The lock and key and induced fit models of enzyme action.

The lock and key theory
An enzyme has a active site which compliments (fits with) a specific substrate exactly (before binding). It states that the enzyme has a fixed shape. A substrate will go into an enzyme like a key goes into a lock and form an enzyme-substrate complex; this then changes the bonds in the substrate to form the products.

http://chsweb.lr.k12.nj.us

The induced fit theory
This states that an enzyme has an active site which is not perfect for a substrate, but that it is flexible.
A substrate will alter the shape of the active site to make it complimentary, the changed shape will alter the bonds in the substrate creating the products.

http://chsweb.lr.k12.nj.us

Enzymes as catalysts lowering activation energy through the formation of enzyme-substrate complexes.

Enzymes form temporary bonds to substrates, this creates an enzyme substrate complex. When the complex is made it alters the bonds in the substrate(s) to create products:

Enzyme + substrate > enzyme-substrate complex > enzyme + products.

Enzymes allow a reaction to happen just by having the necessary substrates in its active site. This needs less energy than if the reaction were to happen with out the enzyme as substrates need to collide with enough force to change their bonds.

boundless.com

A catalyst (like an enzyme) will lower the activation energy of a reaction, but will not change the energy of the reactants or products. The catalyst itself will remain unchanged by a reaction.

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html