[Charkol]

I have been told by both my teacher and book that enzymes do not affect the equilibrium constant of the reaction that they catalyze.  I can accept this if I must, but I don't understand it.

Some enzymes totally encapsulate their substrate, secluding it away from the solution from which it it was plucked.  If this is the case in some instances, wouldn't the enzyme do it's thing regardless of the concentration of products as long as a substrate(s) found their way into the enzyme?

[fledarmus]

Catalysts don't drive reactions - what they do is lower the energy of the transition state of the reaction, so if the reaction is going to occur, it will occur faster. The equilibrium of a reaction is determined solely by the difference in energy between the reactant and the product. The speed with which the equilibrium is established is determined by the energy required to reach the transition state.

In most useful enzymatic reactions, the product is enough more stable than the reactant that the equilibrium concentration of the reactant is very small. Once you lower the barrier to the reaction far enough to get the reaction to occur, it goes essentially to completion.

[Yggdrasil]

You also have to remember that enzymes will catalyze the reverse reaction as well.  So the enzyme will also do its thing when products find their way into the enzyme.