uncompetitive inhibition

2020 ◽  
Author(s):  
P. S. Sears
Keyword(s):  
Uncompetitive Inhibition

scholarly journals Molecular mechanism of uncompetitive inhibition of human placental and germ-cell alkaline phosphatase

1992 ◽  
Vol 286 (1) ◽  
pp. 23-30 ◽  
Author(s):  
M F Hoylaerts ◽  
T Manes ◽  
J L Millán
Keyword(s):  
Germ Cell ◽  
Active Site ◽  
Molecular Model ◽  
Inhibition Mechanism ◽  
Side Group ◽  
Alkaline Phosphatases ◽  
Uncompetitive Inhibition ◽  
Guanidinium Group ◽  
Hydrolysis Of ◽  
Site Binding

Placental (PLAP) and germ-cell (GCAP) alkaline phosphatases are inhibited uncompetitively by L-Leu and L-Phe. Whereas L-Phe inhibits PLAP and GCAP to the same extent, L-Leu inhibits GCAP 17-fold more strongly than it does PLAP. This difference has been attributed [Hummer & Millán (1991) Biochem. J 274, 91-95] to a Glu----Gly substitution at position 429 in GCAP. The D-Phe and D-Leu enantiomorphs are also inhibitory through an uncompetitive mechanism but with greatly decreased efficiencies. Replacement of the active-site residue Arg-166 by Ala-166 changes the inhibition mechanism of the resulting PLAP mutant to a more complex mixed-type inhibition, with decreased affinities for L-Leu and L-Phe. The uncompetitive mechanism is restored on the simultaneous introduction of Gly-429 in the Ala-166 mutant, but the inhibitions of [Ala166,Gly429]PLAP and even [Lys166,Gly429]PLAP by L-Leu and L-Phe are considerably decreased compared with that of [Gly429]PLAP. These findings point to the importance of Arg-166 during inhibition. Active-site binding of L-Leu requires the presence of covalently bound phosphate in the active-site pocket, and the inhibition of PLAP by L-Leu is pH-sensitive, gradually disappearing when the pH is decreased from 10.5 to 7.5. Our data are compatible with the following molecular model for the uncompetitive inhibition of PLAP and GCAP by L-Phe and L-Leu: after binding of a phosphorylated substrate to the active site, the guanidinium group of Arg-166 (normally involved in positioning phosphate) is redirected to the carboxy group of L-Leu (or L-Phe), thus stabilizing the inhibitor in the active site. Therefore leucinamide and leucinol are weaker inhibitors of [Gly429]PLAP than is L-Leu. During this Arg-166-regulated event, the amino acid side group is positioned in the loop containing Glu-429 or Gly-429, leading to further stabilization. Replacement of Glu-429 by Gly-429 eliminates steric constraints experienced by the bulky L-Leu side group during its positioning and also increases the active-site accessibility for the inhibitor, providing the basis for the 17-fold difference in inhibition efficiency between PLAP and GCAP. Finally, the inhibitor's unprotonated amino group co-ordinates with the active-site Zn2+ ion 1, interfering with the hydrolysis of the phosphoenzyme intermediate, a phenomenon that determines the uncompetitive nature of the inhibition.

scholarly journals Species-specific mechanisms of tumor suppression are fundamental drivers of vertebrate speciation: critical implications for the ‘war on cancer’

2019 ◽  
Vol 26 (2) ◽  
pp. C1-C5
Author(s):  
Jonathan W Nyce
Keyword(s):  
Tumor Suppression ◽  
De Novo ◽  
Human Cancer ◽  
Sequence Motif ◽  
Vertebrate Species ◽  
Specific Sequence ◽  
Anthropoid Primate ◽  
Uncompetitive Inhibition ◽  
High Concentrations ◽  
Suppression System

We recently reported our detection of an anthropoid primate-specific, ‘kill switch’ tumor suppression system that reached its greatest expression in humans, but that is fully functional only during the first twenty-five years of life, corresponding to the primitive human lifespan that has characterized the majority of our species' existence. This tumor suppression system is based upon the kill switch being triggered in cells in which p53 has been inactivated; such kill switch consisting of a rapid, catastrophic increase in ROS caused by the induction of irreversible uncompetitive inhibition of glucose-6- phosphate dehydrogenase (G6PD), which requires high concentrations of both inhibitor (DHEA) and G6P substrate. While high concentrations of intracellular DHEA are readily available in primates from the importation and subsequent de-sulfation of circulating DHEAS into p53-affected cells, both an anthropoid primate-specific sequence motif (GAAT) in the glucose-6-phosphatase (G6PC) promoter, and primate-specific inactivation of de novo synthesis of vitamin C by deletion of gulonolactone oxidase (GLO) were required to enable accumulation of G6P to levels sufficient to enable irreversible uncompetitive inhibition of G6PD. Malignant transformation acts as a counterforce opposing vertebrate speciation, particularly increases in body size and lifespan that enable optimized exploitation of particular niches. Unique mechanisms of tumor suppression that evolved to enable niche exploitation distinguish vertebrate species, and prevent one vertebrate species from serving as a valid model system for another. This here-to-fore unrecognized element of speciation undermines decades of cancer research data, using murine species, which presumed universal mechanisms of tumor suppression, independent of species. Despite this setback, the potential for pharmacological reconstitution of the kill switch tumor suppression system that distinguishes our species suggests that ‘normalization’ of human cancer risk, from its current 40% to the 4% of virtually all other large, long-lived species, represents a realistic near-term goal.

Application of the median method to estimate the kinetic constants of the substrate uncompetitive inhibition equation

2017 ◽  
Vol 418 ◽  
pp. 122-128 ◽  
Author(s):  
Pedro L. Valencia ◽  
Carolina Astudillo-Castro ◽  
Diego Gajardo ◽  
Sebastián Flores
Keyword(s):  
Kinetic Constants ◽  
Uncompetitive Inhibition

scholarly journals Species specificity of an adrenal androgen-mediated kill-switch triggered by p53 inactivation

2017 ◽  
Vol 1 (3) ◽  
Author(s):  
Jonathan W. Nyce
Keyword(s):  
Tumor Regression ◽  
Housekeeping Gene ◽  
High Dose ◽  
Adrenal Androgen ◽  
Rac Gtpase ◽  
Natural Systems ◽  
Uncompetitive Inhibition ◽  
Switch Mechanism ◽  
High Concentrations ◽  
P53 Inactivation

Glucose-6-phosphate dehydrogenase (G6PD) is an oncoprotein that is regulated by the p53 tumor suppressor. Mutant p53 loses the ability to inhibit G6PD, and loss of G6PD control clearly plays a role in oncogenesis. The steroid hormone precursor dehydroepiandrosterone (DHEA) is an endogenous uncompetitive inhibitor of G6PD. In humans, and a few other species, the sulfated circulatory form of DHEA (DHEAS) is present at extremely high concentrations – much higher than can be accounted for by DHEA’s function as a precursor to steroid hormones. Uncompetitive inhibition is extremely rare in natural systems because it is irreversible in the presence of high concentrations of substrate and inhibitor. What has gone unappreciated is that such uncompetitive inhibition can quickly lead to cell death when the target is an obligatory housekeeping gene such as G6PD. Cells with inactivated p53 not only lose control over G6PD, but also over hexokinase (HK), the enzyme that converts glucose into glucose-6-phosphate (G6P), the substrate of G6PD. Furthermore, loss of p53 function de-represses NFκB activity, resulting in the upregulation of steroid sulfatase (SS) which converts circulating DHEAS into active DHEA. We propose that inactivation of p53 rapidly elevates G6P and DHEA concentrations in affected cells, driving uncompetitive inhibition of G6PD to lethal irreversibility. In animals with circulating DHEAS, this kill-switch mechanism may prevent most cases of p53 inactivation from becoming tumorigenic. Tumors would thus represent instances in which this mechanism had not been triggered, but which might still be triggered by application of DHEA sufficient to uncompetitively inhibit tumor G6PD. To test this hypothesis, we performed a pilot study in which dogs with cardiac hemangiosarcoma were treated with high dose (HD) DHEA supplemented with isoprene precursors to maintain geranylation of Rac GTPase. Tumor regression and longevity observed in these dogs supported the concept that some tumors retain extraordinary sensitivity to uncompetitive inhibition by DHEA.

Methylenetetrahydrofolate dehydrogenase – methenyltetrahydrofolate cyclohydrolase –formyltetrahydrofolate synthetase from porcine liver: evidence to support a common dehydrogenase–cyclohydrolase site

1983 ◽  
Vol 61 (11) ◽  
pp. 1166-1171 ◽  
Author(s):  
D. Drummond S. Smith ◽  
Robert E. MacKenzie
Keyword(s):  
Chemical Modification ◽  
Binding Site ◽  
Porcine Liver ◽  
Formyltetrahydrofolate Synthetase ◽  
Uncompetitive Inhibition ◽  
Dead End ◽  
Methylenetetrahydrofolate Dehydrogenase

The cyclohydrolase activity of the trifunctional enzyme methylenetetrahydrofolate dehydrogenase – methenyltetrahydrofolate cyclohydrolase – formyltetrahydrofolate synthetase is inhibited by NADP+, a substrate of the dehydogenase. This uncompetitive inhibition, shown also by 3-aminopyridine adenine dinucleotide phosphate (AADP), indicates formation of dead-end complexes consisting of enzyme–nucleotide–methenyltetrahydrofolate. Chemical modification with diethylpyrocarbonate inactivates the dehydrogenase and cyclohydrolase but not the synthetase. Folate, but neither NADP+ nor AADP, protects both activities against modification. However, NADP+ potentiates the protection by folate by decreasing the apparent Kd for that ligand approximately sixfold. Chemical modification with phenylglyoxal also inactivates both the dehydrogenase and cyclohydrolase activities. Neither activity was protected by NADP+ or folate alone; however, the combination of NADP+ and folate protected both activities. These results are consistent with a model in which the dehydrogenase and cyclohydrolase activities share a common folate binding site.

Partial uncompetitive inhibition of horseradish peroxidase by a water-miscible ionic liquid [BMIM][MeSO4]

2011 ◽  
Vol 33 (8) ◽  
pp. 1657-1662 ◽  
Author(s):  
Jung Hee Park ◽  
Ik Keun Yoo ◽  
O. Yul Kwon ◽  
Keungarp Ryu
Keyword(s):  
Ionic Liquid ◽  
Horseradish Peroxidase ◽  
Uncompetitive Inhibition

scholarly journals The role of alkyl chain length in the inhibitory effect n-alkyl xanthates on mushroom tyrosinase activities.

2007 ◽  
Vol 54 (1) ◽  
pp. 183-191 ◽  
Author(s):  
Ali Akbar Saboury ◽  
Mahdi Alijanianzadeh ◽  
Hasan Mansoori-Torshizi
Keyword(s):  
Competitive Inhibition ◽  
Inhibitory Effect ◽  
Uv Spectrophotometry ◽  
Negative Cooperativity ◽  
Mushroom Tyrosinase ◽  
Uncompetitive Inhibition ◽  
Sodium Phosphate Buffer ◽  
Catecholase Activity ◽  
Hydrophobic Tail

Sodium salts of four n-alkyl xanthate compounds, C2H5OCS2Na (I), C3H7OCS2Na (II), C4H9OCS2Na (III), and C6H13OCS2Na (IV) were synthesized and examined for inhibition of both cresolase and catecholase activities of mushroom tyrosinase (MT) in 10 mM sodium phosphate buffer, pH 6.8, at 293 K using UV spectrophotometry. 4-[(4-Methylbenzo)azo]-1,2-benzendiol (MeBACat) and 4-[(4-methylphenyl)azo]-phenol (MePAPh) were used as synthetic substrates for the enzyme for catecholase and cresolase reactions, respectively. Lineweaver-Burk plots showed different patterns of mixed, competitive or uncompetitive inhibition for the four xanthates. For the cresolase activity, I and II showed uncompetitive inhibition but III and IV showed competitive inhibition pattern. For the catecholase activity, I and II showed mixed inhibition but III and IV showed competitive inhibition. The synthesized compounds can be classified as potent inhibitors of MT due to their Ki values of 13.8, 11, 8 and 5 microM for the cresolase activity, and 1.4, 5, 13 and 25 microM for the catecholase activity for I, II, III and IV, respectively. For the catecholase activity both substrate and inhibitor can be bound to the enzyme with negative cooperativity between the binding sites (alpha > 1) and this negative cooperativity increases with increasing length of the aliphatic tail of these compounds. The length of the hydrophobic tail of the xanthates has a stronger effect on the Ki values for catecholase inhibition than for cresolase inhibition. Increasing the length of the hydrophobic tail leads to a decrease of the Ki values for cresolase inhibition and an increase of the Ki values for catecholase inhibition.

scholarly journals Role of Phosphoenolpyruvate in the NADP-Isocitrate Dehydrogenase and Isocitrate Lyase Reaction in Escherichia coli

2006 ◽  
Vol 189 (3) ◽  
pp. 1176-1178 ◽  
Author(s):  
Tadashi Ogawa ◽  
Keiko Murakami ◽  
Hirotada Mori ◽  
Nobuyoshi Ishii ◽  
Masaru Tomita ◽  
...  
Keyword(s):  
Escherichia Coli ◽  
Isocitrate Dehydrogenase ◽  
Isocitrate Lyase ◽  
Tricarboxylic Acid Cycle ◽  
Tricarboxylic Acid ◽  
Glyoxylate Shunt ◽  
Acid Cycle ◽  
Uncompetitive Inhibition

ABSTRACT Phosphoenolpyruvate inhibited Escherichia coli NADP-isocitrate dehydrogenase allosterically (Ki of 0.31 mM) and isocitrate lyase uncompetitively (Ki ′ of 0.893 mM). Phosphoenolpyruvate enhances the uncompetitive inhibition of isocitrate lyase by increasing isocitrate, which protects isocitrate dehydrogenase from the inhibition, and contributes to the control through the tricarboxylic acid cycle and glyoxylate shunt.

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