Heat is the energy of random molecular motion. Entropy is a tendency toward disorder in the universe. Free energy is the energy present in the chemical bonds of a molecule, minus the energy lost to entropy. In exergonic reactions, products contain less energy or more disorder than the reactants, release energy, and proceed spontaneously. In endergonic reactions, products contain more free energy or more disorder than reactants and require an input of energy to proceed.
Activation energy is the energy required to get spontaneous reactions going.
It keeps spontaneous reactions under some control to prevent reactions from immediately going to the maximum. Catalysts lower the activation energy required for a reaction to get started and will provide for a greater amount of reactant converted into product. When an enzyme is denatured, it unfolds and loses its catalytic ability. The active site on an enzyme is where the substrate binds. An allosteric site is where a non-competitive enzyme inhibitor would bind, effecting a change in the shape of the enzyme so that it could no longer bind the substrate.