'Click cyclic ADP-ribose': a neutral second messenger mimic

Small Molecule CD38 Inhibitors: Synthesis of 8-Amino-N1-inosine 5'-monophosphate, Analogues and Early Structure-Activity Relationship

Although a monoclonal antibody targeting the multifunctional ectoenzyme CD38 is an FDA-approved drug, few small molecule inhibitors exist for this enzyme that catalyzes inter alia the formation and metabolism of the N1-ribosylated, Ca²⁺-mobilizing, second messenger cyclic adenosine 5′-diphosphoribose (cADPR). N1-Inosine 5′-monophosphate (N1-IMP) is a fragment directly related to cADPR. 8-Substituted-N1-IMP derivatives, prepared by degradation of cyclic parent compounds, inhibit CD38-mediated cADPR hydrolysis more efficiently than related cyclic analogues, making them attractive for inhibitor development. We report a total synthesis of the N1-IMP scaffold from adenine and a small initial compound series that facilitated early delineation of structure-activity parameters, with analogues evaluated for inhibition of CD38-mediated hydrolysis of cADPR. The 5′-phosphate group proved essential for useful activity, but substitution of this group by a sulfonamide bioisostere was not fruitful. 8-NH₂-N1-IMP is the most potent inhibitor (IC₅₀ = 7.6 μM) and importantly HPLC studies showed this ligand to be cleaved at high CD38 concentrations, confirming its access to the CD38 catalytic machinery and demonstrating the potential of our fragment approach.

Probing the Ca2+ mobilizing properties on primary cortical neurons of a new stable cADPR mimic

Cyclic adenosine diphosphate ribose (cADPR) is a second messenger involved in the Ca²⁺ homeostasis. Its chemical instability prompted researchers to tune point by point its structure, obtaining stable analogues featuring interesting biological properties. One of the most challenging derivatives is the cyclic inosine diphosphate ribose (cIDPR), in which the hypoxanthine isosterically replaces the adenine. As our research focuses on the synthesis of N1 substituted inosines, in the last few years we have produced new flexible cIDPR analogues, where the northern ribose has been replaced by alkyl chains. Interestingly, some of them mobilized Ca²⁺ ions in PC12 cells. To extend our SAR studies, herein we report on the synthesis of a new stable cIDPR derivative which contains the 2″S,3″R dihydroxypentyl chain instead of the northern ribose. Interestingly, the new cyclic derivative and its open precursor induced an increase in intracellular calcium concentration ([Ca²⁺]_i) with the same efficacy of the endogenous cADPR in rat primary cortical neurons.

Roles of NAD+ and Its Metabolites Regulated Calcium Channels in Cancer

Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor for redox enzymes, but also moonlights as a regulator for ion channels, the same as its metabolites. Ca²⁺ homeostasis is dysregulated in cancer cells and affects processes such as tumorigenesis, angiogenesis, autophagy, progression, and metastasis. Herein, we summarize the regulation of the most common calcium channels (TRPM2, TPCs, RyRs, and TRPML1) by NAD⁺ and its metabolites, with a particular focus on their roles in cancers. Although the mechanisms of NAD⁺ metabolites in these pathological processes are yet to be clearly elucidated, these ion channels are emerging as potential candidates of alternative targets for anticancer therapy.

Synthesis of phosphonoacetate analogues of the second messenger adenosine 5'-diphosphate ribose (ADPR)

Adenosine 5′-diphosphate ribose (ADPR) is an intracellular signalling molecule generated from nicotinamide adenine dinucleotide (NAD⁺). Synthetic ADPR analogues can shed light on the mechanism of activation of ADPR targets and their downstream effects. Such chemical biology studies, however, are often challenging due to the negatively charged pyrophosphate that is also sensitive to cellular pyrophosphatases. Prior work on an initial ADPR target, the transient receptor potential cation channel TRPM2, showed complete pyrophosphate group replacement to be a step too far in maintaining biological activity. Thus, we designed ADPR analogues with just one of the negatively charged phosphate groups removed, by employing a phosphonoacetate linker. Synthesis of two novel phosphonoacetate ADPR analogues is described via tandem N,N′-dicyclohexylcarbodiimide coupling to phosphonoacetic acid. Neither analogue, however, showed significant agonist or antagonist activity towards TRPM2, underlining the importance of a complete pyrophosphate motif in activation of this particular receptor.

Pyrophosphate replacement using phosphonoacetate isosteres – tools to study biological targets of ADPR.

Synthesis of Camphecene and Cytisine Conjugates Using Click Chemistry Methodology and Study of Their Antiviral Activity

A series of camphecene and quinolizidine alkaloid (-)-cytisine conjugates has been obtained for the first time using 'click' chemistry methodology. The cytotoxicity and virus-inhibiting activity of compounds were determined against MDCK cells and influenza virus A/Puerto Rico/8/34 (H1N1), correspondingly, in in vitro tests. Based on the results obtained, values of 50 % cytotoxic dose (CC₅₀ ), 50 % inhibition dose (IC₅₀ ) and selectivity index (SI) were determined for each compound. It has been shown that the antiviral activity is affected by the length and nature of linkers between cytisine and camphor units. Conjugate 13 ((1R,5S)-3-(6-{4-[(2-{(E)-[(1R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene]amino}ethoxy)methyl]-1H-1,2,3-triazol-1-yl}hexyl)-1,2,3,4,5,6-hexahydro-8H-1,5-methanopyrido[1,2-a][1,5]diazocin-8-one), which contains cytisine fragment separated from triazole ring by -C₆ H₁₂ - aliphatic linker, showed the highest activity at relatively low toxicity (CC₅₀ =168 μmol, IC₅₀ =8 μmol, SI=20). Its selectivity index appeared higher than that of reference compound, rimantadine. According to theoretical calculations, the antiviral activity of the lead compound 13 can be explained by its influence on the functioning of neuraminidase.

New Linear Precursors of cIDPR Derivatives as Stable Analogs of cADPR: A Potent Second Messenger with Ca2+-Modulating Activity Isolated from Sea Urchin Eggs

Herein, we report on the synthesis of a small set of linear precursors of an inosine analogue of cyclic ADP-ribose (cADPR), a second messenger involved in Ca2+ mobilization from ryanodine receptor stores firstly isolated from sea urchin eggs extracts. The synthesized compounds were obtained starting from inosine and are characterized by an N1-alkyl chain replacing the "northern" ribose and a phosphate group attached at the end of the N1-alkyl chain and/or 5'-sugar positions. Preliminary Ca2+ mobilization assays, performed on differentiated C2C12 cells, are reported as well.

Niacin in the Central Nervous System: An Update of Biological Aspects and Clinical Applications

Niacin (also known as "vitamin B₃" or "vitamin PP") includes two vitamers (nicotinic acid and nicotinamide) giving rise to the coenzymatic forms nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). The two coenzymes are required for oxidative reactions crucial for energy production, but they are also substrates for enzymes involved in non-redox signaling pathways, thus regulating biological functions, including gene expression, cell cycle progression, DNA repair and cell death. In the central nervous system, vitamin B₃ has long been recognized as a key mediator of neuronal development and survival. Here, we will overview available literature data on the neuroprotective role of niacin and its derivatives, especially focusing especially on its involvement in neurodegenerative diseases (Alzheimer's, Parkinson's, and Huntington's diseases), as well as in other neuropathological conditions (ischemic and traumatic injuries, headache and psychiatric disorders).

Cyclic ADP-Carbocyclic-Ribose and -4-Thioribose, as Stable Mimics of Cyclic ADP-Ribose, a Ca2+-Mobilizing Second Messenger

Cyclic ADP-ribose (cADPR), a general mediator involved in Ca²⁺ signaling, has the characteristic 18-membered ring consisting of an adenine, two riboses and a pyrophosphate, in which the two primary hydroxy groups of the riboses are linked by a pyrophosphate unit. This review focuses on chemical synthetic studies of cADPR analogues of biological importance. Although cADPR analogues can be synthesized by enzymatic and chemo-enzymatic methods using ADP-ribosyl cyclase, the analogues obtained by these methods are limited due to the substrate-specificity of the enzymes. Consequently, chemical synthetic methods providing a greater variety of cADPR analogues are required. Although early chemical synthetic studies demonstrated that construction of the large 18-membered ring structure is difficult, the construction was achieved using the phenylthiophosphate-type substrates by treating with AgNO₃ or I₂. This is now a general method for synthesizing these types of biologically important cyclic nucleotides. Using this method as the key step, the chemically and biologically stable cADPR mimic, cADP-carbocyclic-ribose (cADPcR) and -4-thioribose (cADPtR), were synthesized.

Nucleoside macrocycles formed by intramolecular click reaction: efficient cyclization of pyrimidine nucleosides decorated with 5'-azido residues and 5-octadiynyl side chains

Copper(I)-promoted "click" cyclization in the presence of TBTA afforded nucleoside macrocycles in very high yields (≈70%) without using protecting groups. To this end, dU and dC derivatives functionalized at the 5-position of the nucleobase with octadiynyl side chains and with azido groups at the 5'-position of the sugar moieties were synthesized. The macrocycles display freely accessible Watson-Crick recognition sites. The conformation of the 16-membered macrocycle was deduced from X-ray analysis and ¹H,¹H-NMR coupling constants. The sugar conformation (N vs S) was different in solution as compared to the solid state.

Calcium-Mobilizing Behaviors of Neutral Cyclic ADP-Ribose Mimics that Integrate Modifications to the Nucleobase, Northern Ribose and Pyrophosphate

Cyclic adenosine diphosphate ribose (cADPR) is an endogenous Ca²⁺ mobilizer involved in diverse cellular processes. Mimics of cADPR play a crucial role in investigating the molecular mechanism(s) of cADPR-mediated signaling. Here, compound 3, a mimic of cADPR in which a neutral triazole moiety and an ether linkage were introduced to substitute the pyrophosphate and "northern" ribose components, respectively, was synthesized for the first time. The pharmacological activities in Jurkat cells indicated that this mimic is capable of penetrating plasma membrane and inciting Ca²⁺ release from the endoplasmic reticulum (ER) through the action of ryanodine receptors (RyRs) and triggering Ca²⁺ influx. Furthermore, a uridine moiety was introduced in place of adenine and the new cADPR mimics 4 and 5 were synthesized. The results of biological investigation showed that these mimics also targeted RyRs and retained moderate Ca²⁺ agonistic activities. The results indicated that the neutral cADPR mimics had the same targets for inducing Ca²⁺ signaling.

Selective inhibition of TRPM2 channel by two novel synthesized ADPR analogues

Transient receptor potential melastatin-2 (TRPM2) channel critical for monitoring internal body temperature is implicated in the pathological processes such as neurodegeneration. However, lacking selective and potent TRPM2 inhibitors impedes investigation and validation of the channel as a drug target. To discover novel and selective TRPM2 inhibitors, a series of adenosine 5'-diphosphoribose analogues were synthesized, and their activities and selectivity were evaluated. Whole-cell patch-clamp recordings were employed for screen and evaluation of synthesized compounds. Two compounds, 7i and 8a, were identified as TRPM2 inhibitors with IC50 of 5.7 and 5.4 μm, respectively. Both 7i and 8a inhibited TRPM2 current without affecting TRPM7, TRPM8, TRPV1 and TRPV3. These two TRPM2 inhibitors can serve as new pharmacological tools for further investigation and validation of TRPM2 channel as a drug target, and the summarized structure-activity relationship (SAR) may also provide insights into further improving existing inhibitors as potential lead compounds.

Macrocyclic derivatives with a sucrose scaffold: insertion of a long polyhydroxylated linker between the terminal 6,6′-positions

A series of five new macrocyclic hybrids with a sucrose scaffold were prepared by the reaction of activated 1′,2,3,3′,4,4′-hexa-O-methylsucrose with diversely functionalized d-mannitols.

Second messenger analogues highlight unexpected substrate sensitivity of CD38: total synthesis of the hybrid "L-cyclic inosine 5'-diphosphate ribose"

The multifunctional, transmembrane glycoprotein human CD38 catalyses the synthesis of three key Ca²⁺-mobilising messengers, including cyclic adenosine 5′-diphosphate ribose (cADPR), and CD38 knockout studies have revealed the relevance of the related signalling pathways to disease. To generate inhibitors of CD38 by total synthesis, analogues based on the cyclic inosine 5′-diphosphate ribose (cIDPR) template were synthesised. In the first example of a sugar hybrid cIDPR analogue, “L-cIDPR”, the natural “northern” N1-linked D-ribose of cADPR was replaced by L-ribose. L-cIDPR is surprisingly still hydrolysed by CD38, whereas 8-Br-L-cIDPR is not cleaved, even at high enzyme concentrations. Thus, the inhibitory activity of L-cIDPR analogues appears to depend upon substitution of the base at C-8; 8-Br-L-cIDPR and 8-NH₂-L-cIDPR inhibit CD38-mediated cADPR hydrolysis (IC₅₀ 7 μM and 21 µM respectively) with 8-Br-L-cIDPR over 20-fold more potent than 8-Br-cIDPR. In contrast, L-cIDPR displays a comparative 75-fold reduction in activity, but is only ca 2-fold less potent than cIDPR itself. Molecular modelling was used to explore the interaction of the CD38 catalytic residue Glu-226 with the “northern” ribose. We propose that Glu226 still acts as the catalytic residue even for an L-sugar substrate. 8-Br-L-cIDPR potentially binds non-productively in an upside-down fashion. Results highlight the key role of the “northern” ribose in the interaction of cADPR with CD38.

Synthesis of 8-Substituted Analogues of Cyclic ADP-4-Thioribose and Their Unexpected Identification as Ca2+-Mobilizing Full Agonists

A series of 8-substituted analogues of cyclic ADP-4-thioribose (cADPtR, 3), which is a stable equivalent of Ca²⁺-mobilizing second messenger cyclic ADP-ribose (cADPR, 1), were designed as potential pharmacological tools for studies on cADPR-modulated Ca²⁺ signaling pathways. These 8-amino analogue (8-NH₂-cADPtR, 4), 8-azido analogue (8-N₃-cADPtR, 5), and 8-chloro analogue (8-Cl-cADPtR, 6) were efficiently synthesized, where the stereoselective N1-β-thioribosyladenine ring closure reaction via an α/β-equilibrium of the 1-aminothioribose derivative and construction of the characteristic 18-membered pyrophosphate ring by Ag⁺-promoted activation of a phenyl phosphorothioate type substrate were the two key steps. Although 8-NH₂-cADPR (2) is a well-known potent antagonist against cADPR-inducing Ca²⁺-release, the 4-thioribose congener 8-NH₂-cADPtR turned out unexpectedly to be a full agonist in sea urchin egg homogenate evaluation system. This important finding suggested that the ring-oxygen in the N1-ribose of cADPR analogues is essential for the antagonistic activity in the Ca²⁺-signaling pathway, which can contribute to clarify the structure-agonist/antagonist activity relationship.

Synthesis of aza-crown analogues and macrocyclic bis-lactams with sucrose scaffold

2,3,3',4,4'-Penta-O-benzylsucrose was converted into the corresponding diaminoalcohol which was used as a key building block in the synthesis of the analogues of aza-crown ethers and bis-lactams.

Identifying Glyceraldehyde 3-Phosphate Dehydrogenase as a Cyclic Adenosine Diphosphoribose Binding Protein by Photoaffinity Protein-Ligand Labeling Approach

Cyclic adenosine diphosphoribose (cADPR), an endogenous nucleotide derived from nicotinamide adenine dinucleotide (NAD⁺), mobilizes Ca²⁺ release from endoplasmic reticulum (ER) via ryanodine receptors (RyRs), yet the bridging protein(s) between cADPR and RyRs remain(s) unknown. Here we synthesized a novel photoaffinity labeling (PAL) cADPR agonist, PAL-cIDPRE, and subsequently applied it to purify its binding proteins in human Jurkat T cells. We identified glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as one of the cADPR binding protein(s), characterized the binding affinity between cADPR and GAPDH in vitro by surface plasmon resonance (SPR) assay, and mapped cADPR's binding sites in GAPDH. We further demonstrated that cADPR induces the transient interaction between GAPDH and RyRs in vivo and that GAPDH knockdown abolished cADPR-induced Ca²⁺ release. However, GAPDH did not catalyze cADPR into any other known or novel compound(s). In summary, our data clearly indicate that GAPDH is the long-sought-after cADPR binding protein and is required for cADPR-mediated Ca²⁺ mobilization from ER via RyRs.

Design, Synthesis, and Identification of 4″α-Azidoethyl-cyclic ADP-Carbocyclic-ribose as a Highly Potent Analogue of Cyclic ADP-Ribose, a Ca(2+)-Mobilizing Second Messenger

Cyclic adenosine diphosphate-carbocyclic-ribose (cADPcR, 2) is a stable equivalent of cyclic adenosine diphosphate-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. On the basis of the structure-activity relationship of cADPR-related compounds and three-dimensional structural modeling of cADPcR, we designed and synthesized cyclic-ADP-4″α-azidoethyl carbocyclic-ribose (N3-cADPcR, 3) to demonstrate that it has a highly potent Ca(2+)-mobilizing activity (EC50 = 24 nM). N3-cADPcR will be a useful precursor for the preparation of biological tools effective to investigate cADPR-mediated signaling pathways.

Triazolophostins: a library of novel and potent agonists of IP3 receptors

IP₃R initiate most cellular Ca²⁺ signaling. AdA is the most potent agonist of IP₃R. The structural complexity of AdA makes synthesis of its analogs cumbersome. We report an easy method for generating a library of potent triazole-based analogs of AdA, triazolophostins, which are the most potent AdA analogs devoid of a nucleobase.

IP₃ receptors are channels that mediate the release of Ca²⁺ from the intracellular stores of cells stimulated by hormones or neurotransmitters. Adenophostin A (AdA) is the most potent agonist of IP₃ receptors, with the β-anomeric adenine contributing to the increased potency. The potency of AdA and its stability towards the enzymes that degrade IP₃ have aroused interest in AdA analogs for biological studies. The complex structure of AdA poses problems that have necessitated optimization of synthetic conditions for each analog. Such lengthy one-at-a-time syntheses limit access to AdA analogs. We have addressed this problem by synthesizing a library of triazole-based AdA analogs, triazolophostins, by employing click chemistry. An advanced intermediate having all the necessary phosphates and a β-azide at the anomeric position was reacted with various alkynes under Cu(i) catalysis to yield triazoles, which upon deprotection gave triazolophostins. All eleven triazolophostins synthesized are more potent than IP₃ and some are equipotent with AdA in functional analyses of IP₃ receptors. We show that a triazole ring can replace adenine without compromising the potency of AdA and provide facile routes to novel AdA analogs.

Designer small molecules to target calcium signalling

Synthetic compounds open up new avenues to interrogate and manipulate intracellular Ca2+ signalling pathways. They may ultimately lead to drug-like analogues to intervene in disease. Recent advances in chemical biology tools available to probe Ca2+ signalling are described, with a particular focus on those synthetic analogues from our group that have enhanced biological understanding or represent a step towards more drug-like molecules. Adenophostin (AdA) is the most potent known agonist at the inositol 1,4,5-trisphosphate receptor (IP3R) and synthetic analogues provide a binding model for receptor activation and channel opening. 2-O-Modified inositol 1,4,5-trisphosphate (IP3) derivatives that are partial agonists at the IP3R reveal key conformational changes of the receptor upon ligand binding. Biphenyl polyphosphates illustrate that simple non-inositol surrogates can be engineered to give prototype IP3R agonists or antagonists and act as templates for protein co-crystallization. Cyclic adenosine 5'-diphosphoribose (cADPR) can be selectively modified using total synthesis, generating chemically and biologically stable tools to investigate Ca2+ release via the ryanodine receptor (RyR) and to interfere with cADPR synthesis and degradation. The first neutral analogues with a synthetic pyrophosphate bioisostere surprisingly retain the ability to release Ca2+, suggesting a new route to membrane-permeant tools. Adenosine 5'-diphosphoribose (ADPR) activates the Ca2+-, Na+- and K+-permeable transient receptor potential melastatin 2 (TRPM2) cation channel. Synthetic ADPR analogues provide the first structure-activity relationship (SAR) for this emerging messenger and the first functional antagonists. An analogue based on the nicotinic acid motif of nicotinic acid adenine dinucleotide phosphate (NAADP) antagonizes NAADP-mediated Ca2+ release in vitro and is effective in vivo against induced heart arrhythmia and autoimmune disease, illustrating the therapeutic potential of targeted small molecules.

Nucleoside azide-alkyne cycloaddition reactions under solvothermal conditions or using copper vials in a ball mill

Novel nucleoside analogues containing photoswitchable moieties were prepared using 'click' cycloaddition reactions between 5'-azido-5'-deoxythymidine and mono- or bis-N-propargylamide-substituted azobenzenes. In solution, high to quantitative yields were achieved using 5 mol% Cu(I) in the presence of a stabilizing ligand. 'Click' reactions using the monopropargylamides were also effected in the absence of added cuprous salts by the application of liquid assisted grinding (LAG) in metallic copper reaction vials. Specifically, high speed vibration ball milling (HSVBM) using a 3/32″ (2.38 mm) diameter copper ball (62 mg) at 60 Hz overnight in the presence of ethyl acetate lead to complete consumption of the 5'-azido nucleoside with clean conversion to the corresponding 1,3-triazole.

Synthesis of 7-Deaza-cyclic Adenosine-5'-diphosphate-carbocyclic-ribose and Its 7-Bromo Derivative as Intracellular Ca(2+)-Mobilizing Agents

Cyclic ADP-carbocyclic-ribose (cADPcR, 3) is a biologically and chemically stable equivalent of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. We became interested in the biological activity of the 7-deaza analogues of cADPcR, i.e., 7-deaza-cADPcR (7) and its 7-bromo derivative, i.e., 7-deaza-7-Br-cADPcR (8), because 7-deazaadenosine is an efficient bioisostere of adenosine. The synthesis of 7 and 8 required us to construct the key N1-carbocyclic-ribosyl-7-deazaadenosine structure. Therefore, we developed a general method for preparing N1-substituted 7-deazaadenosines by condensing a 2,3-disubstituted pyrrole nucleoside with amines. Using this method, we prepared the N1-carbocyclic ribosyl 7-deazaadenosine derivative 10a, from which we then synthesized the target 7-deaza-cADPcR (7) via an Ag(+)-promoted intramolecular condensation to construct the 18-membered pyrophosphate ring structure. The corresponding 7-bromo derivative 8, which was the first analogue of cADPR with a substitution at the 7-position, was similarly synthesized. Biological evaluation for Ca(2+)-mobilizing activity in the sea urchin egg homogenate system indicated that 7-deaza-cADPcR (7) and 7-deaza-7-Br-cADPcR (8) acted as a full agonist and a partial agonist, respectively.

Molecular bases of catalysis and ADP-ribose preference of human Mn2+-dependent ADP-ribose/CDP-alcohol diphosphatase and conversion by mutagenesis to a preferential cyclic ADP-ribose phosphohydrolase

Among metallo-dependent phosphatases, ADP-ribose/CDP-alcohol diphosphatases form a protein family (ADPRibase-Mn-like) mainly restricted, in eukaryotes, to vertebrates and plants, with preferential expression, at least in rodents, in immune cells. Rat and zebrafish ADPRibase-Mn, the only biochemically studied, are phosphohydrolases of ADP-ribose and, somewhat less efficiently, of CDP-alcohols and 2´,3´-cAMP. Furthermore, the rat but not the zebrafish enzyme displays a unique phosphohydrolytic activity on cyclic ADP-ribose. The molecular basis of such specificity is unknown. Human ADPRibase-Mn showed similar activities, including cyclic ADP-ribose phosphohydrolase, which seems thus common to mammalian ADPRibase-Mn. Substrate docking on a homology model of human ADPRibase-Mn suggested possible interactions of ADP-ribose with seven residues located, with one exception (Cys253), either within the metallo-dependent phosphatases signature (Gln27, Asn110, His111), or in unique structural regions of the ADPRibase-Mn family: s2s3 (Phe37 and Arg43) and h7h8 (Phe210), around the active site entrance. Mutants were constructed, and kinetic parameters for ADP-ribose, CDP-choline, 2´,3´-cAMP and cyclic ADP-ribose were determined. Phe37 was needed for ADP-ribose preference without catalytic effect, as indicated by the increased ADP-ribose Km and unchanged kcat of F37A-ADPRibase-Mn, while the Km values for the other substrates were little affected. Arg43 was essential for catalysis as indicated by the drastic efficiency loss shown by R43A-ADPRibase-Mn. Unexpectedly, Cys253 was hindering for cADPR phosphohydrolase, as indicated by the specific tenfold gain of efficiency of C253A-ADPRibase-Mn with cyclic ADP-ribose. This allowed the design of a triple mutant (F37A+L196F+C253A) for which cyclic ADP-ribose was the best substrate, with a catalytic efficiency of 3.5´104 M-1s-1 versus 4´103 M-1s-1 of the wild type.