In a TPR book, it says that they do NOT act on the transition state, yet it "stabilizes the transition state". Anyone?

Enzymes lower down the activation energy of a reaction by stabilizing the transition state.

Well enzymes bind more tightly to transition states than either the reactants or the products

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enzymes bind the reactants and have them in favorable positions to achieve the post-transition state

Enzymes lower down the activation energy of a reaction by stabilizing the transition state.

That's the explanation that I'm familiar with.

The simple explanation is:

The larger the difference between starting point and the transition state (the top of the curve), the less stable the transition state is (being so high in energy); Therefore if you lower activation energy (as an enzyme does by favourable aligning two molecules to react in a lock and key sort of way), the peak drops since activation energy drops therefore the transition state becomes more stable.

That kinda puts everything together mentioned before.

Well enzymes bind more tightly to transition states than either the reactants or the products

this is true. in biochem, i learned that analogs most resembling the t-state had the greatest affinity for the enzyme active site.

stabilizing the transition state simply means that the transition state WITHOUT the enzyme is less stable than the the transition state WITH the enzyme.

usually, instability arises from partial +/- charges which are stabilized by charged residues at the enzyme's active site

For purposes of the MCAT, know that they lower the activation energy and do not change delta G - KISS (keep it simple, stupid).

For the joy of learning, take a graduate level class solely on enzyme kinetics and then try your best to forget it all over winter break. :)