Enzymes are the biological equivalent of a catalyst. They make sure organic systems are working properly and at the correct speed. Often due to the size of the chemicals involved the physical qualities, such as shape of the enzyme, are just as important as the chemistry. The active site of an enzyme is the physical part of it that bonds, normally to a protein, during the reaction. This active site is unique and ensures that only the correct enzyme stimulates the right reaction. If an enzyme is heated too much the structure unravels and the active site is distorted beyond repair, the enzyme is then described as denatured (this is the thing that will kill you during hyperthermia).
Like all catalysts the aim is to lower the activation energy of the reaction by offering a different chemical path for the reaction to take. A study has recently showed that one aspect of this involves polarising the active site by providing it with excess electrons. This makes electron transport across the site easier and so reduces the energy required for the biological reaction. It is also suggested that the proteins may have also evolved ways of distorting the electric field surrounding the site to favour the reaction as well but this is an area that still needs to be researched.