Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity of the enzyme and binds equally well to the enzyme regardless of whether it has already bound the substrate. This is unlike competitive inhibition, where binding affinity for the substrate in the enzyme is decreased in the presence of an inhibitor. The inhibitor may bind to the enzyme regardless of whether the substrate has already been bound, but if it has a higher affinity for binding the enzyme in one state or the other, it is called a mixed inhibitor.
History During his years working as a physician Leonor Michaelis and a friend Peter Rona built a compact lab, in the hospital, and over the course of five years – Michaelis successfully became published over 100 times. During his research in the hospital, he was the first to view the different types of inhibition; specifically using fructose and glucose as inhibitors of maltase activity. Maltase breaks maltose into two units of glucose. Findings from that experiment allowed for the divergence of non-competitive and competitive inhibition. Non-competitive inhibition affects the kcat value (but not the Km) on any given graph; this inhibitor binds to a site that has specificity for the certain molecule. Michaelis determined that when the inhibitor is bound, the enzyme would become inactivated. Like many other scientists of their time, Leonor Michaelis and Maud Menten worked on a reaction that was used to change the composition of sucrose and make it lyse into two products – fructose and glucose. The enzyme involved in this reaction is called invertase, and it is the enzyme the kinetics of which have been supported by Michaelis and Menten to be revolutionary for the kinetics of other enzymes. While expressing the rate of the reaction studied, they derived an equation that described the rate in a way which suggested that it is mostly dependent on the enzyme concentration, as well as on presence of the substrate, but only to a certain extent. Adrian John Brown and Victor Henri laid the groundwork for the discoveries in enzyme kinetics that Michaelis and Menten are known for. Brown theoretically envisioned the mechanism now accepted for enzyme kinetics, but did not have the quantitative data to make a claim. Victor Henri made significant contributions to enzyme kinetics during his doctoral thesis, however he lacked noting the importance of hydrogen ion concentration and mutarotation of glucose. The goal of Henri's thesis was to compare his knowledge of enzyme-catalysed reactions to the recognized laws of physical chemistry. Henri is credited with being the first to write the equation that is now known as the Michaelis-Menten equation. Using glucose and fructose in the catalytic reactions controlled by maltase and invertase, Leonor Michaelis was the first scientist to distinguish the different types of inhibition by using the pH scale which did not exist in Henri's time. Particularly during their work on describing the rate of this reaction they also tested and extrapolated on the idea of another scientist, Victor Henri, that enzyme they were using had some affinity for both products of this reaction – fructose and glucose. Using Henri's methods, Michaelis and Menten nearly perfected this concept of initial-rate method for steady-state experiments. They were studying inhibition when they found that non-competitive (mixed) inhibition is characterized by its effect on kcat (catalyst rate) while competitive is characterized by its effect on velocity (V). In the Michaelis and Menten experiments they heavily focused on pH effects of invertase using hydrogen ions. Invertase is an enzyme found in extracellular yeast and catalyzed reactions by hydrolysis or inverting a sucrose (mixture of sucrose and fructose) to “invert sugar.” The main reason for using invertase was that it could be easily assayed and experiments could be done in quicker manner. Sucrose rotates in polarimeter as dextroratatory-D whereas invert sugar is levorotatory-L. This made tracking the inversion of sugar relatively simple. They also found that α-D-glucose is released in reactions catalyzed by invertase which is very unstable and spontaneously changes to β-D-glucose. Although, these are both in the dextrorotatory form, this is where they noted that glucose can change spontaneously, also known as mutarotation. Failing to take this into consideration was one of the main reasons Henri's experiments fell short. Using invertase to catalyze sucrose inversion, they could see how fast the enzyme was reacting by polarimetry; therefore, non-competitive inhibition was found to occur in the reaction where sucrose was inverted with invertase.
Terminology While all non-competitive inhibitors bind the enzyme at allosteric sites (i.e. locations other than its active site)—not all inhibitors that bind at allosteric sites are non-competitive inhibitors. In fact, allosteric inhibitors may act as competitive, non-competitive, or uncompetitive inhibitors. Many sources continue to conflate these two terms, or state the definition of allosteric inhibition as the definition for non-competitive inhibition.
Mechanism
… excerpt ends here. Continue reading the full article.
