Enzymes are three-dimensional protein structures with an active site. They convert a chemical or a substrate denoted as S into a product denoted by P. One or more products can be formed from a single or many substrates. Enzymes help in catalysing the reactions but they do not participate in the reaction themselves.
The multi-step reaction mechanism involving enzymes as catalysts is called the catalytic cycle. First, the substrate S binds to the active site of the enzyme and fits itself inside a given cleft or pocket. The binding of the substrate forces the enzyme to change its shape and fit more tightly around the substrate, forming a highly reactive enzyme-substrate (ES) complex. This complex is unstable and this temporarily formed complex is called the transition-state structure. The active site of the enzyme breaks and a new enzyme-product (EP) complex is formed, which is also unstable. Finally, the structure of the substrate is transformed into the structure of the products.
Here, ‘ES’ and ‘EP’ complexes with a high-energy status are unstable, while ‘S’ and ‘P’ have a low-energy status and are stable. Activation energy is the minimum amount of energy required by the substrates to form products.
The graph shows how the enzymes lower the activation energy during reactions, thereby increasing the rate of reaction. The y-axis represents the potential energy while the x-axis represents the formation of products via complex structures. If ‘P’ is at a lower level than ‘S’, it implies an exothermic reaction releasing heat energy. In the graph, the red curve indicates that in the absence of an enzyme, the activation energy for a reaction is high. On the other hand, the pink curve shows that in the presence of an enzyme, the activation energy gets lowered, due to which the substrate transforms into a product more quickly.