Enzymes are proteins that are assembled by one or more polypeptides. They are very huge (in terms of number of aminoacids, they usually have hundreds or thousands!). They have the critical role of facilitating chemical reactions, that otherwise would not happen. The reactions they facilitate require energy/heat, so they could not happen alone. Enzimes have a huge and complex structure, but the site where they do "the magic" (that is the chemical/catalytic reaction) is a tiny space called "active site". The active site has special chemical properties that allow the reaction to happen, against the energetic demand it imposes. Enzimes often need a metalic element to work, this is called the cofactor. An important cofactor is Mg+, this cofactor is essential for DNA polymerase (the enzyme that copies DNA) proper function. Other enzimes require bigger molecules to aid in their proper function, in this case, this molecules are called "coenzimes". Either cofactors or coenzimes attach to the enzyme and modify its 3D structure. Usually this modification allows the activation of the active site, that is, opens it. Substrate is the molecule that fits this active site. Usually enzymes are specific for the active site, nevertheless there are molecules with identical chemical properties (antagonists) to several substrates that can interfere with the enzyme-substrate interaction.