Most biological enzymes are proteins . They perform the chemical reactions in cells. Not all proteins are enzymes, but most enzymes are proteins (the exception is catalytic RNA). A catalyst is a molecule which increases the rate of a reaction but is not the substrate or product of that reaction. A substrate is a molecule upon which an enzyme acts to yield a product.
A ------> B EnzThe free energy of this reaction is not changed by the presence of the enzyme, but, for a favored reaction (where delta G is negative), the enzyme can speed it up.
Delta G* is the activation energy
Delta G is negative overall for forward reaction
Enzymes have affinity for the substrate in a transition state. They get
the substrate into the right conformation which will lead to the
breakdown into products.
Alternatively, for a reaction such as the one shown below, the enzyme may increase the local concentration of the two substrates A and B, driving the reaction forward:
A + B ---> CThe part of the enzyme that does the work is called the active site . The residues in this site are in the right 3D conformation to accomplish the enzyme's work.