One way that a catalyst can speed up a reaction is by putting steric strain on the substrate in a reagent. For example, when digesting protein by the Kjeldahl method, an ionic species like copper, provided to the reaction in the form of copper sulfate, interacts electrostatically with charged areas on a protein molecule, literally bending it out of shape. This makes it much easier for the protein molecule to be broken down by the digesting reagent (concentrated sulfuric acid). The catalyst takes part in the reaction, speeding it up, but is not itself altered by the reaction.
