Heat slowing down chemical reaction?

[Forensicsarefun]

I was doing a science experiment involving enzymes and hydrogen peroxide, and by adding heat to the catalase, it should have sped up the reaction, but instead it slowed it down. Is there a reason why this happened?? Isn't heat supposed to speed up the reaction, instead of slowing it?

 

[Charles Link]

If the reaction is exothermic, introducing heat will shift the equilibrium so that the reaction won't occur so completely. In some cases, it could reduce the rate that the reaction proceeds.

 

[TeethWhitener]

Is there a reason why this happened??

At some temperature, you'll start to denature (unfold) the protein and it will lose its catalytic function dramatically (and often irreversibly). Google "enzyme kinetics temperature dependence" for more info.

 

[Forensicsarefun]

ok thank you. But would this happen at 20oC??

 

[TeethWhitener]

But would this happen at 20oC

Are you saying you heated the reaction from some low temperature up to 20°C? Did you cool the reaction first? The reason I ask is because 20°C is a little on the cool side of room temperature. I'd need more experimental details to understand what was going on.

 

[Forensicsarefun]

The catalase was a mixture of ice, water, potato, and salt. The ice in the mixture cooled it below room temp. The control test, was just leaving the catalase as it was, but the second test was heating it to 20oC.

 

[TeethWhitener]

It still sounds like maybe something is denaturing the protein (though I doubt it's heat: protein can be denatured through any number of chemical means as well). Once the mixture is warmed to room temp, if you cool it back down, does the catalytic activity speed back up?

 

[Forensicsarefun]

yes it did - for another test, I cooled it down to room temp, and it sped back up

 

[TeethWhitener]

yes it did - for another test, I cooled it down to room temp, and it sped back up

This seems to imply that it was heated above room temperature, whereas before it was cooled below room temperature. In one instance, it sounds like the reaction slowed down as room temperature was approached from below, and in another instance it sped up as room temperature was approached from above. I would be curious to know if the rates at room temperature were the same for each experiment. To me, it sounds like some sort of reversible denaturation: essentially, as you heat a protein up, it can undergo conformational changes (changes in shape) that affect its catalytic activity. Usually these are irreversible (an example would be cooking an egg: once the proteins are denatured, there's no way to "uncook" the egg). However, it's not out of the realm of possibility that some mild temperature changes would result in reversible conformational changes, such that as the temperature was restored to its original value, the observed reaction rate would approach its original value as well. That's just my best guess, though. But keep in mind that since enzymes are so sensitive to lots of different experimental parameters, the experiments are particularly unforgiving in terms of sloppy lab technique. You have to be careful to make sure the variable you want to observe is actually isolated.