Noncompetitive inhibition is distinct from competitive, uncompetitive, and mixed-type inhibition. Noncompetitive inhibition occurs when an inhibitor attaches to the allosteric site of an enzyme without binding to the substrate. Noncompetitive inhibition differs from uncompetitive inhibition, when an inhibitor only binds to the enzyme-substrate complex. The inhibitor prevents the enzyme from producing its product. Because the inhibitor does not compete with the substrate for active site binding, the inhibition is independent of substrate concentration. However, it does not affect the enzyme’s affinity for its substrate.
Cellular
All living species use cellular enzymes as catalysts for chemical processes. Enzymes and their inhibitors are important in all aspects of human physiology. Metabolism is a series of processes designed to suit the body’s metabolic needs. Demand varies amongst tissues, even cells, and can fluctuate with time or environmental conditions. Enzymes and their regulators allow cells to precisely control metabolic processes. Non-competitive inhibition is important in the regulation of metabolism via feedback inhibition. In feedback inhibition, metabolic products function as inhibitors of their own enzymes. For example, alanine and ATP both block pyruvate kinase, the enzyme that catalyses the final glycolytic step. This prevents overproduction and energy waste by inhibiting pyruvate kinase. Hexokinase phosphorylates glucose to form glucose-6-phosphate in the first phase of glycolysis. Glucose-6-phosphate inhibits hexokinase noncompetitively, stopping the activity if the glycolytic pathway has already broken down enough glucose. Noncompetitive inhibition can affect many metabolic processes, including glycolysis.
Mechanism
For over a century, the Michaelis-Menten model of enzyme kinetics has been used to explain noncompetitive inhibition. Noncompetitive inhibition occurs when the inhibitor binds to an allosteric location distinct from the substrate active site. In noncompetitive inhibition, the inhibitor can bind the enzyme even if the substrate is bound. In other words, the inhibitor has the same affinity for both enzyme and substrate. The inhibitor binds to the enzyme or enzyme-substrate complex, inactivating it and preventing product synthesis. Inactivation of the enzyme reduces the maximal reaction rate (Vmax), which is the rate at which all active sites of the enzyme are fully saturated. The Michaelis constant (Km) is the substrate concentration where the reaction rate is half that of Vmax (Vmax = Km is also an inverse measure of enzyme-substrate affinity. Noncompetitive inhibition maintains the enzyme’s substrate affinity (Km) since the inhibitor does not compete for the active site. Increasing substrate concentration to offset noncompetitive inhibition’s effect on Vmax is futile, as competition for the active site between inhibitor and substrate is not the issue. Noncompetitive inhibition is distinguished from competitive inhibition (no direct change in Vmax, increased Km) by a drop in Vmax and a rise in Km (decreased Vmax and Km).
Enzymatic activity and inhibition data were shown on graphs before the convenience of powerful software utilised today in enzyme kinetics. A Lineweaver-Burk plot is the most common in teaching. On the y-axis is 1/V (velocity) and on the x-axis is 1/[S] (substrate concentration). The x-intercept represents 1/-Km and the y-intercept represents 1/Vmax. In non-competitive inhibition, the y-intercept increases between pre- and post-inhibition plots . This graph correlates with inhibition’s decrease in Vmax (increase in 1/Vmax). The x-intercept is unchanged since the enzyme’s apparent affinity for its substrate (Km) is unchanged. On the Lineweaver-Burk plot, changes in Vmax and Km are used to distinguish between noncompetitive and competitive inhibition.
Clinical Importance of Cyanide Poisoning
Ingestion of cyanide can be lethal. Most cyanide toxicity is caused by smoke inhalation from home fires. Toxicity occurs when oxidative phosphorylation stops (the production of ATP via the use of oxygen). Cyanide inhibits cytochrome c oxidase, the final enzyme in the electron transport chain. Rather than overcoming the enzyme’s inhibition, current cyanide toxicity treatments intercept or displace the cyanide before it reaches the enzyme ( potentially due to the non-competitive nature of inhibition).
Heavy Metals
Mercury, cadmium, and lead are all poisonous to humans.
Non-competitive enzyme inhibition has relationships with all three. Uncertainty surrounds the contribution of these individual inhibitors to the overall toxicity of these metals.
Aside from anticancer medicines, noncompetitive inhibition’s application as a therapeutic agent has only begun. Copper and mercury are powerful noncompetitive inhibitors of a light-chain protease in botulinum neurotoxin intoxication. Their noncompetitive suppression of neurotoxic enzymes delayed death in rodents poisoned with botulinum neurotoxin.
Chronic Disease
T2DM is one of the most frequent chronic diseases causing considerable morbidity and death in the healthcare system. Medical therapy for T2DM has taken a long time to develop and is not without adverse effects. Less intrusive blood sugar control methods are still being researched. Inhibition of intestinal enzymes involved for sugar breakdown (such alpha-glucosidase and alpha-amylase) has been shown to reduce postprandial hyperglycemia in diabetics. The traditional Indian therapy for T2DM, rosha grass (Cymbopogon martinii), has been proven to be a noncompetitive alpha-glucosidase inhibitor. Similarly, an Indonesian folk remedy for T2DM has been shown to have a noncompetitive alpha-amylase inhibitor that effectively lowers blood glucose levels in diabetic rats.
Conclusion
Also, pharmaceutical companies have stressed the need to discover new molecular targets for diabetes mellitus treatment. Gluconeogenesis is a well-defined condition in Type 2 diabetics, yet no treatment treats it. Fructose-1,6-bisphosphatase regulates gluconeogenesis, and noncompetitive inhibitors of the enzyme may be beneficial in T2DM.
Finally, noncompetitive inhibition is implicated in disulfiram’s suppression of pro-hepatocellular carcinoma enzymes, benzodiazepines’ inhibition of certain CYP450 enzyme families, and neuraminidase inhibition in the treatment of avian flu.
A quick PubMed search reveals thousands of novel implications for noncompetitive inhibition in disease knowledge, pharmacological therapy, and anti-cancer research.