Characterization and inactivation of an agmatine deiminase from Helicobacter pylori

Insights into the action of the pharmaceutical metformin: Targeted inhibition of the gut microbial enzyme agmatinase

Metformin is the first-line treatment for type 2 diabetes, yet its mechanism of action is not fully understood. Recent studies suggest metformin's interactions with gut microbiota are responsible for exerting therapeutic effects. In this study, we report that metformin targets the gut microbial enzyme agmatinase, as a competitive inhibitor, which may impair gut agmatine catabolism. The metformin inhibition constant (Ki) of E. coli agmatinase is 1 mM and relevant in the gut where the drug concentration is 1-10 mM. Metformin analogs phenformin, buformin, and galegine are even more potent inhibitors of E. coli agmatinase (Ki = 0.6, 0.1, and 0.007 mM, respectively) suggesting a shared mechanism. Agmatine is a known effector of human host metabolism and has been reported to augment metformin's therapeutic effects for type 2 diabetes. This gut-derived inhibition mechanism gives new insights on metformin's action in the gut and may lead to significant discoveries in improving metformin therapy.

A unique aromatic cluster near the active site of H. pylori CPA is essential for catalytic function

Polyamines are essential for cell growth and proliferation. In plants and many bacteria, including Helicobacter pylori, the parent polyamine putrescine is only produced through the metabolism of N-carbamoylputrescine by N-carbamoylputrescine amidase (CPA). Thus, CPA is a crucial intermediate enzyme. Moreover, the absence of CPA in humans makes its presence in H. pylori a potential target for the development of new therapeutics against this pathogen. Despite this enzyme's presence in plants and bacteria, its function is not completely explored. Using structure-guided biochemical and biophysical studies on H. pylori CPA, we discovered an aromatic cluster containing four conserved tryptophans near the catalytic site and elucidated its role. Mutational studies revealed that they are individually vital to enzyme function. Unlike wild-type, which forms a hexamer, the Trp to Ala mutants only formed dimers. Interestingly, two other conserved residues, Gln155 and Asp278, interact with the tryptophan cluster and perform similar roles. Our results indicate that aromatic-aromatic and H-bonding contacts between the residues (Trp156-Trp273, Trp196-Gln155, and Trp153-Asp278) play a crucial role in stimulating activity through hexamer formation. Additionally, Trp156 is essential to generating a catalytically efficient hexamer. These results suggest dual roles for the tryptophans; in hexamer formation and in generating its functionally active form, thereby providing a mechanistic understanding into the role of the cluster. We also elucidated the catalytic roles of Glu43, Lys115, and Cys152, which are present at the active site. Our findings highlight, for the first time, the importance of a tryptophan cluster in H. pylori CPA that can be exploited to design therapeutic inhibitors.

Three Related Enzymes in Candida albicans Achieve Arginine- and Agmatine-Dependent Metabolism That Is Essential for Growth and Fungal Virulence

Amino acid metabolism is crucial for fungal growth and development. Ureohydrolases produce amines when acting on l-arginine, agmatine, and guanidinobutyrate (GB), and these enzymes generate ornithine (by arginase), putrescine (by agmatinase), or GABA (by 4-guanidinobutyrase or GBase). Candida albicans can metabolize and grow on arginine, agmatine, or guanidinobutyrate as the sole nitrogen source. Three related C. albicans genes whose sequences suggested that they were putative arginase or arginase-like genes were examined for their role in these metabolic pathways. Of these, Car1 encoded the only bona fide arginase, whereas we provide evidence that the other two open reading frames, orf19.5862 and orf19.3418, encode agmatinase and guanidinobutyrase (Gbase), respectively. Analysis of strains with single and multiple mutations suggested the presence of arginase-dependent and arginase-independent routes for polyamine production. CAR1 played a role in hyphal morphogenesis in response to arginine, and the virulence of a triple mutant was reduced in both Galleria mellonella and Mus musculus infection models. In the bloodstream, arginine is an essential amino acid that is required by phagocytes to synthesize nitric oxide (NO). However, none of the single or multiple mutants affected host NO production, suggesting that they did not influence the oxidative burst of phagocytes.IMPORTANCE We show that the C. albicans ureohydrolases arginase (Car1), agmatinase (Agt1), and guanidinobutyrase (Gbu1) can orchestrate an arginase-independent route for polyamine production and that this is important for C. albicans growth and survival in microenvironments of the mammalian host.

Structural Study of Agmatine Iminohydrolase From Medicago truncatula, the Second Enzyme of the Agmatine Route of Putrescine Biosynthesis in Plants

Plants are unique eukaryotes that can produce putrescine (PUT), a basic diamine, from arginine via a three-step pathway. This process starts with arginine decarboxylase that converts arginine to agmatine. Then, the consecutive action of two hydrolytic enzymes, agmatine iminohydrolase (AIH) and N-carbamoylputrescine amidohydrolase, ultimately produces PUT. An alternative route of PUT biosynthesis requires ornithine decarboxylase that catalyzes direct putrescine biosynthesis. However, some plant species lack this enzyme and rely only on agmatine pathway. The scope of this manuscript concerns the structural characterization of AIH from the model legume plant, Medicago truncatula. MtAIH is a homodimer built of two subunits with a characteristic propeller fold, where five αββαβ repeated units are arranged around the fivefold pseudosymmetry axis. Dimeric assembly of this plant AIH, formed by interactions of conserved structural elements from one repeat, is drastically different from that observed in dimeric bacterial AIHs. Additionally, the structural snapshot of MtAIH in complex with 6-aminohexanamide, the reaction product analog, presents the conformation of the enzyme during catalysis. Our structural results show that MtAIH undergoes significant structural rearrangements of the long loop, which closes a tunnel-shaped active site over the course of the catalytic event. This conformational change is also observed in AIH from Arabidopsis thaliana, indicating the importance of the closed conformation of the gate-keeping loop for the catalysis of plant AIHs.

Structural and Functional Basis for Targeting Campylobacter jejuni Agmatine Deiminase To Overcome Antibiotic Resistance

Campylobacter jejuni is the most common bacterial cause of gastroenteritis and a major contributor to infant mortality in the developing world. The increasing incidence of antibiotic-resistant C. jejuni only adds to the urgency to develop effective therapies. Because of the essential role that polyamines play, particularly in protection from oxidative stress, enzymes involved in the biosynthesis of these metabolites are emerging as promising antibiotic targets. The recent description of an alternative pathway for polyamine synthesis, distinct from that in human cells, in C. jejuni suggests this pathway could be a target for novel therapies. To that end, we determined X-ray crystal structures of C. jejuni agmatine deiminase (CjADI) and demonstrated that loss of CjADI function contributes to antibiotic sensitivity, likely because of polyamine starvation. The structures provide details of key molecular features of the active site of this protein. Comparison of the unliganded structure (2.1 Å resolution) to that of the CjADI-agmatine complex (2.5 Å) reveals significant structural rearrangements that occur upon substrate binding. The shift of two helical regions of the protein and a large conformational change in a loop near the active site generate a narrow binding pocket around the bound substrate. This change optimally positions the substrate for catalysis. In addition, kinetic analysis of this enzyme demonstrates that CjADI is an iminohydrolase that effectively deiminates agmatine. Our data suggest that C. jejuni agmatine deiminase is a potentially important target for combatting antibiotic resistance, and these results provide a valuable framework for guiding future drug development.

Helicobacter pylori does not use spermidine synthase to produce spermidine

Helicobacter pylori is the primary pathogen associated to gastritis and gastric cancer. Growth of H. pylori depends on the availability of spermidine in vivo. Interestingly, the genome of H. pylori contains an incomplete set of genes for the classical pathway of spermidine biosynthesis. It is thus not clear whether some other genes remained in the pathway would have any functions in spermidine biosynthesis. Here, we study spermidine synthase, which is responsible for the final catalytic process in the classical route. Protein sequence alignment reveals that H. pylori SpeE (HpSpeE) lacks key residues for substrate binding. By using isothermal titration calorimetry, we show that purified recombinant HpSpeE does not interact with the putative substrates putrescine and decarboxylated S-adenosylmethionine, and the product spermidine. High performance liquid chromatography analysis further demonstrates that HpSpeE has no detectable in vitro enzymatic activity. Additionally, intracellular spermidine level in speE-null mutant strain is comparable to that in the wild type strain. Collectively, our results suggest that HpSpeE is functionally distinct from spermidine production. H. pylori may instead employ the alternative pathway for spermidine synthesis which is dominantly exploited by other human gut microbes.

Mechanistic studies of the agmatine deiminase from Listeria monocytogenes

Listeria monocytogenes is a Gram-positive food-borne pathogen that is capable of living within extreme environments (i.e. low temperatures and pH). This ability to survive in such conditions may arise, at least in part, from agmatine catabolism via the agmatine deiminase system (AgDS). This catabolic pathway utilizes an agmatine deiminase (AgD) to hydrolyse agmatine into N-carbamoylputrescine (NCP), with concomitant release of ammonia, which increases the pH, thus mitigating the ill effects of the acidic environment. Given the potential significance of this pathway for cell survival, we set out to study the catalytic mechanism of the AgD encoded by L. monocytogenes In the present paper, we describe the catalytic mechanism employed by this enzyme based on pH profiles, pKa measurements of the active site cysteine and solvent isotope effects (SIE). In addition, we report inhibition of this enzyme by two novel AgD inhibitors, i.e. N-(4-aminobutyl)-2-fluoro-ethanimidamide (ABFA) and N-(4-aminobutyl)-2-chloro-ethanimidamide (ABCA). In contrast with other orthologues, L. monocytogenes AgD does not use the reverse protonation or substrate-assisted mechanism, which requires an active site cysteine with a high pKa and has been commonly seen in other members of the guanidinium-modifying enzyme (GME) superfamily. Instead, the L. monocytogenes AgD has a low pKa cysteine in the active site leading to an alternative mechanism of catalysis. This is the first time that this mechanism has been observed in the GME superfamily and is significant because it explains why previously developed mechanism-based inactivators of AgDs are ineffective against this orthologue.

Vaginal biogenic amines: biomarkers of bacterial vaginosis or precursors to vaginal dysbiosis?

Bacterial vaginosis (BV) is the most common vaginal disorder among reproductive age women. One clinical indicator of BV is a “fishy” odor. This odor has been associated with increases in several biogenic amines (BAs) that may serve as important biomarkers. Within the vagina, BA production has been linked to various vaginal taxa, yet their genetic capability to synthesize BAs is unknown. Using a bioinformatics approach, we show that relatively few vaginal taxa are predicted to be capable of producing BAs. Many of these taxa (Dialister, Prevotella, Parvimonas, Megasphaera, Peptostreptococcus, and Veillonella spp.) are more abundant in the vaginal microbial community state type (CST) IV, which is depleted in lactobacilli. Several of the major Lactobacillus species (L. crispatus, L. jensenii, and L. gasseri) were identified as possessing gene sequences for proteins predicted to be capable of putrescine production. Finally, we show in a small cross sectional study of 37 women that the BAs putrescine, cadaverine and tyramine are significantly higher in CST IV over CSTs I and III. These data support the hypothesis that BA production is conducted by few vaginal taxa and may be important to the outgrowth of BV-associated (vaginal dysbiosis) vaginal bacteria.

Combining phase information in reciprocal space for molecular replacement with partial models

ARCIMBOLDO allows ab initio phasing of macromolecular structures below atomic resolution by exploiting the location of small model fragments combined with density modification in a multisolution frame. The model fragments can be either secondary-structure elements predicted from the sequence or tertiary-structure fragments. The latter can be derived from libraries of typical local folds or from related structures, such as a low-homology model that is unsuccessful in molecular replacement. In all ARCIMBOLDO applications, fragments are searched for sequentially. Correct partial solutions obtained after each fragment-search stage but lacking the necessary phasing power can, if combined, succeed. Here, an analysis is presented of the clustering of partial solutions in reciprocal space and of its application to a set of different cases. In practice, the task of combining model fragments from an ARCIMBOLDO run requires their referral to a common origin and is complicated by the presence of correct and incorrect solutions as well as by their not being independent. The F-weighted mean phase difference has been used as a figure of merit. Clustering perfect, non-overlapping fragments dismembered from test structures in polar and nonpolar space groups shows that density modification before determining the relative origin shift enhances its discrimination. In the case of nonpolar space groups, clustering of ARCIMBOLDO solutions from secondary-structure models is feasible. The use of partially overlapping search fragments provides a more favourable circumstance and was assessed on a test case. Applying the devised strategy, a previously unknown structure was solved from clustered correct partial solutions.

Structure and mechanism of a bacterial host-protein citrullinating virulence factor, Porphyromonas gingivalis peptidylarginine deiminase

Citrullination is a post-translational modification of higher organisms that deiminates arginines in proteins and peptides. It occurs in physiological processes but also pathologies such as multiple sclerosis, fibrosis, Alzheimer’s disease and rheumatoid arthritis (RA). The reaction is catalyzed by peptidylarginine deiminases (PADs), which are found in vertebrates but not in lower organisms. RA has been epidemiologically associated with periodontal disease, whose main infective agent is Porphyromonas gingivalis. Uniquely among microbes, P. gingivalis secretes a PAD, termed PPAD (Porphyromonas peptidylarginine deiminase), which is genetically unrelated to eukaryotic PADs. Here, we studied function of PPAD and its substrate-free, substrate-complex, and substrate-mimic-complex structures. It comprises a flat cylindrical catalytic domain with five-fold α/β-propeller architecture and a C-terminal immunoglobulin-like domain. The PPAD active site is a funnel located on one of the cylinder bases. It accommodates arginines from peptide substrates after major rearrangement of a “Michaelis loop” that closes the cleft. The guanidinium and carboxylate groups of substrates are tightly bound, which explains activity of PPAD against arginines at C-termini but not within peptides. Catalysis is based on a cysteine-histidine-asparagine triad, which is shared with human PAD1-PAD4 and other guanidino-group modifying enzymes. We provide a working mechanism hypothesis based on 18 structure-derived point mutants.

Development of a clickable activity-based protein profiling (ABPP) probe for agmatine deiminases

Agmatine deiminases (AgDs) catalyze the hydrolytic conversion of agmatine (decarboxylated arginine) to N-carbamoylputrescine with concomitant release of ammonia. These enzymes, which are encoded by some pathogenic bacterial species, confer a competitive survival advantage by virtue of energy production and acid tolerance through agmatine catabolism. Herein we report the development of a clickable activity-based protein profiling (ABPP) probe that targets the AgD encoded by Streptococcus mutans with high selectivity and sensitivity.

Design, synthesis, and in vitro evaluation of an activity-based protein profiling (ABPP) probe targeting agmatine deiminases

Agmatine deiminases (AgDs) belong to a family of enzymes known as guanidinium group modifying enzymes (GMEs). Many pathogenic bacteria encode an AgD that participates in the catabolism of agmatine (decarboxylated arginine). This catabolism may confer a competitive survival advantage, by virtue of energy production and increased acid tolerance, making this sub-family of enzymes a potential therapeutic target that warrants further study. Herein we report the development of an activity-based protein profiling (ABPP) probe that selectively targets the AgD from Streptococcus mutans. Due to the selectivity and covalent nature of the modification, this probe could prove to be a valuable tool for the study of other AgD family members.

Mechanistic studies of protein arginine deiminase 2: evidence for a substrate-assisted mechanism

Citrullination, which is catalyzed by protein arginine deiminases (PADs 1–4 and 6), is a post-translational modification (PTM) that effectively neutralizes the positive charge of a guanidinium group by its replacement with a neutral urea. Given the sequence similarity of PAD2 across mammalian species and the genomic organization of the PAD2 gene, PAD2 is predicted to be the ancestral homologue of the PADs. Although PAD2 has long been known to play a role in myelination, it has only recently been linked to other cellular processes, including gene transcription and macrophage extracellular trap formation. For example, PAD2 deiminates histone H3 at R26, and this PTM leads to the increased transcription of more than 200 genes under the control of the estrogen receptor. Given that our understanding of PAD2 biology remains incomplete, we initiated mechanistic studies on this enzyme to aid the development of PAD2-specific inhibitors. Herein, we report that the substrate specificity and calcium dependence of PAD2 are similar to those of PADs 1, 3, and 4. However, unlike those isozymes, PAD2 appears to use a substrate-assisted mechanism of catalysis in which the positively charged substrate guanidinium depresses the pK_a of the nucleophilic cysteine. By contrast, PADs 1, 3, and 4 use a reverse-protonation mechanism. These mechanistic differences will aid the development of isozyme-specific inhibitors.

Nontraditional therapies to treat Helicobacter pylori infection

The Gram-negative pathogen Helicobacter pylori is increasingly more resistant to the three major antibiotics (metronidazole, clarithromycin and amoxicillin) that are most commonly used to treat infection. As a result, there is an increased rate of treatment failure; this translates into an overall higher cost of treatment due to the need for increased length of treatment and/or the requirement for combination or sequential therapy. Given the rise in antibiotic resistance, the complicated treatment regime, and issues related to patient compliance that stem from the duration and complexity of treatment, there is clearly a pressing need for the development of novel therapeutic strategies to combat H. pylori infection. As such, researchers are actively investigating the utility of antimicrobial peptides, small molecule inhibitors and naturopathic therapies. Herein we review and discuss each of these novel approaches as a means to target this important gastric pathogen.

The nonenzymatic decomposition of guanidines and amidines

To establish the rates and mechanisms of decomposition of guanidine and amidine derivatives in aqueous solution and the rate enhancements produced by the corresponding enzymes, we examined their rates of reaction at elevated temperatures and used the Arrhenius equation to extrapolate the results to room temperature. The similar reactivities of methylguanidine and 1,1,3,3-tetramethylguanidine and their negative entropies of activation imply that their decomposition proceeds by hydrolysis rather than elimination. The influence of changing pH on the rate of decomposition is consistent with attack by hydroxide ion on the methylguanidinium ion (k2 = 5 × 10(-6) M(-1) s(-1) at 25 °C) or with the kinetically equivalent attack by water on uncharged methylguanidine. At 25 °C and pH 7, N-methylguanidine is several orders of magnitude more stable than acetamidine, urea, or acetamide. Under the same conditions, the enzymes arginase and agmatinase accelerate substrate hydrolysis 4 × 10(14)-fold and 6 × 10(12)-fold, respectively, by mechanisms that appear to involve metal-mediated water attack. Arginine deiminase accelerates substrate hydrolysis 6 × 10(12)-fold by a mechanism that (in contrast to the mechanisms employed by arginase and agmatinase) is believed to involve attack by an active-site cysteine residue.

Targeting the arginine phosphatase YwlE with a catalytic redox-based inhibitor

Protein phosphatases are critical regulators of cellular signaling in both eukaryotes and prokaryotes. The majority of protein phosphatases dephosphorylate phosphoserine/phosphothreonine or phosphotyrosine residues. Recently, however, YwlE, a member of the low-molecular weight protein tyrosine phosphatase (LMW-PTP) family, was shown to efficiently target phosphoarginine. YwlE shares several sequence motifs with this family including the C(X)4 CR(S/T) motif that is crucial for catalysis and redox regulation of the enzyme. Herein we confirm that Cys9 and Cys14 play important roles in YwlE catalysis and regulation. On the basis of these observations, we designed and synthesized a YwlE inhibitor, denoted cyc-SeCN-amidine, that irreversibly inhibits YwlE (kinact/KI = 310 M(-1) min(-1)) by inducing disulfide bond formation between the two active site cysteine residues. Interestingly, inactivation appears to be catalytic, since the compound is neither destroyed nor altered after enzyme inhibition. Although the exact mechanism of disulfide induction remains elusive, we propose several potential mechanisms accounting for the cyc-SeCN-amidine mediated inhibition of YwlE. These findings could stimulate the design of similar selenium-based compounds targeting other redox-sensitive enzymes.

Listeria monocytogenes aguA1, but not aguA2, encodes a functional agmatine deiminase: biochemical characterization of its catalytic properties and roles in acid tolerance

Listeria monocytogenes is adaptable to low pH environments and therefore crosses the intestinal barrier to establish systemic infections. L. monocytogenes aguA1 and aguA2 encode putative agmatine deiminases (AgDIs) AguA1 and AguA2. Transcription of aguA1 and aguA2 was significantly induced at pH 5.0. Deletion of aguA1 significantly impaired its survival both in gastric fluid at pH 2.5 and in mouse stomach, whereas aguA2 deletion did not show significant defect of survival in gastric fluid. With agmatine as the sole substrate, AguA1 expressed in Escherichia coli was optimal at 25 °C and over a wide range of pH from 3.5 to 10.5. Recombinant AguA2 showed no deiminase activity. Site-directed mutagenesis revealed that all nine AguA1 mutants completely lost enzymatic activity. AguA2 acquired AgDI activity only when Cys-157 was mutated to glycine. AguA1 mutation at the same site, G157C, also inactivated the enzyme. Thus, we have discovered Gly-157 as a novel residue other than the known catalytic triad (Cys-His-Glu/Asp) in L. monocytogenes that is critical for enzyme activity. Of the two putative AgDIs, we conclude that only AguA1 functionally participates in the AgDI pathway and mediates acid tolerance in L. monocytogenes.

N-Carbamoylputrescine, a citrulline-derived polyamine, is not a significant citrulline metabolite in rats

Citrulline, a key amino acid of the urea cycle, has been shown to play a regulatory role in protein and energy metabolism in mammals. We questioned whether N-carbamoyl-putrescine (NCP), the decarboxylated derivative of citrulline, could play a role in the biological properties of this amino acid. To evidence the presence of NCP in mammalian tissues, we developed a sensitive reverse-phase high-performance liquid chromatography (HPLC) with fluorimetric detection method with precolumn dansyl derivatization and solid-phase extraction for the determination of NCP together with polyamines in biological samples. Dansyl NCP was identified with a 5.85-min retention time. Linearity was obtained in a concentration range of 0.125 to 12.5 μM. Intraday and day-to-day relative coefficients of variation ranged from 8.9% to 12.3% and from 14% to 14.3%, respectively. Recovery rates in serum ranged from 75% to 83%. Thereafter, we used this method to search for the presence of NCP in serum, muscle, liver, jejunum, and ileum in rats after both short-term intraperitoneal injection and long-term oral citrulline supplementation. We failed to detect NCP in these animals. These data suggest that NCP is not a significant citrulline metabolite in rats.

Activity-based protein profiling of protein arginine methyltransferase 1

The protein arginine methyltransferases (PRMTs) are SAM-dependent enzymes that catalyze the mono- and dimethylation of peptidyl arginine residues. PRMT1 is the founding member of the PRMT family, and this isozyme is responsible for methylating ∼85% of the arginine residues in mammalian cells. Additionally, PRMT1 activity is aberrantly upregulated in heart disease and cancer. As a part of a program to develop isozyme-specific PRMT inhibitors, we recently described the design and synthesis of C21, a chloroacetamidine bearing histone H4 tail analogue that acts as an irreversible PRMT1 inhibitor. Given the covalent nature of the interaction, we set out to develop activity-based probes (ABPs) that could be used to characterize the physiological roles of PRMT1. Herein, we report the design, synthesis, and characterization of fluorescein-conjugated C21 (F-C21) and biotin-conjugated C21 (B-C21) as PRMT1-specific ABPs. Additionally, we provide the first evidence that PRMT1 activity is negatively regulated in a spatial and temporal fashion.

Mechanistic studies of agmatine deiminase from multiple bacterial species

One subfamily of guanidino group-modifying enzymes (GMEs) consists of the agmatine deiminases (AgDs). These enzymes catalyze the conversion of agmatine (decarboxylated arginine) to N-carbamoyl putrescine and ammonia. In plants, viruses, and bacteria, these enzymes are thought to be involved in energy production, biosynthesis of polyamines, and biofilm formation. In particular, we are interested in the role that this enzyme plays in pathogenic bacteria. Previously, we reported the initial kinetic characterization of the agmatine deiminase from Helicobacter pylori and described the synthesis and characterization the two most potent AgD inactivators. Herein, we have expanded our initial efforts to characterize the catalytic mechanisms of AgD from H. pylori as well as Streptococcus mutans and Porphyromonas gingivalis. Through the use of pH rate profiles, pK(a) measurements of the active site cysteine, solvent isotope effects, and solvent viscosity effects, we have determined that the AgDs, like PADs 1 and 4, utilize a reverse protonation mechanism.

Mechanistic similarity and diversity among the guanidine-modifying members of the pentein superfamily

The pentein superfamily is a mechanistically diverse superfamily encompassing both noncatalytic proteins and enzymes that catalyze hydrolase, dihydrolase and amidinotransfer reactions on guanidine substrates. Despite generally low sequence identity, they possess a conserved structural fold and display common mechanistic themes in catalysis. The structurally characterized catalytic penteins possess a conserved core of residues that include a Cys, His and two polar, guanidine-binding residues. All known catalytic penteins use the core Cys to attack the substrate's guanidine moiety to form a covalent thiouronium adduct and all cleave one or more of the guanidine C--N bonds. The mechanistic information compiled to date supports the hypothesis that this superfamily may have evolved divergently from a catalytically promiscuous ancestor.