A substance that accelerates a chemical reaction, and that is not a reagent, is called a catalyst. The catalysts of the biochemical reactions that occur in living organisms are found as enzymes. These proteins are also proteins, although some ribonucleic acid (RNA) molecules also act as enzymes.
Enzymes executed the fundamental task of decreasing activation energy, that is the amount of energy that a reaction must be added in order for this audience. Enzymes operated by binding to the reagent molecules and sustaining them in such a way that the processes that form and break chemical bonds happen more easily.
Let's clarify an important point, enzymes do not change the ∆G value of a reaction. That is, they do not change if a reaction releases or absorbs energy in general. This is because enzymes do not affect the free energy of reagents or products.
Instead, enzymes decrease the energy of the transition state, an unstable state through which reagents must pass to become products. The transition state is at the top of the "hill" of energy in the previous diagram.
Active sites and substrate specificity
To catalyze a reaction, an enzyme sticks (binds) to one or more reagent molecules. These molecules are the substrates of the enzyme.
In some reactions, a substrate breaks into several products. In others, two substrates join together to create a larger molecule or to exchange parts. In fact, for any biological reaction that can occur to you, there is probably an enzyme to accelerate it.
The part of the enzyme where the substrate binds is called the active site (since that is where the catalytic "action" occurs).
