Qbook, biochem, q543

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Qbook, biochem, q543

Post by khadeja » Sat Apr 07, 2018 11:04 am

the explanation says that the lowering of activation energy increases the rate of BOTH the forward and reverse reaction.
is that always the case? because the reverse reaction can be different; so does the enzyme have this effect always and is that what should be assumed?
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Re: Qbook, biochem, q543

Post by NS_Tutor_Andrew » Tue Apr 10, 2018 12:10 pm

Hi khadeja,

Yes! Absolutely. This is one of my favorite questions and I always try to mention this in Biochem office hours when enzyme kinetics comes up. The idea that reducing the activation energy increases the rate of both the forward and reverse reactions may seem counterintuitive, but it both follows from a graphical analysis of reaction coordinates and is logically necessary for a catalyst or enzyme to be able to speed up a reaction without affecting the equilibrium concentration (which is a point they LOVE to test).

Let's start with the graphical analysis. Draw a simple reaction coordinate, and then lower the Ea. You'll notice that the amount of energy that you have to increase to get over the "hump" of the reaction coordinate diagram (i.e., how far you have to go up the "hill") is less when the Ea is reduced, both when you go from left to right (as in the forward reaction) and when you go from right to left (as in the reverse reaction).

Logically, imagine what would happen if this wasn't the case. If the forward reaction sped up but the reverse reaction didn't, then you would get a shift in the equilibrium to favor more products. But this contradicts what we know about catalysts not being able to change the equilibrium constant, which itself reflects the balance in concentrations between products and reactants. So for the equilibrium constant to remain the same, it must be the case that both the forward and reverse rates are sped up.

I hope this helps clarify this conceptual issue! It's a small one in some ways but it does matter, because they like to try to trick you into ways of mistakenly saying that enzymes change thermodynamic properties of a reaction. I'm a little curious about what you meant by saying that the reverse reaction can be different, though. To clarify, this refers to the forward and reverse directions of a single reaction, not a situation where you can use multiple distinct reaction pathways to get to and from a single compound, like what occurs when gluconeogenesis bypasses the committed steps of glycolysis.
Andrew D.
Content Manager, Next Step Test Prep.
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