References

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Proton pumps just like any mechanochemical process generates heat, normally much of this heat is reclaimed in the production of ATP. Without thermogenin proton pumps produce a proton gradient, the gradient is used to power atp synthase. thermogenin is a channel protein that creates holes in the membrane preventing a gradient from building up. The pumps never stops or slows so they continuously generate higher amounts of heat, which yields a higher temperature. It is basically the reason an electric motor running full throttle generates more heat than the same one barely running.

Basically brown adipose tissue evolved a way to generate heat by running a "motor" that is not attached to anything. Its not efficient but it is better than not having a way to consistently generate heat.

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  • $\begingroup$ This answer is not correct, the ETC/proton pumps don't slow down normally - because they ARE coupled to the ATP syntahse which 'uses up' the proton gradient, thus a stable equilibrium forms. Also the ETC/proton pumps themselves don't generate a lot of heat, that would be pretty inefficient (and would mean that ALL eukaryotic lifeforms always generate heat, which is not the case). $\endgroup$ – Nicolai Oct 20 '18 at 14:06
  • $\begingroup$ Its coupled to ATP synthase in mitochondria not primarily used in thermoregulation. In brown adipose tissue which has mitochondria focused on the production of heat it is not coupled to ATP synthase or has minimal coupling. the proton pumps are indeed the port generating the heat and it is very inefficient. $\endgroup$ – John Oct 21 '18 at 14:07
  • $\begingroup$ I still don't believe that the ETC (electorn transfer chain / respiratory chain / proton pump) is generating the heat itself, since that would still mean that ALL mitochondria generate heat. Also non of your sources seem to support that claim, they mostly describe that thermogenin allows thermogenesis. $\endgroup$ – Nicolai Oct 21 '18 at 16:45

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