New S-Adenosyl-l-methionine Analogues: Synthesis and Reactivity Studies

Analogs of S-Adenosyl-L-Methionine in Studies of Methyltransferases

Methyltransferases (MTases) play an important role in the functioning of living systems, catalyzing the methylation reactions of DNA, RNA, proteins, and small molecules, including endogenous compounds and drugs. Many human diseases are associated with disturbances in the functioning of these enzymes; therefore, the study of MTases is an urgent and important task. Most MTases use the cofactor S‑adenosyl‑L‑methionine (SAM) as a methyl group donor. SAM analogs are widely applicable in the study of MTases: they are used in studies of the catalytic activity of these enzymes, in identification of substrates of new MTases, and for modification of the substrates or substrate linking to MTases. In this review, new synthetic analogs of SAM and the problems that can be solved with their usage are discussed.

Synthesis and Biological Evaluation of 1,3-Dideazapurine-Like 7-Amino-5-Hydroxymethyl-Benzimidazole Ribonucleoside Analogues as Aminoacyl-tRNA Synthetase Inhibitors

Aminoacyl-tRNA synthetases (aaRSs) have become viable targets for the development of antimicrobial agents due to their crucial role in protein translation. A series of six amino acids were coupled to the purine-like 7-amino-5-hydroxymethylbenzimidazole nucleoside analogue following an optimized synthetic pathway. These compounds were designed as aaRS inhibitors and can be considered as 1,3-dideazaadenine analogues carrying a 2-hydroxymethyl substituent. Despite our intentions to obtain N¹-glycosylated 4-aminobenzimidazole congeners, resembling the natural purine nucleosides glycosylated at the N⁹-position, we obtained the N³-glycosylated benzimidazole derivatives as the major products, resembling the respective purine N⁷-glycosylated nucleosides. A series of X-ray crystal structures of class I and II aaRSs in complex with newly synthesized compounds revealed interesting interactions of these “base-flipped” analogues with their targets. While the exocyclic amine of the flipped base mimics the reciprocal interaction of the N³-purine atom of aminoacyl-sulfamoyl adenosine (aaSA) congeners, the hydroxymethyl substituent of the flipped base apparently loses part of the standard interactions of the adenine N¹ and the N⁶-amine as seen with aaSA analogues. Upon the evaluation of the inhibitory potency of the newly obtained analogues, nanomolar inhibitory activities were noted for the leucine and isoleucine analogues targeting class I aaRS enzymes, while rather weak inhibitory activity against the corresponding class II aaRSs was observed. This class bias could be further explained by detailed structural analysis.

Parallel Solution Phase Synthesis and Preliminary Biological Activity of a 5'-Substituted Cytidine Analog Library

A 109-membered library of 5'-substituted cytidine analogs was synthesized, via funding through the NIH Roadmap Initiative and the Pilot Scale Library (PSL) Program. Reaction core compounds contained -NH₂ (2) and -COOH (44 and 93) groups that were coupled to a diversity of reactants in a parallel, solution phase format to produce the target library. The assorted reactants included -NH₂, -CHO, -SO₂Cl, and -COOH functional groups, and condensation with the intermediate core materials 2 and 44 followed by acidic hydrolysis produced 3-91 in good yields and high purity. Linkage of the amino terminus of d-phenylalanine methyl ester to the free 5'-COOH of 44 and NaOH treatment led to core library -COOH precursor 93. In a libraries from libraries approach, compound 93 served as the vital building block for our unique library of dipeptidyl cytidine analogs 94-114 through amide coupling of the -COOH group with numerous commercial amines followed by acidic deprotection. Initial screening of the complete final library through the MLPCN program revealed a modest number of hits over diverse biological processes. These hits might be considered as starting points for hit-to-lead optimization and development studies.

Novel adenosine-derived inhibitors of Cryptosporidium parvum inosine 5'-monophosphate dehydrogenase

We have found cyclophane-type adenosine derivatives having p-quinone amide moieties (1 and 2) as weak inhibitors of Cryptosporidium parvum inosine 5'-monophosphate dehydrogenase (CpIMPDH) from the Hokkaido University Chemical Library via the luciferase-based high-throughput screening. To obtain more potent inhibitors, we synthesized four new derivatives free from cyclophane rings (3-6). The N-H derivatives 3 and 5 showed more potent activities (24.4 and 11.1 μM, respectively) in the presence of dithiothreitol (DTT), whereas the N-methyl derivative 4 indicated more potent activity (2.1 μM) without DTT. Conformational analysis of compounds 3 and 4 suggested that N-H amide 3 binds to IMP-binding site in the DTT mediated manner.

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.

Methyltransferase-Directed Labeling of Biomolecules and its Applications

Methyltransferases (MTases) form a large family of enzymes that methylate a diverse set of targets, ranging from the three major biopolymers to small molecules. Most of these MTases use the cofactor S-adenosyl-l-Methionine (AdoMet) as a methyl source. In recent years, there have been significant efforts toward the development of AdoMet analogues with the aim of transferring moieties other than simple methyl groups. Two major classes of AdoMet analogues currently exist: doubly-activated molecules and aziridine based molecules, each of which employs a different approach to achieve transalkylation rather than transmethylation. In this review, we discuss the various strategies for labelling and functionalizing biomolecules using AdoMet-dependent MTases and AdoMet analogues. We cover the synthetic routes to AdoMet analogues, their stability in biological environments and their application in transalkylation reactions. Finally, some perspectives are presented for the potential use of AdoMet analogues in biology research, (epi)genetics and nanotechnology.

Construction of a library of structurally diverse ribonucleopeptides with catalytic groups

Functional screening of structurally diverse libraries consisting of proteins or nucleic acids is an effective method to obtain receptors or aptamers with unique molecular recognition characteristics. However, further modification of these selected receptors to exert a newly desired function is still a challenging task. We have constructed a library of structurally diverse ribonucleopeptides (RNPs) that are modified with a catalytic group, in which the catalytic group aligns with various orientations against the ATP binding pocket of RNA subunit. As a proof-of-principle, the screening of the constructed RNP library for the catalytic reaction of ester hydrolysis was successfully carried out. The size of both the substrate-binding RNA library and the catalytic group modified peptide library are independently expandable, and thus, the size of RNPs library could be enlarged by a combination of these two subunits. We anticipate that the library of functionalized and structurally diverse RNPs would be expanded for various other catalytic reactions.

Nucleophilic displacement reactions of 5'-derivatised nucleosides in a vibration ball mill

Vibration ball-milling in a zirconia-lined vessel afforded clean and quantitative nucleophilic displacement reactions between 4-methoxybenzylthiolate salts and nucleoside 5′-halides or 5′-tosylates in five to 60 minutes. Under these conditions, commonly-encountered nucleoside cyclisation byproducts (especially of purine nucleosides) were not observed. Liquid-assisted grinding of the same 5'-iodide and 5′-tosylate substrates with potassium selenocyanate in the presence of DMF produced the corresponding 5′-selenocyanates in variable yields over the course of between one and eleven hours thereby avoiding the preparation and use of hygroscopic tetrabutylammonium salts.

SAM/SAH Analogs as Versatile Tools for SAM-Dependent Methyltransferases

S-Adenosyl-L-methionine (SAM) is a sulfonium molecule with a structural hybrid of methionine and adenosine. As the second largest cofactor in the human body, its major function is to serve as methyl donor for SAM-dependent methyltransferases (MTases). The resultant transmethylation of biomolecules constitutes a significant biochemical mechanism in epigenetic regulation, cellular signaling, and metabolite degradation. Recently, numerous SAM analogs have been developed as synthetic cofactors to transfer the activated groups on MTase substrates for downstream ligation and identification. Meanwhile, new compounds built upon or derived from the SAM scaffold have been designed and tested as selective inhibitors for important MTase targets. Here, we summarized the recent development and application of SAM analogs as chemical biology tools for MTases.

DNA Labeling Using DNA Methyltransferases

DNA methyltransferases (MTases) uniquely combine the ability to recognize and covalently modify specific target sequences in DNA using the ubiquitous cofactor S-adenosyl-L-methionine (AdoMet). Although DNA methylation plays important roles in biological signaling, the transferred methyl group is a poor reporter and is highly inert to further biocompatible derivatization. To unlock the biotechnological power of these enzymes, two major types of cofactor AdoMet analogs were developed that permit targeted MTase-directed attachment of larger moieties containing functional or reporter groups onto DNA. One such approach (named sequence-specific methyltransferase-induced labeling, SMILing) uses reactive aziridine or N-mustard mimics of the cofactor AdoMet, which render targeted coupling of a whole cofactor molecule to the target DNA. The second approach (methyltransferase-directed transfer of activated groups, mTAG) uses AdoMet analogs with a sulfonium-bound extended side chain replacing the methyl group, which permits MTase-directed covalent transfer of the activated side chain alone. As the enlarged cofactors are not always compatible with the active sites of native MTases, steric engineering of the active site has been employed to optimize their alkyltransferase activity. In addition to the described cofactor analogs, recently discovered atypical reactions of DNA cytosine-5 MTases involving non-cofactor-like compounds can also be exploited for targeted derivatization and labeling of DNA. Altogether, these approaches offer new powerful tools for sequence-specific covalent DNA labeling, which not only pave the way to developing a variety of useful techniques in DNA research, diagnostics, and nanotechnologies but have already proven practical utility for optical DNA mapping and epigenome studies.

Design, synthesis, and kinetic analysis of potent protein N-terminal methyltransferase 1 inhibitors

The protein N-terminal methyltransferase 1 (NTMT1) methylates the α-N-terminal amines of proteins. NTMT1 is upregulated in a variety of cancers and knockdown of NTMT1 results in cell mitotic defects. Therefore, NTMT1 inhibitors could be potential anticancer therapeutics. This study describes the design and synthesis of the first inhibitor targeting NTMT1. A novel bisubstrate analogue (NAM-TZ-SPKRIA) was shown to be a potent inhibitor (Ki = 0.20 μM) for NTMT1 and was selective versus protein lysine methyltransferase G9a and arginine methyltransferase 1. NAM-TZ-SPKRIA was found to exhibit a competitive inhibition pattern for both substrates, and mass spectrometry experiments revealed that the inhibitor substantially suppressed the methylation progression. Our results demonstrate the feasibility of using a triazole group to link an S-adenosyl-L-methionine analog with a peptide substrate to construct bisubstrate analogues as NTMT1 potent and selective inhibitors. This study lays a foundation to further discover small molecule NTMT1 inhibitors to interrogate its biological functions, and suggests a general strategy for the development of selective protein methyltransferase inhibitors.

5'-(N-aminoacyl)-sulfonamido-5'-deoxyadenosine: attempts for a stable alternative for aminoacyl-sulfamoyl adenosines as aaRS inhibitors

Synthesis of aminoacyl-sulfamoyl adenosines (aaSAs) and their peptidyl conjugates as aminoacyl tRNA synthetase (aaRS) inhibitors remains problematic due to the low yield of the aminoacylation and the subsequent conjugation reaction causing concomitant formation of a cyclic adenosine derivative. In an effort to reduce this undesirable side reaction, we aimed to prepare the corresponding aminoacyl sulfonamide (aaSoA) analogues as more stable alternatives for aaSA derivatives. Deletion of the 5'-oxygen in aaSA analogues should render the C-5' less electrophilic and therefore improve the stability of the aminoacyl sulfamate analogues. We therefore synthesized six sulfonamides and compared their activity against the respective aaSA analogues. However, except for the aspartyl derivative, the new compounds are not able to inhibit the corresponding aaRS. Possible reasons for this loss of activity are discussed by modeling and comparison of the newly synthesized aaSoA derivatives with their parent aaSA analogues.

An efficient method for the synthesis of selenium modified nucleosides: its application in the synthesis of Se-adenosyl-l-selenomethionine (SeAM)

A versatile method for the synthesis of 5′-selenium modified nucleosides has been explored on the basis of a 2-(trimethylsilyl)ethyl (TSE) selenyl group.

Synthesis of novel peptidyl adenosine antibiotic analogs

A small library of peptidyl adenosine antibiotic analogs was synthesized, under the Pilot Scale Library Program of the NIH Roadmap initiative, from 2',3'-O-isoproylideneadenosine-5'-carboxylic acid 2 in excellent yield. The coupling of the amino terminus of L-2-aminophenylbutyric methyl ester to a free 5'-carboxylic acid moiety of 2 followed by sodium hydroxide treatment led to carboxylic acid analog 4. Hydrolysis of this latter gave unprotected nucleoside analog 5. Intermediate 4 served as the precursor for the preparation of novel peptidyl adenosine analogs 6-18 in good yields and high purity through peptide coupling reactions to diverse amine derivatives. No marked anticancer and antimalaria activity was noted on preliminary cellular testing; however these analogs should be useful candidates for other types of biological activity.

Parallel solution-phase synthesis and general biological activity of a uridine antibiotic analog library

A small library of ninety four uridine antibiotic analogs was synthesized, under the Pilot Scale Library (PSL) Program of the NIH Roadmap initiative, from amine 2 and carboxylic acids 33 and 77 in solution-phase fashion. Diverse aldehyde, sulfonyl chloride, and carboxylic acid reactant sets were condensed to 2, leading after acid-mediated hydrolysis, to the targeted compounds 3–32 in good yields and high purity. Similarly, treatment of 33 with diverse amines and sulfonamides gave 34–75. The coupling of the amino terminus of d-phenylalanine methyl ester to the free 5′-carboxylic acid moiety of 33 followed by sodium hydroxide treatment led to carboxylic acid analog 77. Hydrolysis of this material gave analog 78. The intermediate 77 served as the precursor for the preparation of novel dipeptidyl uridine analogs 79–99 through peptide coupling reactions to diverse amine reactants. None of the described compounds show significant anticancer or antimalarial acivity. A number of samples exhibited a variety of promising inhibitory, agonist, antagonist, or activator properties with enzymes and receptors in primary screens supplied and reported through the NIH MLPCN program.

Parallel solution-phase synthesis of an adenosine antibiotic analog library

A library of eighty one adenosine antibiotic analogs was prepared under the Pilot Scale Library Program of the NIH Roadmap initiative from 5'-amino-5'-deoxy-2',3'-O-isopropylidene-adenosine 3. Diverse aldehyde, sulfonyl chloride and carboxylic acid reactant sets were condensed to 3, in solution-phase fashion, leading after acid-mediated hydrolysis to the targeted compounds in good yields and high purity. No marked antituberculosis or anticancer activity was noted on preliminary cellular testing, but these nucleoside analogs should be useful candidates for other types of biological activity.

Photoinitiated release of an aziridinium ion precursor for the temporally controlled alkylation of nucleophiles

A photo-activatable aziridinium precursor has been developed to investigate the possibility of a photo-initiated traditional nucleophilic reaction. The photolysis of a quaternary amine yields a tertiary amine and has allowed us to temporally control aziridinium formation and subsequent alkylation of a colorimetric nucleophilic reporter molecule. We have also used this photo-initiated reaction to alkylate a sulfhydryl group. This new photo-initiated alkylation strategy is water-soluble and expands the toolkit of photo-activated crosslinkers for protein labeling research.

6'-Oxa Analogs of S-Adenosylhomocysteine

S-Adenosylmethionine (AdoMet) is a ubiquitous cofactor in biomethylations and, in that role, becomes S-adenosylhomocysteine (AdoHcy), which serves as a biofeedback inhibitor of the methylation process. In seeking to avail unexplored structural variations of AdoHcy for biological studies, its 6'-oxa analog and two corresponding carbocyclic nucleosides (based on aristeromycin and neplanocin) have been prepared via common convergent syntheses.