A catalyst is not altered by the reaction. A catalyst does not change the final result of a reaction. The activation energy is the amount of energy needed by the reactant molecules to participate in a reaction. In the absence of a catalyst, only a small proportion of the reactants possess the activation energy to participate in the reaction. By lowering the activation energy, enzymes allow a larger proportion of the reactants to participate in the reaction, thus increasing the reaction rate.
Most enzymes are proteins.
Protein enzymes have specific three-dimensional shapes, which are determined by the amino acid sequence and, ultimately, by the genes. The reactants in an enzyme-catalyzed reaction, called the substrates of the enzyme, fit into a specific pocket in the enzyme called the active site. By forming an enzyme-substrate complex, substrate molecules are brought into proper orientation and existing bonds are weakened. This allows new bonds to be formed more easily. The activity of an enzyme is affected by a variety of factors.