Enzymes are biological catalysts that decrease the activation energy and increase the rate of chemical reactions.
Enzymes are very specific and have an area called an active site which substrates bind to. This binding process on the active site is called the enzyme-substrate complex. When a substrate binds to an active site it induces a conformational shift in the substrate.
This shift causes the activation energy to decrease, substrate bonds to be broken, and new bonds to be formed. These changes allow new products to be made. Enzyme inhibitors are substances that bind to a site on an enzyme, and compete with substrates to block these chemical reactions from occurring. There are 2 types of inhibitors; competitive and non-competitive inhibitors. As the name implies, competitive inhibitors compete with substrates to block chemical reactions from occurring.
Competitive inhibitors work by replicating a substrate and binding to an enzyme active site, thereby blocking a substrate from binding to that same active site.Non-competitive inhibitors are also able to block substrates but do so in a different way. In addition to having an active site, some enzymes also contain a secondary site for binding called an allosteric site. Non-competitive inhibitors bind to these secondary allosteric sites, and this binding causes a conformational shift in the enzyme and the enzyme active site. The change to the active site means that substrates are no longer able to bind there, and the substrate chemical reaction can no longer occur. A common drug that acts as an enzyme inhibitor is sildenafil (Viagra). This medication which is commonly used to treat erectile dysfunction and pulmonary hypertension, inhibits the enzyme phosphodiesterase type 5 (PDE5).
This inhibition allows stimulation of vasodilation and increased blood flow to occur.