Uncompetitive inhibition (which Laidler and Bunting preferred to call anti-competitive inhibition,[1] but this term has not been widely adopted) is a type of inhibition in which the apparent values of the Michaelis–Menten parameters and are decreased in the same proportion.
It can be recognized by two observations: first, it cannot be reversed by increasing the substrate concentration , and second, linear plots show effects on and , seen, for example, in the Lineweaver–Burk plot as parallel rather than intersecting lines. It is sometimes explained by supposing that the inhibitor can bind to the enzyme-substrate complex but not to the free enzyme. This type of mechanism is rather rare,[2] and in practice uncompetitive inhibition is mainly encountered as a limiting case of inhibition in two-substrate reactions in which one substrate concentration is varied and the other is held constant at a saturating level.[3][4]
Mathematical definition
editIn uncompetitive inhibition at an inhibitor concentration of the Michaelis–Menten equation takes the following form:[4]
in which is the rate at concentrations of substrate and of inhibitor, for limiting rate , Michaelis constant and uncompetitive inhibition constant .
This has exactly the form of the Michaelis–Menten equation, as may be seen by writing it in terms of apparent kinetic constants:
in which
It is important to note that and decrease in the same proportions as a result of the inhibition.
This is apparent when viewing a Lineweaver-Burk plot of uncompetitive enzyme inhibition: the ratio between V and Km remains the same with or without an inhibitor present.
This may be seen in any of the common ways of plotting Michaelis–Menten data, such as the Lineweaver–Burk plot, for which for uncompetitive inhibition produces a line parallel to the original enzyme-substrate plot, but with a higher intercept on the ordinate:[5][6]
Implications and uses in biological systems
editThe unique traits of uncompetitive inhibition lead to a variety of implications for the inhibition's effects within biological and biochemical systems. Uncompetitive inhibition is present within biological systems in a number of ways. In fact, it often becomes clear that the traits of inhibition specific to uncompetitive inhibitors, such as their tendency to act at their best at high concentrations of substrate, are essential to some important bodily functions operating properly.[7]
Involvement in cancer mechanisms
editUncompetitive mechanisms are involved with certain types of cancer. Some human alkaline phosphatases have been found to be over-expressed in certain types of cancers, and those phosphatases often operate via uncompetitive inhibition. It has also been found that a number of the genes that code for human alkaline phosphatases are inhibited uncompetitively by amino acids such as leucine and phenylalanine.[8] Studies of the involved amino acid residues have been undertaken in attempts to regulate alkaline phosphatase activity and learn more about said activity's relevance to cancer.[9]
Additionally, uncompetitive inhibition works alongside transformation-related protein 53 to help repress the activity of cancer cells and prevent tumorigenesis in certain forms of the illness, as it inhibits glucose-6-phosphate dehydrogenase, an enzyme of the pentose phosphate pathway). One of the side roles this enzyme is responsible for is helping to regulate is the control of reactive oxygen levels, as reactive oxygen species must be kept at appropriate levels to allow cells to survive. When concentration of glucose 6-phosphate, the substrate of the enzyme, is high, uncompetitive inhibition of the enzyme becomes far more effective.[10] This extreme sensitivity to substrate concentration within the cancer mechanism implicates uncompetitive inhibition rather than mixed inhibition, which displays similar traits but is often less sensitive to substrate concentration due to some inhibitor binding to free enzymes regardless of the substrate's presence.[7] As such, the extreme strength of uncompetitive inhibitors at high substrate concentrations and the overall sensitivity to substrate amount indicates that only uncompetitive inhibition can make this type of process possible.
Importance in cell and organelle membranes
editAlthough uncompetitive inhibition is present in various diseases within biological systems, it does not necessarily only relate to pathologies. It can be involved in typical bodily functions. For example, active sites capable of uncompetitive inhibition appear to be present in membranes, as removing lipids from cell membranes and making active sites more accessible through conformational changes has been shown to invoke elements resembling the effects of uncompetitive inhibition (i.e. both and decrease). In mitochondrial membrane lipids specifically, removing lipids decreases the
This presence of uncompetitive enzymes in membranes has also been supported in a number of other studies. For example, in studies of the protein ADP ribosylation factor, which is involved in regulating membrane activity, it was found that brefeldin A, a lactone antiviral trapped one of the protein's intermediates via uncompetitive inhibition. This made it clear that this type of inhibition exists within various types of cells and organelles as opposed to just in pathological cells. In fact, brefeldin A was found to relate to the activity of the Golgi apparatus and its role in regulating movement across the cell membrane.[12]
Presence in the cerebellar granule layer
editUncompetitive inhibition can play roles in various other parts of the body as well. It is part of the mechanism by which N-methyl-D-aspartate glutamate receptors are inhibited in the brain, for example. Specifically, this type of inhibition impacts the granule cells that make up a layer of the cerebellum. These cells have the receptors mentioned, and their activity typically increases as ethanol is consumed. This often leads to withdrawal symptoms if ethanol is removed. Various uncompetitive blockers act as antagonists at the receptors and modify the process, with one example being the inhibitor memantine.[13] In fact, in similar cases (involving over-expression of N-methyl-D-aspartate glutamate receptors, though not necessarily via ethanol), uncompetitive inhibition helps in nullifying the over-expression due to its particular properties. Since uncompetitive inhibitors block high concentrations of substrates very efficiently, their traits alongside the innate characteristics of the receptors themselves lead to very effective blocking of N-methyl-D-aspartate glutamate channels when they are excessively open due to massive amounts of agonists.[14]
Examples for uncompetitive inhibition
editInvestigations of phenol-based HSD17B13 inhibitors indicate an uncompetitive mode of inhibition against NAD+.[15]
References
edit- ^ Laidler, Keith J.; Bunting, Peter S. (1973). The Chemical Kinetics of Enzyme Action. Clarendon Press, Oxford.
- ^ Cornish-Bowden, A. (1986). "Why is uncompetitive inhibition so rare? A possible explanation, with implications for the design of drugs and pesticides". FEBS Lett. 203 (1): 3–6. doi:10.1016/0014-5793(86)81424-7.
- ^ Cleland, W. W. "The kinetics of enzyme-catalyzed reactions with two or more substrates or products: II. Inhibition: Nomenclature and theory". Biochim. Biophys. Acta. 67 (2): 173–187. doi:10.1016/0926-6569(63)90226-8.
- ^ a b Cornish-Bowden, Athel (2012). Fundamentals of Enzyme Kinetics (4th ed.). Wiley-Blackwell, Weinheim. pp. 25–75. ISBN 978-3-527-33074-4.
- ^ Rhodes D. "Enzyme Kinetics - Single Substrate, Uncompetitive Inhibition, Lineweaver-Burk Plot". Purdue University. Retrieved 31 August 2013.
- ^ Cornish-Bowden A (January 1974). "A simple graphical method for determining the inhibition constants of mixed, uncompetitive and non-competitive inhibitors". The Biochemical Journal. 137 (1): 143–4. doi:10.1042/bj1370143. PMC 1166095. PMID 4206907.
- ^ a b Nahorski SR, Ragan CI, Challiss RA (August 1991). "Lithium and the phosphoinositide cycle: an example of uncompetitive inhibition and its pharmacological consequences". Trends in Pharmacological Sciences. 12 (8): 297–303. doi:10.1016/0165-6147(91)90581-C. PMID 1658998.
- ^ Millán JL (July 1992). "Alkaline phosphatase as a reporter of cancerous transformation". Clinica Chimica Acta; International Journal of Clinical Chemistry. 209 (1–2): 123–9. doi:10.1016/0009-8981(92)90343-O. PMID 1395034.
- ^ Millán JL, Fishman WH (1995). "Biology of human alkaline phosphatases with special reference to cancer". Critical Reviews in Clinical Laboratory Sciences. 32 (1): 1–39. doi:10.3109/10408369509084680. PMID 7748466.
- ^ Nyce JW (November 2018). "Detection of a novel, primate-specific 'kill switch' tumor suppression mechanism that may fundamentally control cancer risk in humans: an unexpected twist in the basic biology of TP53". Endocrine-Related Cancer. 25 (11): R497–R517. doi:10.1530/ERC-18-0241. PMC 6106910. PMID 29941676.
- ^ Lenaz G, Curatola G, Mazzanti L, Parenti-Castelli G (November 1978). "Biophysical studies on agents affecting the state of membrane lipids: biochemical and pharmacological implications". Molecular and Cellular Biochemistry. 22 (1): 3–32. doi:10.1007/bf00241467. PMID 154058. S2CID 28599836.
- ^ Zeghouf M, Guibert B, Zeeh JC, Cherfils J (December 2005). "Arf, Sec7 and Brefeldin A: a model towards the therapeutic inhibition of guanine nucleotide-exchange factors". Biochemical Society Transactions. 33 (Pt 6): 1265–8. doi:10.1042/BST20051265. PMID 16246094.
- ^ Tabakoff B, Hoffman PL (March 1993). "Ethanol, sedative hypnotics, and glutamate receptor function in brain and cultured cells". Behavior Genetics. 23 (2): 231–6. doi:10.1007/BF01067428. PMID 8390239. S2CID 12640658.
- ^ Nakamura T, Lipton SA (January 2008). "Emerging roles of S-nitrosylation in protein misfolding and neurodegenerative diseases". Antioxidants & Redox Signaling. 10 (1): 87–101. doi:10.1089/ars.2007.1858. PMID 17961071.
- ^ Thamm, Sven; Willwacher, Marina K.; Aspnes, Gary E.; Bretschneider, Tom; Brown, Nicholas F.; Buschbom-Helmke, Silke; Fox, Thomas; Gargano, Emanuele M.; Grabowski, Daniel; Hoenke, Christoph; Matera, Damian; Mueck, Katja; Peters, Stefan; Reindl, Sophia; Riether, Doris (2023-02-23). "Discovery of a Novel Potent and Selective HSD17B13 Inhibitor, BI-3231, a Well-Characterized Chemical Probe Available for Open Science". Journal of Medicinal Chemistry. 66 (4): 2832–2850. doi:10.1021/acs.jmedchem.2c01884. ISSN 0022-2623. PMC 9969402. PMID 36727857.