Enzymes Accelerate Chemical Reactions

In the body, cells must continually carry out a remarkable assortment of chemical reaction, which much proceed at a rate that is fast enough to be useful to the cell. All living things therefore depend on molecules called enzymes that accelerate chemical reactions, and a proccess known as catalysis. The enzymes catalyze chemical reactions by lowering the activation energy barrier. Most enzymes are proteins, although some are composed of RNA. Enzymes vary in degree to which they accelerate reaction rates. Some accelerate a chemical reaction by just several orders of magnitude, while some enhance reaction rates by a remarkable factor of 10¹⁷.

There are several features that are common to all enzymes. An enzyme binds to one or more molecules known as substrates, promotes a chemical reaction in the substrate molecules and then realeases the products of reaction. The enzyme isn’t permanently modified in a proccess, so it’s then able to bind another substrate molecule and perform the reaction again. Enzymes generally catalyze just one type of chemical reaction and are usually highly specific for the types of substrates on which they act. For this reason, cells produce many different types of enzymes, each uniquely designed for a particular chemical task.

A chemical reaction with negative free energy change should occur spontaneously, given enough time. However, in the course of the chemical reaction, the substrates must undergo transient changes in stereochemistry, charge, or covalent stucture before completing the necessary rearrangements to produce the final products. These changes are energetically unfavorable, which means that the first steps in the chemical reaction generally involve a positive free energy change. These are then followed by chemical steps with a negative free energy change of even greater magnitude, resulting in a chemical reaction whose net free energy change, when summed over all the steps is negative.

A consequence of this energetic pathway is that there is a free energy barrier between the initial and final states of the chemical reaction. The transition state, which is the highest point in free energy on the reaction pathway from substrate to product, marks the top of the free energy barrier. This species exists only transiently (for perhaps as little as 10⁻¹⁵ seconds) and is generally different in stereochemistry and charge configuration from either the substrate or the product. It is the energy difference between the ground state and the transition state that constitutes the activation energy.

Without the aid of an enzyme, few substrate molecules at any one time typically have sufficient energy to reach the transition state, and, as a result, the uncatalyzed reaction proceeds slowly. Enzymes achieve their catalytic effects by lowering the activation-energy barrier using several different strategies (Craig, dkk: 2010).