[site]

I read that no enzyme shifts the equilibrium of a chemical reaction.  This implies that every enzyme increases the rate of the forward reaction (reactants>>products) exactly as much as it increases the rate of the reverse reaction (products>>reactants). 

How is this possible?  The standard model for an enzyme has one active site, which accepts exactly one set of reactants and facilitates their transformation into products.  Is there always also a corresponding active site that turns those same products back into reactants that is just as effective as the principal active site?  I've never heard of this second active site, but if it does not exist it seems like enzymes would only increase the rate of the forward reaction and thus shift equilibrium. 

[Borek]

Active site fits both reactants and products.

[Dan]

Catalysts lower the activation energy by stabilising the transition state of the reaction.

Consider:



A is the starting materials
B is the transition state
C is the products

and the equilibrium A B

Reaction rate depends on the activation energy.

1. How does this plot change if a catalyst is added?
2. What effect does this have on the forward reaction (A B)?
3. What effect does this have on the backward reaction (B A)?

[site]

I assumed that the lower activation energy referred only to the forward reaction, as the substrate complex is molds itself around the reactants.  So it is true that the active site will receive either the reactant set or the product set?

[Dan]

The transition state is the same in both directions. If the product must be able to fit the active site, otherwise it could not form inside the active site in the first place.

[AWK]

Catalase works irreversibly

[site]

Let's say hydrogen peroxide and water are in equilibrium.  Then if you add catalase, the forward reaction (H2O2 >> water) would speed up but the reverse reaction would not speed up.  So wouldn't equilibrium shift to favor water?

[Borek]

I doubt catalase irreversibility has other reasons than those commonly seen in the irreversible reactions - like huge increase in entropy, separation of products and so on. So no, equilibrium doesn't shift. Even without catalase it lies very far to the right.