Enzymes and substrates are related in two key ways because they interact frequently with each other in many biological processes. First, enzymes and substrates are often specific for one another, possessing complimentary shapes that allow them to bind. Second, enzymes can alter substrates by catalyzing chemical reactions or modifying structures. Together they undergo transformations, including the enzyme substrate (ES) complex, intermediate states, and transition states.
There are thought to be about 75,000 enzymes in the human body, many of which are specific to certain substrates. Enzymes and substrates bind to each other associating through various interactions, including hydrogen bonds, hydrophobic interactions, and covalent bonds. This initial binding is referred to as an induced fit model, rather than a lock-and-key model, because each molecule changes in response to binding with the other molecule to form a new three-dimensional shape. Together, the enzyme and the substrate comprise the ES complex, in which chemical reactions can occur.
During a catalyzed reaction, enzymes and substrates interact to yield new products. While the enzyme remains unchanged after the reaction is complete, the substrate is often modified, sometimes changing completely into a new molecule or molecules. Despite remaining unchanged, the enzyme plays a major role in the chemical reactions taking place because it lowers the activation energy for the reaction to proceed. This means that the energy present in the enzyme’s and substrate’s chemical interactions is enough to overcome the reaction’s energy hurdle.
Throughout the reaction, enzymes and substrates pass through different stages. Some of these, known as intermediate stages or chemical intermediates, involve the formation of new, transient molecules. The enzyme is able to stabilize these and catalyze further reactions to change these molecules into the desired products.
Transition states are other complexes formed between enzymes and substrates that are incredibly short-lived and high-energy. They frequently occur at the moment of bond breakage, reformation, or rearrangement in the reaction. The enzymes are often structured to reduce the energy of the transition states through stabilizing bonds, and often this reduction in energy is what lowers the activation energy of the reaction.
Although there are many enzyme substrate pairs, many enzymes can correspond to multiple substrates. In fact, many enzymes are needed to join two substrates together into a single molecule or to break a single substrate into two resulting product molecules. There are also many enzymes that are non-specific, meaning that they can target more than one substrate, depending on conditions within the cell.