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Vern L. Schramm - One of the best experts on this subject based on the ideXlab platform.
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Structural and metabolic specificity of methylthioCoformycin for malarial adenosine deaminases.
Biochemistry, 2009Co-Authors: Maria B. Cassera, Dennis C. Madrid, Li Min Ting, Peter C. Tyler, Kami Kim, Steven C. Almo, Vern L. SchrammAbstract:Plasmodium falciparum is a purine auxotroph requiring hypoxanthine as a key metabolic precursor. Erythrocyte adenine nucleotides are the source of the purine precursors, making adenosine deaminase (ADA) a key enzyme in the pathway of hypoxanthine formation. Methylthioadenosine (MTA) is a substrate for most malarial ADAs, but not for human ADA. The catalytic site specificity of malarial ADAs permits methylthioCoformycin (MT-Coformycin) to act as a Plasmodium-specific transition state analogue with low affinity for human ADA [Tyler, P. C., Taylor, E. A., Frohlich, R. G. G., and Schramm, V. L. (2007) J. Am. Chem. Soc. 129, 6872−6879]. The structural basis for MTA and MT-Coformycin specificity in malarial ADAs is the subject of speculation [Larson, E. T., et al. (2008) J. Mol. Biol. 381, 975−988]. Here, the crystal structure of ADA from Plasmodium vivax (PvADA) in a complex with MT-Coformycin reveals an unprecedented binding geometry for 5′-methylthioribosyl groups in the malarial ADAs. Compared to malarial A...
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Synthesis of 5‘-Methylthio Coformycins: Specific Inhibitors for Malarial Adenosine Deaminase
Journal of the American Chemical Society, 2007Co-Authors: Peter C. Tyler, Erika A. Taylor, Richard F.g. Fröhlich, Vern L. SchrammAbstract:Transition state theory suggests that enzymatic rate acceleration (kcat/knon) is related to the stabilization of the transition state for a given reaction. Chemically stable analogues of a transition state complex are predicted to convert catalytic energy into binding energy. Because transition state stabilization is a function of catalytic efficiency, differences in substrate specificity can be exploited in the design of tight-binding transition state analogue inhibitors. Coformycin and 2‘-deoxyCoformycin are natural product transition state analogue inhibitors of adenosine deaminases (ADAs). These compounds mimic the tetrahedral geometry of the ADA transition state and bind with picomolar dissociation constants to enzymes from bovine, human, and protozoan sources. The purine salvage pathway in malaria parasites is unique in that Plasmodium falciparum ADA (PfADA) catalyzes the deamination of both adenosine and 5‘-methylthioadenosine. In contrast, neither human adenosine deaminase (HsADA) nor the bovine e...
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Molecular Electrostatic Potential Analysis for Enzymatic Substrates, Competitive Inhibitors, and Transition-State Inhibitors
Journal of the American Chemical Society, 1996Co-Authors: Carey K. Bagdassarian, Vern L. Schramm, Steven D. SchwartzAbstract:Recent advances in the application of kinetic isotope effects to enzyme-catalyzed reactions have provided reliable information for enzymatic transition state structures. A method is presented for quantifying the similarity of substrates and inhibitors with their enzyme-stabilized transition states. On the basis of transition-state stabilization theory for enzymatic reactions, molecules most similar to the transition state structure bind with greatest affinity. Molecular similarity measures are applied to compare substrates, competitive inhibitors, and transition state inhibitors with the transition state structures stabilized by the enzymes AMP deaminase, adenosine deaminase, and AMP nucleosidase. (R)- and (S)-Coformycin 5‘-phosphate are inhibitors for AMP deaminase, with the R-species superior to its enantiomer. Formycin 5‘-phosphate 4-aminopyrazolo[3,4-d]pyrimidine-1-ribonucleotide, and tubercidin 5‘-phosphate inhibit AMP nucleosidase. The transition state for adenosine deaminase is analogous to that fo...
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Electrostatic potential surfaces of the transition state for AMP deaminase and for (R)-Coformycin, a transition state inhibitor.
The Journal of biological chemistry, 1994Co-Authors: Paul C. Kline, Vern L. SchrammAbstract:The transition state for the hydrolysis of AMP by AMP deaminase has been characterized by heavy atom kinetic isotope effects (Merkler, D.J., Kline, P.C., Weiss, P., and Schramm, V.L. (1993) Biochemistry 32, 12993-13001). The experimentally established transition state includes a bond order of 0.8 to the attacking water nucleophile, a full bond order to the exocyclic 6-amino group, rehybridization of C-6 of the purine ring to sp3 and protonation of N-1 by Glu633. The transition state is one the path to formation of an unstable tetrahedral intermediate in which the exocyclic amine undergoes rapid protonation followed by its departure. In this mechanism, the highest energetic barrier on the reaction coordinate is the attack of the zinc-activated water. In a further test of this transition state structure, the electrostatic potential surface for the purine ring of the transition state has been determined by molecular orbital calculations and compared to that of the base of (R)-Coformycin 5'-monophosphate, a slow onset, tight binding inhibitor of AMP deaminase that binds with an overall dissociation constant of 10(-11) M. The electrostatic potential surfaces of the aglycones of the transition state and (R)-Coformycin are compared to the adenine ring of the substrate and to an alternative transition state structure in which the transition state is late, with fully bonded hydroxyl and fully protonated exocyclic amine. The results indicate a near-match of the electrostatic potential surfaces for the early transition state and (R)-Coformycin. The electrostatic nature of the late transition state with a protonated amine leaving group differs both from the transition state determine by kinetic isotope effects and from that of (R)-Coformycin analogues. The results provide evidence that the nature of the enzyme-stabilized transition state for adenine deamination involves an early transition state with a partially bonded hydroxyl group. The observed tight binding inhibition by (R)-Coformycin analogues as transition state inhibitors results from the similarity of the partial charges on the inhibitors to that of the enzymatic transition state stabilized by AMP deaminase.
B. Mark Britt - One of the best experts on this subject based on the ideXlab platform.
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Direct measurement of local and global contributions in the binding of Coformycin to bovine adenosine deaminase.
Journal of enzyme inhibition and medicinal chemistry, 2002Co-Authors: Esther A. Strohmeyer, Janel R. Beckley, B. Mark BrittAbstract:A general method is outlined that determines quantitatively the extent to which tight ligand binding to an enzyme active site is facilitated by the adoption of a stabler macromolecular conformation in the complex. The method therefore rejects the general assumption that competitive inhibitor binding to enzyme active sites involves only local (active site) interactions. The procedure involves comparing the unfolding transition state free energies of the free and complexed enzyme from physiological conditions. For the interaction of the transition state analog Coformycin with bovine adenosine deaminase we observed that the binding free energy by the physiological enzyme was ~92% due to the assumption of a stabler enzyme conformation in the complex. The significance of these findings in terms of general enzyme catalysis is discussed.
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Binding thermodynamics of the transition state analogue Coformycin and of the ground state analogue 1-deazaadenosine to bovine adenosine deaminase.
Journal of enzyme inhibition, 2001Co-Authors: Christian Castro, B. Mark BrittAbstract:Binding of the transition state analogue Coformycin and the ground state analogue 1-deaazadenosine to bovine adenosine deaminase have been thermody-namically characterized. The heat capacity changes for Coformycin and 1-deazaadenosine binding are - 4.7 × 0.8 kJ/mole-K and -1.2 × 0.1 kJ/mole-K, respectively. Since the predominant source of heat capacity change in enzyme interactions are changes in the extent of exposure of nonpolar amino acid side chains to the aqueous environment and the hydrophobic effect is the predominant factor in native structure stabilization, we propose that the binding of either class of ligand is associated with a stabilizing enzyme conformational change with Coformycin producing the far greater effect Analysis of the T dependence of the second order rate constant for formation of the enzyme/Coformycin complex further reveals that the conformational change is not rate limiting. We propose that the enzyme may facilitate catalysis via the formation of a stabilizing conformation at ...
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Evidence for a low temperature transition state binding preference in bovine adenosine deaminase.
Biophysical chemistry, 1998Co-Authors: Christian Castro, B. Mark BrittAbstract:Abstract Arrhenius plots of the interactions of bovine adenosine deaminase (ADA) and of Coformycin-inhibited ADA with adenosine are non-linear and reveal that Coformycin significantly increases the activation energy for reaction only at temperatures well below the normal operating temperature of the enzyme (38.3°C). This apparent enhanced affinity of the enzyme for the transition state analog at low temperature is confirmed from determinations of Coformycin biding at 38.3°C (K1 = 5.3 × 10−11 M) and at 21°C (K1 = 1.1 × 10−11 M). It is suggested that these data are inconsistent with a model for general enzyme catalysis that requires an initial transition state complementary active site. Instead, it is suggested that an initial active site transition state complementarity is undesirable and the tendency of the enzyme to exist in this conformer at low temperatures is responsible for its inefficient interaction with adenosine substrate.
Osamu Hayaishi - One of the best experts on this subject based on the ideXlab platform.
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adenosine in the tuberomammillary nucleus inhibits the histaminergic system via a1 receptors and promotes non rapid eye movement sleep
Proceedings of the National Academy of Sciences of the United States of America, 2008Co-Authors: Yo Oishi, Yoshihiro Urade, Zhili Huang, Bertil B. Fredholm, Osamu HayaishiAbstract:Adenosine has been proposed to promote sleep through A1 receptors (A1R's) and/or A2A receptors in the brain. We previously reported that A2A receptors mediate the sleep-promoting effect of prostaglandin D2, an endogenous sleep-inducing substance, and that activation of these receptors induces sleep and blockade of them by caffeine results in wakefulness. On the other hand, A1R has been suggested to increase sleep by inhibition of the cholinergic region of the basal forebrain. However, the role and target sites of A1R in sleep–wake regulation remained controversial. In this study, immunohistochemistry revealed that A1R was expressed in histaminergic neurons of the rat tuberomammillary nucleus (TMN). In vivo microdialysis showed that the histamine release in the frontal cortex was decreased by microinjection into the TMN of N6-cyclopentyladenosine (CPA), an A1R agonist, adenosine or Coformycin, an inhibitor of adenosine deaminase, which catabolizes adenosine to inosine. Bilateral injection of CPA into the rat TMN significantly increased the amount and the delta power density of non-rapid eye movement (non-REM; NREM) sleep but did not affect REM sleep. CPA-promoted sleep was observed in WT mice but not in KO mice for A1R or histamine H1 receptor, indicating that the NREM sleep promoted by A1R-specific agonist depended on the histaminergic system. Furthermore, the bilateral injection of adenosine or Coformycin into the rat TMN increased NREM sleep, which was completely abolished by coadministration of 1,3-dimethyl-8-cyclopenthylxanthine, a selective A1R antagonist. These results indicate that endogenous adenosine in the TMN suppresses the histaminergic system via A1R to promote NREM sleep.
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Dominant localization of adenosine deaminase in leptomeninges and involvement of the enzyme in sleep.
Biochemical and biophysical research communications, 2003Co-Authors: Tetsuya Okada, Yoshihiro Urade, Zhili Huang, Takatoshi Mochizuki, Naomi Eguchi, Yoshiro Sugita, Osamu HayaishiAbstract:Adenosine is an endogenous hypnotic molecule. However, the mechanism by which the level of extracellular adenosine is regulated remains to be elucidated. We found by Northern hybridization and enzyme assay that ecto-5(')-nucleotidase and adenosine deaminase (ADA), major enzymes responsible for the production and degradation of adenosine, respectively, were localized most abundantly in the leptomeninges within the rat brain. Immunohistochemical study showed that ADA was dominantly localized in arachnoid barrier and trabecular cells of the leptomeninges. In vivo microdialysis demonstrated that externally applied adenosine was rapidly metabolized by ADA to inosine in the subarachnoid space. Perfusion of an ADA inhibitor, Coformycin, increased the extracellular adenosine level in the subarachnoid space under the rostral basal forebrain. When Coformycin was continuously infused into the subarachnoid space, non-rapid eye movement sleep was increased with prolonged duration of the sleep episode. These results demonstrate that the leptomeninges control the extracellular level of adenosine in the subarachnoid space by their high 5(')-nucleotidase and ADA activities and regulate non-rapid eye movement sleep.
Ramachandra S. Hosmane - One of the best experts on this subject based on the ideXlab platform.
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Inhibitors of adenosine deaminase: continued studies of structure-activity relationships in analogues of Coformycin.
Nucleosides nucleotides & nucleic acids, 2004Co-Authors: Ayub Reayi, Ramachandra S. HosmaneAbstract:Synthesis and adenosine deaminase (ADA) inhibitory activity of two analogues of Coformycin, containing the imidazo[4,5-e][1,2,4]triazepine ring system, have been reported as part of the structure-activity relationship (SAR) studies to explore the factors responsible for the extremely tight-binding characteristics of Coformycins to ADA.
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Ring-expanded ("Fat") nucleosides as broad-spectrum anticancer and antiviral agents.
Current topics in medicinal chemistry, 2002Co-Authors: Ramachandra S. HosmaneAbstract:Ring-expanded (ldauo;fat") nucleosides (RENs) described in this review are analogues of purine nucleosides containing a 5:7-fused imidazodiazepine or imidazotriazepine ring system. They are both of natural and synthetic origin, and are of chemical, biochemical, biophysical, as well as medicinal interest. The important natural RENs include Coformycin, pentostatin, azepinomycin, adechlorin, and adecypenol. A majority of them are synergistic antitumor and/or antiviral antibiotics which potentiate the effects of other antitumor or antiviral compounds through inhibition of key enzymes such as adenosine deaminase or guanase which would otherwise metabolically degrade the active compounds into therapeutically less potent or totally inactive counterparts. However, despite the fact that some of the natural RENs such as Coformycins are the strongest known enzyme inhibitors, they have not been proven as effective clinically as anticipated because of the extremely high toxicity associated with their use. Nevertheless, pentostatin (2'-deoxyCoformycin) is a conspicuous exception as it is gaining wide attention in recent years as a clinically effective therapeutic agent against leukemias and lymphomas. Many of the recently reported synthetic RENs, by contrast, possess biological activities of their own, in particular against a wide spectrum of cancers and viruses with little toxicity to the host cells, and thus hold considerable promise as chemotherapeutic agents. The promising preliminary in vitro data concerning the effects of RENs on human cancers, in particular prostate and breast cancer cells, support their further pursuit in animal and clinical studies. RENs also carry promise against many viral infections belonging to the families of hepatitis, herpes, and respiratory infections, most notable being the hepatitis B (HBV), hepatitis C (HCV), and the West Nile (WNV) viruses.
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How Important Is theN-3 Sugar Moiety in the Tight-Binding Interaction of Coformycin with Adenosine Deaminase?☆
Biochemical and biophysical research communications, 1997Co-Authors: Ramachandra S. Hosmane, Mikyung HongAbstract:Preliminary findings on the possible important role of the N-3 sugar moiety of Coformycin in its tight-binding interaction with adenosine deaminase (ADA) are reported. The compound 3-β-D-Ribofuranosyl-5,6,7,8tetrahydro-4H-imidazo[4,5-d][1,3]diazepin-5-one-8-ol (1), its 3-benzyl analogue (6), and the aglycon (7) served as probes. The first two were both found to be competitive inhibitors of ADA with Ki's in the range of 10−5M,while the last one was inactive.
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Irreversible, Tight-Binding Inhibition of Adenosine Deaminase by Coformycins: Inhibitor Structural Features That Contribute to the Mode of Enzyme Inhibition
Nucleosides and Nucleotides, 1997Co-Authors: Mikyung Hong, Ramachandra S. HosmaneAbstract:Abstract Coformycin analogues 1–6 were synthesized and biochemically screened against adenosine deaminase in order to assess the relative contributions of N-4, N-6, and the N-3 sugar moiety to the mode of enzyme inhibition. Our results indicate that N-4 plays a relatively greater role than N-6 in enzyme tight-binding, and that a benzyl group can substitute for the sugar moiety at N-3. The absence of a sugar or benzyl group at N-3, however, leads to loss of activity. The hydroxyl group at C-8, while crucial for activity, does not alone confer the tight-binding characteristics to Coformycins.
Stephen D. Lindell - One of the best experts on this subject based on the ideXlab platform.
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The design and synthesis of inhibitors of adenosine 5'-monophosphate deaminase.
Bioorganic & Medicinal Chemistry Letters, 1999Co-Authors: Stephen D. Lindell, Brian A. Moloney, Brian D. Hewitt, Christopher G. Earnshaw, Philip J. Dudfield, Jane E. DancerAbstract:Abstract Carbocylic Coformycin (4) is a potent herbicide whose primary mode of action involves inhibition of adenosine 5′-monophosphate deaminase (AMPDA) following phosphorylation of the 5′-hydroxyl group in vivo. The search for more stable and accessible structures led to the synthesis of carbocyclic nebularine (8) and deaminoformycin (10). The latter compound is a good herbicide and its corresponding 5′-monophosphate 14 is a strong inhibitor of plant AMPDA (IC50 100 nM).
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Adenosine-5[prime]-Phosphate Deaminase (A Novel Herbicide Target)
Plant physiology, 1997Co-Authors: J. E. Dancer, R. G. Hughes, Stephen D. LindellAbstract:The isolation of carbocyclic Coformycin as the herbicidally active component from a fermentation of Saccharothrix species was described previously (B.D. Bush, G.V. Fitchett, D.A. Gates, D. Langley [1993] Phytochemistry 32: 737-739). Here we report that the primary mode of action of carbocyclic Coformycin has been identified as inhibition of the enzyme AMP deaminase (EC 3.5.4.6) following phosphorylation at the 5' hydroxyl on the carbocyclic ring in vivo. When pea (Pisum sativum L. var Onward) seedlings are treated with carbocyclic Coformycin, there is a very rapid and dramatic increase in ATP levels, indicating a perturbation in purine metabolism. Investigation of the enzymes of purine metabolism showed a decrease in the extractable activity of AMP deaminase that correlates with a strong, noncovalent association of the phosphorylated natural product with the protein. The 5'-phosphate analog of the carbocyclic Coformycin was synthesized and shown to be a potent, tight binding inhibitor of AMP deaminase isolated from pea seedlings. Through the use of a synthetic radiolabeled marker, rapid conversion of carbocyclic Coformycin to the 5'-phosphate analog could be demonstrated in vivo. It is proposed that inhibition of AMP deaminase leads to the death of the plant through perturbation of the intracellular ATP pool.
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Total synthesis of carbocyclic analogues of Coformycin
Tetrahedron, 1994Co-Authors: E. A. Saville‐stones, Stephen D. Lindell, Richard M. Turner, N. S. Jennings, J. C. Head, David S. CarverAbstract:Abstract Four carbocyclic analogues of the ribonucleoside Coformycin, including the recently isolated natural product 2, have been synthesised in racemic form. The syntheses were achieved in a convergent and direct manner via palladium(0) catalysed coupling between diazepinones 15 and 16 and the allylic acetate 5.