Enzymes are responsible for speeding up the rate of a reaction, but not changing whether or not a reaction is favorable.

Enzymes act to reduce the activation energy of a reaction by

1. increasing the local concentration of the reactant
2. putting the reactants in the proper conformation
3. straining the reactants to favor the transition state

Enzymes have an active site where the reactants bind and may have additional sites affecting their activity.

Enzymes' level of activity is affected by the presence of substrate and can eventually reach a maximum if all active sites are saturated.