Tyler, Peter C.; Taylor, Erika A.; Fröhlich, Richard F. G.; Schramm, Vern L.
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. Since 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, human adenosine deaminase (HsADA) does not deaminate 5’-methylthioadenosine. 5′-Methylthio coformycin and 5’-meththio-2′-deoxycoformycin were synthesized to be specific transition state mimics of the P. falciparum enzyme. These analogues inhibited PfADA with dissociation constants of 430 and 790 pM, respectively. Remarkably, they gave no detectable inhibition of the human and bovine enzymes. Adenosine deamination is involved in the essential pathway of purine salvage in P. falciparum and prior studies have shown that inhibition of purine salvage results in parasite death. Inhibitors of HsADA are known to cause toxicity in humans and the availability of parasite-specific ADA inhibitors may prevent this side-effect. The potent and P. falciparum-specific inhibitors described here have potential for development as antimalarials without inhibition of host ADA. PMID:17488013
Ho, M.; Cassera, M; Madrid, D; Ting, L; Tyler, P; Kim, K; Almo, S; Schramm, V
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. The structural basis for MTA and MT-coformycin specificity in malarial ADAs is the subject of speculation. 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 ADA complexes with adenosine or deoxycoformycin, 5?-methylthioribosyl groups are rotated 130 degrees. A hydrogen bonding network between Asp172 and the 3?-hydroxyl of MT-coformycin is essential for recognition of the 5?-methylthioribosyl group. Water occupies the 5?-hydroxyl binding site when MT-coformycin is bound. Mutagenesis of Asp172 destroys the substrate specificity for MTA and MT-coformycin. Kinetic, mutagenic, and structural analyses of PvADA and kinetic analysis of five other Plasmodium ADAs establish the unique structural basis for its specificity for MTA and MT-coformycin. Plasmodium gallinaceum ADA does not use MTA as a substrate, is not inhibited by MT-coformycin, and is missing Asp172. Treatment of P. falciparum cultures with coformycin or MT-coformycin in the presence of MTA is effective in inhibiting parasite growth.
Snyder, F F; Mendelsohn, J; Seegmiller, J E
The association of a human genetic deficiency of adenosine deaminase activity with combined immunodeficiency prompted a study of the effects of adenosine and of inhibition of adenosine deaminase activity on human lymphocyte transformation and a detailed study of adenosine metabolism throughout phytohemagglutinin-induced blastogenesis. The adenosine deaminase inhibitor, coformycin, at a concentration that inhibited adenosine deaminase activity more than 95%, or 50 muM adenosine, did not prevent blastogenesis by criteria of morphology or thymidine incorporation into acid-precipitable material. The combination of coformycin and adenosine, however, substantially reduced both the viable cell count and the incorporation of thymidine into DNA in phytohemagglutinin-stimulated lymphocytes. Incubation of lymphocytes with phytohemagglutinin for 72 h produced a 12-fold increase in the rate of deamination and a 6-fold increase in phosphorylation of adenosine by intact lymphocytes. There was no change in the apparent affinity for adenosine with either deamination or phosphorylation. The increased rates of metabolism, apparent as early as 3 h after addition of mitogen, may be due to increased entry of the nucleoside into stimulated lymphocytes. Increased adenosine metabolism was not due to changes in total enzyme activity; after 72 h in culture, the ratios of specific activities in extracts of stimulated to unstimulated lymphocytes were essentially unchanged for adenosine kinase, 0.92, and decreased for adenosine deaminase, 0.44. As much as 38% of the initial lymphocyte adenosine deaminase activity accumulated extracellularly after a 72-h culture with phytohemagglutinin. In phytohemagglutinin-stimulated lymphocytes, the principal route of adenosine metabolism was phosphorylation at less than 5 muM adenosine, and deamination at concentrations greater than 5 muM. In unstimulated lymphocytes, deamination was the principal route of adenosine metabolism over the range of adenosine
Agarwal, R P; Crabtree, G W; Parks, R E; Nelson, J A; Keightley, R; Parkman, R; Rosen, F S; Stern, R C; Polmar, S H
Deficiency of erythrocytic and lymphocytic adenosine deaminase (ADA) occurs in some patients with severe combined immunodeficiency disease (SCID). SCID with ADA deficiency is inherited as an autosomal recessive trait. ADA is markedly reduced or undetectable in affected patients (homozygotes), and approximately one-half normal levels are found in individuals heterozygous for ADA deficiency. The metabolism of purine nucleosides was studied in erythrocytes from normal individuals, four ADA-deficiency patients, and two heterozygous individuals. ADA deficiency in intake erythrocytes was confirmed by a very sensitive ammonia-liberation technique. Erythrocytic ADA activity in three heterozygous individuals (0.07,0.08, and 0.14 mumolar units/ml of packed cells) was between that of the four normal controls (0.20-0.37 mumol/ml) and the ADA-deficient patients (no activity). In vitro, adenosine was incorporated principally into IMP in the heterozygous and normal individuals but into the adenosine nucleotides in the ADa-deficient patients. Coformycin (3-beta-D-ribofuranosyl-6,7,8-trihydroimidazo[4,5-4] [1,3] diazepin-8 (R)-ol), a potent inhibitor of ADA, made possible incorporation of adenosine nucleotides in the ADA-deficient patients... PMID:947948
Hyrien, O; Debatisse, M; Buttin, G; de Saint Vincent, B R
The amplified DNA of HC50474, a Chinese hamster fibroblast cell line selected in three steps for high resistance to coformycin, consists chiefly of 150 copies of a large inverted duplication including the adenylate deaminase gene. Most if not all of these units are more than 2 x 120 kb long. The inverted duplication was first detected in the cells recovered from the second selection step, at the same chromosomal location as the first step amplified units. Its formation and amplification appear to be coupled since the second step cell line already contained 40 copies of this novel structure. Reamplification of the inverted duplication occurred at the third step of selection concomitant with the loss of amplified DNA acquired during the first step. The head-to-head junction has been formed by recombination within a recombinational hotspot described previously [Hyrien, O., Debatisse, M., Buttin, G. and Robert de Saint Vincent, B. (1987) EMBO J., 6, 2401-2408]. Sequences at the joint and in the corresponding wild-type region reveal that the crossover sites, one of which occurs in the putative promoter region of B2 repeat, are located at the top of significant stem-loop structures and that patchy homologies between the parental molecules on one side of the breakpoints allow alignment of these crossover sites. We present a model which explains the formation and amplification of this and other large inverted duplications by errors in DNA replication. Images PMID:3366118
Pornbanlualap, Somchai; Chalopagorn, Pornchanok
The sequencing of the genome of Streptomyces coelicolor A3(2) identified seven putative adenine/adenosine deaminases and adenosine deaminase-like proteins, none of which have been biochemically characterized. This report describes recombinant expression, purification and characterization of SCO4901 which had been annotated in data bases as a putative adenosine deaminase. The purified putative adenosine deaminase gives a subunit Mr=48,400 on denaturing gel electrophoresis and an oligomer molecular weight of approximately 182,000 by comparative gel filtration. These values are consistent with the active enzyme being composed of four subunits with identical molecular weights. The turnover rate of adenosine is 11.5 s⁻¹ at 30 °C. Since adenine is deaminated ∼10³ slower by the enzyme when compared to that of adenosine, these data strongly show that the purified enzyme is an adenosine deaminase (ADA) and not an adenine deaminase (ADE). Other adenine nucleosides/nucleotides, including 9-β-D-arabinofuranosyl-adenine (ara-A), 5'-AMP, 5'-ADP and 5'-ATP, are not substrates for the enzyme. Coformycin and 2'-deoxycoformycin are potent competitive inhibitors of the enzyme with inhibition constants of 0.25 and 3.4 nM, respectively. Amino acid sequence alignment of ScADA with ADAs from other organisms reveals that eight of the nine highly conserved catalytic site residues in other ADAs are also conserved in ScADA. The only non-conserved residue is Asn317, which replaces Asp296 in the murine enzyme. Based on these data, it is suggested here that ADA and ADE proteins are divergently related enzymes that have evolved from a common α/β barrel scaffold to catalyze the deamination of different substrates, using a similar catalytic mechanism. PMID:21511036