The influence of inorganic phosphate on the activity of adenosine kinase

Calcium Phosphate-Reinforced Metal-Organic Frameworks Regulate Adenosine-Mediated Immunosuppression

Long-term accumulation of adenosine (Ado) in tumor tissues helps to establish the immunosuppressive tumor microenvironment and to promote tumor development. Regulation of Ado metabolism is particularly pivotal for blocking Ado-mediated immunosuppression. The activity of adenosine kinase (ADK) for catalyzing the phosphorylation of Ado plays an essential role in regulating Ado metabolism. Specifically, accumulated Ado in the tumor microenvironment occupies the active site of ADK, inhibiting the phosphorylation of Ado. Phosphate can protect ADK from inactivation and restore the activity of ADK. Herein, calcium phosphate-reinforced iron-based metal-organic frameworks (CaP@Fe-MOFs) are designed to reduce Ado accumulation and to inhibit Ado-mediated immunosuppressive response in the tumor microenvironment. CaP@Fe-MOFs are found to regulate the Ado metabolism by promoting ADK-mediated phosphorylation and relieving the hypoxic tumor microenvironment. Moreover, CaP@Fe-MOFs can enhance the antitumor immune response via Ado regulation, including the increase of T lymphocytes and dendritic cells and the decrease of regulatory T lymphocytes. Finally, CaP@Fe-MOFs are used for cancer treatment in mice, alleviating the Ado-mediated immunosuppressive response and achieving tumor suppression. This study may offer a general strategy for blocking the Ado-mediated immunosuppression in the tumor microenvironment and further for enhancing the immunotherapy efficacy in vivo.

Adenosine Kinase couples sensing of cellular potassium depletion to purine metabolism

Adenosine Kinase (ADK) regulates the cellular levels of adenosine (ADO) by fine-tuning its metabolic clearance. The transfer of γ-phosphate from ATP to ADO by ADK involves regulation by the substrates and products, as well as by Mg²⁺ and inorganic phosphate. Here we present new crystal structures of mouse ADK (mADK) binary (mADK:ADO; 1.2 Å) and ternary (mADK:ADO:ADP; 1.8 Å) complexes. In accordance with the structural demonstration of ADO occupancy of the ATP binding site, kinetic studies confirmed a competitive model of auto-inhibition of ADK by ADO. In the ternary complex, a K⁺ ion is hexacoordinated between loops adjacent to the ATP binding site, where Asp310 connects the K⁺ coordination sphere to the ATP binding site through an anion hole structure. Nuclear Magnetic Resonance 2D ¹⁵N-¹H HSQC experiments revealed that the binding of K⁺ perturbs Asp310 and residues of adjacent helices 14 and 15, engaging a transition to a catalytically productive structure. Consistent with the structural data, the mutants D310A and D310P are catalytically deficient and loose responsiveness to K⁺. Saturation Transfer Difference spectra of ATPγS provided evidence for an unfavorable interaction of the mADK D310P mutant for ATP. Reductions in K⁺ concentration diminish, whereas increases enhance the in vitro activity of mADK (maximum of 2.5-fold; apparent K_d = 10.4 mM). Mechanistically, K⁺ increases the catalytic turnover (K_cat) but does not affect the affinity of mADK for ADO or ATP. Depletion of intracellular K⁺ inhibited, while its restoration was accompanied by a full recovery of cellular ADK activity. Together, this novel dataset reveals the molecular basis of the allosteric activation of ADK by K⁺ and highlights the role of ADK in connecting depletion of intracellular K⁺ to the regulation of purine metabolism.

9-(2'-Deoxy-2'-Fluoro-β-d-Arabinofuranosyl) Adenine Is a Potent Antitrypanosomal Adenosine Analogue That Circumvents Transport-Related Drug Resistance

Current chemotherapy against African sleeping sickness, a disease caused by the protozoan parasite Trypanosoma brucei, is limited by toxicity, inefficacy, and drug resistance. Nucleoside analogues have been successfully used to cure T. brucei-infected mice, but they have the limitation of mainly being taken up by the P2 nucleoside transporter, which, when mutated, is a common cause of multidrug resistance in T. brucei We report here that adenine arabinoside (Ara-A) and the newly tested drug 9-(2'-deoxy-2'-fluoro-β-d-arabinofuranosyl) adenine (FANA-A) are instead taken up by the P1 nucleoside transporter, which is not associated with drug resistance. Like Ara-A, FANA-A was found to be resistant to cleavage by methylthioadenosine phosphorylase, an enzyme that protects T. brucei against the antitrypanosomal effects of deoxyadenosine. Another important factor behind the selectivity of nucleoside analogues is how well they are phosphorylated within the cell. We found that the T. brucei adenosine kinase had a higher catalytic efficiency with FANA-A than the mammalian enzyme, and T. brucei cells treated with FANA-A accumulated high levels of FANA-A triphosphate, which even surpassed the level of ATP and led to cell cycle arrest, inhibition of DNA synthesis, and the accumulation of DNA breaks. FANA-A inhibited nucleic acid biosynthesis and parasite proliferation with 50% effective concentrations (EC₅₀s) in the low nanomolar range, whereas mammalian cell proliferation was inhibited in the micromolar range. Both Ara-A and FANA-A, in combination with deoxycoformycin, cured T. brucei-infected mice, but FANA-A did so at a dose 100 times lower than that of Ara-A.

An uncharacterized member of the ribokinase family in Thermococcus kodakarensis exhibits myo-inositol kinase activity

Here we performed structural and biochemical analyses on the TK2285 gene product, an uncharacterized protein annotated as a member of the ribokinase family, from the hyperthermophilic archaeon Thermococcus kodakarensis. The three-dimensional structure of the TK2285 protein resembled those of previously characterized members of the ribokinase family including ribokinase, adenosine kinase, and phosphofructokinase. Conserved residues characteristic of this protein family were located in a cleft of the TK2285 protein as in other members whose structures have been determined. We thus examined the kinase activity of the TK2285 protein toward various sugars recognized by well characterized ribokinase family members. Although activity with sugar phosphates and nucleosides was not detected, kinase activity was observed toward d-allose, d-lyxose, d-tagatose, d-talose, d-xylose, and d-xylulose. Kinetic analyses with the six sugar substrates revealed high Km values, suggesting that they were not the true physiological substrates. By examining activity toward amino sugars, sugar alcohols, and disaccharides, we found that the TK2285 protein exhibited prominent kinase activity toward myo-inositol. Kinetic analyses with myo-inositol revealed a greater kcat and much lower Km value than those obtained with the monosaccharides, resulting in over a 2,000-fold increase in kcat/Km values. TK2285 homologs are distributed among members of Thermococcales, and in most species, the gene is positioned close to a myo-inositol monophosphate synthase gene. Our results suggest the presence of a novel subfamily of the ribokinase family whose members are present in Archaea and recognize myo-inositol as a substrate.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Non-nucleoside inhibitors of human adenosine kinase: synthesis, molecular modeling, and biological studies

Adenosine kinase (AK) catalyzes the phosphorylation of adenosine (Ado) to AMP by means of a kinetic mechanism in which the two substrates Ado and ATP bind the enzyme in a binary and/or ternary complex, with distinct protein conformations. Most of the described inhibitors have Ado-like structural motifs and are nonselective, and some of them (e.g., the tubercidine-like ligands) are characterized by a toxic profile. We have cloned and expressed human AK (hAK) and searched for novel non-substrate-like inhibitors. Our efforts to widen the structural diversity of AK inhibitors led to the identification of novel non-nucleoside, noncompetitive allosteric modulators characterized by a unique molecular scaffold. Among the pyrrolobenzoxa(thia)zepinones (4a-qq) developed, 4a was identified as a non-nucleoside prototype hAK inhibitor. 4a has proapoptotic efficacy, slight inhibition of short-term RNA synthesis, and cytostatic activity on tumor cell lines while showing low cytotoxicity and no significant adverse effects on short-term DNA synthesis in cells.

Structure-activity relationship for adenosine kinase from Mycobacterium tuberculosis II. Modifications to the ribofuranosyl moiety

Adenosine kinase (Ado kinase) from Mycobacterium tuberculosis is structurally and biochemically unique from other known Ado kinases. This purine salvage enzyme catalyzes the first step in the conversion of the adenosine analog, 2-methyl-Ado (methyl-Ado), into a metabolite with antitubercular activity. Methyl-Ado has provided proof of concept that the purine salvage pathway from M. tuberculosis may be utilized for the development of antitubercular compounds with novel mechanisms of action. In order to utilize this enzyme, it is necessary to understand the topography of the active site to rationally design compounds that are more potent and selective substrates for Ado kinase. A previous structure-activity relationship identified modifications to the base moiety of adenosine (Ado) that result in substrate and inhibitor activity. In an extension of that work, 62 Ado analogs with modifications to the ribofuranosyl moiety, modifications to the base and ribofuranosyl moiety, or modifications to the glycosidic bond position have been analyzed as substrates and inhibitors of M. tuberculosis Ado kinase. A subset of these compounds was further analyzed in human Ado kinase for the sake of comparison. Although no modifications to the ribose moiety resulted in compounds as active as Ado, the best substrates identified were carbocyclic-Ado, 8-aza-carbocyclic-Ado, and 9-[alpha-l-lyxofuranosyl]-adenine with 38%, 4.3%, and 3.8% of the activity of Ado, respectively. The most potent inhibitor identified, 5'-amino-5'-deoxy-Ado, had a K(i)=0.8muM and a competitive mode of inhibition. MIC studies demonstrated that poor substrates could still have potent antitubercular activity.

Adenosine kinase mediates high affinity adenosine salvage in Trypanosoma brucei

African sleeping sickness is caused by Trypanosoma brucei. This extracellular parasite lacks de novo purine biosynthesis, and it is therefore dependent on exogenous purines such as adenosine that is taken up from the blood and other body fluids by high affinity transporters. The general belief is that adenosine needs to be cleaved to adenine inside the parasites in order to be used for purine nucleotide synthesis. We have found that T. brucei also can salvage this nucleoside by adenosine kinase (AK), which has a higher affinity to adenosine than the cleavage-dependent pathway. The recombinant T. brucei AK (TbAK) preferably used ATP or GTP to phosphorylate both natural and synthetic nucleosides in the following order of catalytic efficiencies: adenosine > cordycepin > deoxyadenosine > adenine arabinoside (Ara-A) > inosine > fludarabine (F-Ara-A). TbAK differed from the AK of the related intracellular parasite Leishmania donovani by having a high affinity to adenosine (K m = 0.04-0.08 microm depending on [phosphate]) and by being negatively regulated by adenosine (K i = 8-14 microm). These properties make the enzyme functionally related to the mammalian AKs, although a phylogenetic analysis grouped it together with the L. donovani enzyme. The combination of a high affinity AK and efficient adenosine transporters yields a strong salvage system in T. brucei, a potential Achilles' heel making the parasites more sensitive than mammalian cells to adenosine analogs such as Ara-A. Studies of wild-type and AK knockdown trypanosomes showed that Ara-A inhibited parasite proliferation and survival in an AK-dependent manner by affecting nucleotide levels and by inhibiting nucleic acid biosynthesis.

Development of off-line and on-line capillary electrophoresis methods for the screening and characterization of adenosine kinase inhibitors and substrates

Fast and convenient CE assays were developed for the screening of adenosine kinase (AK) inhibitors and substrates. In the first method, the enzymatic reaction was performed in a test tube and the samples were subsequently injected into the capillary by pressure and detected by their UV absorbance at 260 nm. An MEKC method using borate buffer (pH 9.5) containing 100 mM SDS (method A) was suitable for separating alternative substrates (nucleosides). For the CE determination of AMP formed as a product of the AK reaction, a phosphate buffer (pH 7.5 or 8.5) was used and a constant current (95 microA) was applied (method B). The methods employing a fused-silica capillary and normal polarity mode provided good resolution of substrates and products of the enzymatic reaction and a short analysis time of less than 10 min. To further optimize and miniaturize the AK assays, the enzymatic reaction was performed directly in the capillary, prior to separation and quantitation of the product employing electrophoretically mediated microanalysis (EMMA, method C). After hydrodynamic injection of a plug of reaction buffer (20 mM Tris-HCl, 0.2 mM MgCl2, pH 7.4), followed by a plug containing the enzyme, and subsequent injection of a plug of reaction buffer containing 1 mM ATP, 100 microM adenosine, and 20 microM UMP as an internal standard (I.S.), as well as various concentrations of an inhibitor, the reaction was initiated by the application of 5 kV separation voltage (negative polarity) for 0.20 min to let the plugs interpenetrate. The voltage was turned off for 5 min (zero-potential amplification) and again turned on at a constant current of -60 microA to elute the products within 7 min. The method employing a polyacrylamide-coated capillary of 20 cm effective length and reverse polarity mode provided good resolution of substrates and products. Dose-response curves and calculated K(i) values for standard antagonists obtained by CE were in excellent agreement with data obtained by the standard radioactive assay.

Mutational analysis of the active-site residues crucial for catalytic activity of adenosine kinase from Leishmania donovani

Leishmania donovani adenosine kinase (LdAdK) plays a pivotal role in scavenging of purines from the host. Exploiting interspecies homology and structural co-ordinates of the enzyme from other sources, we generated a model of LdAdK that led us to target several amino acid residues (namely Gly-62, Arg-69, Arg-131 and Asp-299). Replacement of Gly-62 with aspartate caused a drastic reduction in catalytic activity, with decreased affinity for either substrate. Asp-299 was found to be catalytically indispensable. Mutation of either Arg-131 or Arg-69 caused a significant reduction in kcat. R69A (Arg-69-->Ala) and R131A mutants exhibited unaltered K(m) for either substrate, whereas ATP K(m) for R69K increased 6-fold. Importance of both of the arginine residues was reaffirmed by the R69K/R131A double mutant, which exhibited approx. 0.5% residual activity with a large increase in ATP K(m). Phenylglyoxal, which inhibits the wild-type enzyme, also inactivated the arginine mutants to different extents. Adenosine protected both of the Arg-69 mutants, but not the R131A variant, from inactivation. Binding experiments revealed that the AMP-binding property of R69K or R69A and D299A mutants remained largely unaltered, but R131A and R69K/R131A mutants lost their AMP binding ability significantly. The G62D mutant did not bind AMP at all. Free energy calculations indicated that Arg-69 and Arg-131 are functionally independent. Thus, apart from the mandatory requirement of flexibility around the diglycyl (Gly-61-Gly-62) motif, our results identified Asp-299 and Arg-131 as key catalytic residues, with the former functioning as the proton abstractor from the 5'-OH of adenosine, while the latter acts as a bidentate electrophile to stabilize the negative charge on the leaving group during the phosphate transfer. Moreover, the positive charge distribution of Arg-69 probably helps in maintaining the flexibility of the alpha-3 helix needed for proper domain movement. These findings provide the first comprehensive biochemical evidence implicating the mechanistic roles of the functionally important residues of this chemotherapeutically exploitable enzyme.

Inhibition of adenosine kinase by phosphonate and bisphosphonate derivatives

The enzyme adenosine kinase (AK) plays a central role in regulating the intracellular and interstitial concentration of the purine nucleoside adenosine (Ado). In view of the beneficial effects of Ado in protecting tissues from ischemia and other stresses, there is much interest in developing AK inhibitors, which can regulate Ado concentration in a site- and event-specific manner. The catalytic activity of AK from different sources is dependent upon the presence of activators such as phosphate (Pi). In this work we describe several new phosphorylated compounds which either activate or inhibit AK. The compounds acetyl phosphate, carbamoyl phosphate, dihydroxyacetone phosphate and imidodiphosphate were found to stimulate AK activity in a dose-dependent manner comparable to that seen with Pi. In contrast, a number of phosphonate and bisphosphonate derivatives, which included clodronate and etidronate, were found to inhibit the activity of purified AK in the presence of Pi. These AK inhibitors (viz. clodronate, etidronate, phosphonoacetic acid, 2-carboxyethylphosphonic acid, N-(phosphonomethyl)-glycine and N-(phosphonomethyl)iminodiacetic acid), at concentrations at which they inhibited AK, were also shown to inhibit the uptake of (3)H-adenosine and its incorporation into macromolecules in cultured mammalian cells, indicating that they were also inhibiting AK in intact cells. The drug concentrations at which these effects were observed showed limited toxicity to the cultured cells, indicating that these effects are not caused by cellular toxicity. These results indicate that the enzyme AK provides an additional cellular target for the clinically widely used bisphosphonates and related compounds, which could possibly be exploited for a new therapeutic application. Our structure-activity studies on different AK activators and inhibitors also indicate that all of the AK activating compounds have a higher partial positive charge (delta(+)) on the central phosphorous atom in comparison to the inhibitors. This information should prove helpful in the design and synthesis of more potent inhibitors of AK.

A systematic high-throughput screen of a yeast deletion collection for mutants defective in PHO5 regulation

In response to phosphate limitation, Saccharomyces cerevisiae induces transcription of a set of genes important for survival. One of these genes is PHO5, which encodes a secreted acid phosphatase. A phosphate-responsive signal transduction pathway (the PHO pathway) mediates this response through three central components: a cyclin-dependent kinase (CDK), Pho85; a cyclin, Pho80; and a CDK inhibitor (CKI), Pho81. While signaling downstream of the Pho81/Pho80/Pho85 complex to PHO5 expression has been well characterized, little is known about factors acting upstream of these components. To identify missing factors involved in the PHO pathway, we carried out a high-throughput, quantitative enzymatic screen of a yeast deletion collection, searching for novel mutants defective in expression of PHO5. As a result of this study, we have identified at least nine genes that were previously not known to regulate PHO5 expression. The functional diversity of these genes suggests that the PHO pathway is networked with other important cellular signaling pathways. Among these genes, ADK1 and ADO1, encoding an adenylate kinase and an adenosine kinase, respectively, negatively regulate PHO5 expression and appear to function upstream of PHO81.

Phosphorylated derivatives that activate or inhibit mammalian adenosine kinase provide insights into the role of pentavalent ions in AK catalysis

The enzyme adenosine kinase (AK) exhibits a nearly complete dependency on the presence of pentavalent ions (PVI) such as phosphate, arsenate, and vanadate. To understand its basis, the effect of a large number of phosphorylated compounds on AK activity was examined. Several compounds, such as phosphoribosyl pyrophosphate, phosphoenol pyruvate, creatine phosphate, phosphorous acid, phosphonoformic acid, and inorganic pyrophosphate, were found to substitute for PVI in stimulating AK activity. Similar to PVI, these compounds lowered the Km of AK for adenosine. In contrast, many other structurally related compounds (i.e., phosphonoacetic acid, 2-carboxyethyl phosphonic acid, N-phosphonomethyl glycine, N-phosphonomethyl iminodiacetic acid) inhibited AK activity. These compounds seemed to compete with the activators for binding to AK. Structural comparisons of different compounds indicate that all activating compounds contain a net positive charge on the pentavalent atom (e.g., phosphorous), which should enable it to act as an acceptor for a nucleophilic group. We suggest that a phosphate (or other activator) bound near the active site participates in AK catalysis by forming a transient pentavalent intermediate with a nonbridging oxygen of the beta-phosphate in ATP. This interaction likely facilitates the transfer of gamma-phosphate from ATP to adenosine, thus accounting for the stimulating role of PVI in AK catalysis. The insight provided by these studies concerning the structural features of activators and inhibitors should also prove helpful in the design of more potent inhibitors of AK.

Identification and characterization of a unique adenosine kinase from Mycobacterium tuberculosis

Adenosine kinase (AK) is a purine salvage enzyme that catalyzes the phosphorylation of adenosine to AMP. In Mycobacterium tuberculosis, AK can also catalyze the phosphorylation of the adenosine analog 2-methyladenosine (methyl-Ado), the first step in the metabolism of this compound to an active form. Purification of AK from M. tuberculosis yielded a 35-kDa protein that existed as a dimer in its native form. Adenosine (Ado) was preferred as a substrate at least 30-fold (Km = 0.8 +/- 0.08 microM) over other natural nucleosides, and substrate inhibition was observed when Ado concentrations exceeded 5 micro M. M. tuberculosis and human AKs exhibited different affinities for methyl-Ado, with Km values of 79 and 960 microM, respectively, indicating that differences exist between the substrate binding sites of these enzymes. ATP was a good phosphate donor (Km = 1100 +/- 140 microM); however, the activity levels observed with dGTP and GTP were 4.7 and 2.5 times the levels observed with ATP, respectively. M. tuberculosis AK activity was dependent on Mg2+, and activity was stimulated by potassium, as reflected by a decrease in the Km and an increase in Vmax for both Ado and methyl-Ado. The N-terminal amino acid sequence of the purified enzyme revealed complete identity with Rv2202c, a protein currently classified as a hypothetical sugar kinase. When an AK-deficient strain of M. tuberculosis (SRICK1) was transformed with this gene, it exhibited a 5,000-fold increase in AK activity compared to extracts from the original mutants. These results verified that the protein that we identified as AK was coded for by Rv2202c. AK is not commonly found in bacteria, and to the best of our knowledge, M. tuberculosis AK is the first bacterial AK to be characterized. The enzyme shows greater sequence homology with ribokinase and fructokinase than it does with other AKs. The multiple differences that exist between M. tuberculosis and human AKs may provide the molecular basis for the development of nucleoside analog compounds with selective activity against M. tuberculosis.

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

Cells of the cardiovascular system generate and release purine nucleoside adenosine in increasing quantities when constituent cells are "stressed" or subjected to injurious stimuli. This increased adenosine can interact with surface receptors in myocardial, vascular, fibroblast, and inflammatory cells to modulate cellular function and phenotype. Additionally, adenosine is rapidly reincorporated back into 5'-AMP to maintain the adenine nucleotide pool. Via these receptor-dependent and independent (metabolic) paths, adenosine can substantially modify the acute response to ischemic insult, in addition to generating a more sustained ischemia-tolerant phenotype (preconditioning). However, the molecular basis for acute adenosinergic cardioprotection remains incompletely understood and may well differ from more widely studied preconditioning. Here we review current knowledge and some controversies regarding acute cardioprotection via adenosine and adenosine receptor activation.

Expression inEscherichia coliof a recombinant adenosine kinase fromSaccharomyces cerevisiae: purification, kinetics and substrate analyses

The Saccharomyces cerevisiae ADO1 gene is known to encode a homologue of eukaryotic adenosine kinases. This gene was expressed in Escherichia coli as a recombinant protein fused to a polyhistidine tag by using the rhamnose-inducible bacterial promoter rhaB. The recombinant protein was purified to apparent homogeneity and its ability to phosphorylate different substrates was evaluated. Adenosine (Km 3 microM) is its primary substrate. In addition, it also phosphorylates, albeit less efficiently, 3'-deoxyadenosine (cordycepin; Km 1.84 mM) and 3'-amino-3'-deoxyadenosine (Km 0.26 mM). Other kinetic properties of the recombinant enzyme have also been determined.

Activation of ribokinase by monovalent cations

Carbohydrate kinases frequently require a monovalent cation for their activity. The physical basis of this phenomenon is, however, usually unclear. We report here that Escherichia coli ribokinase is activated by potassium with an apparent K(d) of 5 mM; the enzyme should therefore be fully activated under physiological conditions. Cesium can be used as an alternative ion, with an apparent K(d) of 17 mM. An X-ray structure of ribokinase in the presence of cesium was solved and refined at 2.34 A resolution. The cesium ion was bound between two loops immediately adjacent to the anion hole of the active site. The buried location of the site suggests that conformational changes will accompany ion binding, thus providing a direct mechanism for activation. Comparison with structures of a related enzyme, the adenosine kinase of Toxoplasma gondii, support this proposal. This is apparently the first instance in which conformational activation of a carbohydrate kinase by a monovalent cation has been assigned a clear structural basis. The mechanism is probably general to ribokinases, to some adenosine kinases, and to other members of the larger family. A careful re-evaluation of the biochemical and structural data is suggested for other enzyme systems.

The effect of inorganic phosphate on the activity of bacterial ribokinase

Ribokinase and adenosine kinase are both members of the PfkB family of carbohydrate kinases. The activity of mammalian adenosine kinase was previously shown to be affected by pentavalent ions (PVI). We now present evidence that the catalytic activity of E. coli ribokinase is also affected by PVI, increasing both the velocity and affinity of the enzyme for D-ribose. The Km, for ribose decreased from 0.61 mM to 0.21, 0.25, and 0.33 mM in the presence of 20 mM phosphate, arsenate, and vanadate, respectively. The activity of ribokinase was stimulated in a hyperbolic fashion, with the maximum velocity increasing 23-fold, 13-fold, and 11-fold under the same conditions, respectively. Activity was also affected upon the addition of phosphoenolpyruvate, suggesting that phosphorylated metabolites could be involved in enzymatic control. The similar effect of PVI on distantly related enzymes suggests that a common mechanism for activity is shared among PfkB family members.

Gene structure for adenosine kinase in Chinese hamster and human: high-frequency mutants of CHO cells involve deletions of several introns and exons

The structure for the adenosine kinase (AK) gene has been determined from Chinese hamster (CH) and human cells. The AK gene in CH is comprised of 11 exons ranging in length from 36 to 765 nt, with the majority <100 nt. The exact lengths of the intervening introns have not been determined, but most of them are indicated to be very large (>15 kb). A 6.6-kb fragment from human cells was also sequenced, and it contained only a single exon corresponding to exon 10 in CH. The BLAST searches of the subsequently released draft human genome sequence have revealed that the AK gene structure in human is identical to that in CH. In the human genome, the AK exons are distributed over four genomic clones totaling 752 kb, providing direct evidence that the AK gene in mammalian species is unusually large. In contrast to CH and human, the AK genes from several other eukaryotic organisms whose complete genomes are now known are quite small (between 1.2 and 2.5 kb) and either contain no introns (Saccharomyces cerevisiae and Schizosaccharomyces pombe) or various numbers of introns (Drosophila melanogaster [2], Caenorhabditis elegans [4], Arabidopsis thaliana [10]). Some of the intron-exon junctions in these species are in the same positions as in mammals. The AK gene in CH and human, as well as mouse, is linked upstream in a head-to-head fashion with the gene for the clathrin adaptor mu3 protein (or beta 3A subunit of the AP-3 protein complex), which is affected in type 2 Hermansky-Pudlak syndrome. These two genes are separated by <200 nt, and it is possible that they have a common or overlapping promoter(s). We have also determined the nature of the genetic alterations in two of the class A AK(-) mutants of CHO cells, which are obtained at a very high spontaneous frequency (10(-3)-10(-4)) in this cell line. Both mutants contained large deletions within the AK gene and greatly shortened AK transcripts. The cloning and sequencing of the transcripts from these mutants showed that the deletion in one of them led to the loss of exons 5 through 8, whereas in the other, all exons from 2 through 8 are deleted. The endpoints of these deletions lie in the large introns within the AK gene.

Structure-activity studies on mammalian adenosine kinase

The structure-activity relationship for Chinese hamster adenosine kinase (AK) was examined by making systematic deletions from the N- and C-terminal ends. The first 16 a.a. residues from the N-terminal end, which likely form a random coil, can be deleted without any effect on AK activity or stability. The successive removal of the next 11 residues, which stabilize the first beta structure of the protein, leads to a progressive loss of AK activity from 100 to about 3%. The loss in activity is accompanied by increasing thermal instability and a slight increase in the K(m) for adenosine. All deletions beyond residue M28, which should cause disruption of the tertiary structure, are devoid of AK activity. The residues at the C-terminal end form a substructure involved in the stability of the "adenosine 2 binding site" and removal of any residues results in significant loss of activity. Successive removal of the first 10 residues from this end causes progressive decrease in AK activity to about the 2% level, accompanied by a five-fold increase in the K(m) for ATP, supporting the view that the adenosine 2 binding site located near the C-terminal end is the ATP binding site. All deletions beyond residue R348, which forms two salt bridges with the ATP binding site, are inactive. Site-directed replacement of an aspartic acid residue (D316), which is postulated to function in the transfer of phosphate from ATP to adenosine by either asparagine or glutamic acid, leads to complete loss of activity, supporting the proposed role of D316 as the catalytic base.