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.

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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.

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