Contribution of Structural Peculiarities of Onconase to Its High Stability and Folding Kinetics

Insight into the Initial Stages of the Folding Process in Onconase Revealed by UNRES

The unfolded state of proteins presents many challenges to elucidate the structural basis for biological function. This state is characterized by a large degree of structural heterogeneity which makes it difficult to generate structural models. However, recent experiments into the initial folding events of the 104-residue ribonuclease homologue onconase (ONC) were able to identify the regions in the protein that participate in the initial folding of this protein. Therefore, to gain additional structural insight into the unfolded state of proteins, this study utilized molecular dynamics simulations using the UNited-RESidue (UNRES) force field to evaluate whether there is a good agreement between the experimentally determined initial structures and the structures identified by computer simulations along a folding pathway. Indeed, these UNRES simulations accurately identified the two regions experimentally observed to form the initial native structure along the folding pathway of ONC. In addition, these regions are determined to be chain folding initiation sites (CFIS) according to methods developed previously. Subsequent self-organization maps (SOM) analysis has revealed key structural states involved in these early folding events.

Replacing a single atom accelerates the folding of a protein and increases its thermostability

The conformational attributes of proline can have a substantial effect on the folding of polypeptide chains into a native structure and on the stability of that structure. Replacing the 4S hydrogen of a proline residue with fluorine is known to elicit stereoelectronic effects that favor a cis peptide bond. Here, semisynthesis is used to replace a cis-proline residue in ribonuclease A with (2S,4S)-4-fluoroproline. This subtle substitution accelerates the folding of the polypeptide chain into its three-dimensional structure and increases the thermostability of that structure without compromising its catalytic activity. Thus, an appropriately situated fluorine can serve as a prosthetic atom in the context of a protein.

Onconase dimerization through 3D domain swapping: structural investigations and increase in the apoptotic effect in cancer cells

Onconase® (ONC), a protein extracted from the oocytes of the Rana pipiens frog, is a monomeric member of the secretory 'pancreatic-type' RNase superfamily. Interestingly, ONC is the only monomeric ribonuclease endowed with a high cytotoxic activity. In contrast with other monomeric RNases, ONC displays a high cytotoxic activity. In this work, we found that ONC spontaneously forms dimeric traces and that the dimer amount increases about four times after lyophilization from acetic acid solutions. Differently from RNase A (bovine pancreatic ribonuclease) and the bovine seminal ribonuclease, which produce N- and C-terminal domain-swapped conformers, ONC forms only one dimer, here named ONC-D. Cross-linking with divinylsulfone reveals that this dimer forms through the three-dimensional domain swapping of its N-termini, being the C-terminus blocked by a disulfide bond. Also, a homology model is proposed for ONC-D, starting from the well-known structure of RNase A N-swapped dimer and taking into account the results obtained from spectroscopic and stability analyses. Finally, we show that ONC is more cytotoxic and exerts a higher apoptotic effect in its dimeric rather than in its monomeric form, either when administered alone or when accompanied by the chemotherapeutic drug gemcitabine. These results suggest new promising implications in cancer treatment.

Activating transcription factor 3 is crucial for antitumor activity and to strengthen the antiviral properties of Onconase

Onconase is a ribonuclease that presents both antitumor and antiviral properties linked to its ribonucleolytic activity and represents a new class of RNA-damaging drugs. It has reached clinical trials for the treatment of several cancers and human papilloma virus warts. Onconase targets different RNAs in the cell cytosol but Onconase-treated cells present features that are different from a simple arrest of protein synthesis. We have used microarray-derived transcriptional profiling to identify Onconase-regulated genes in two ovarian cancer cell lines (NCI/ADR-RES and OVCAR-8). RT-qPCR analyses have confirmed the microarray findings. We have identified a network of up-regulated genes implicated in different signaling pathways that may explain the cytotoxic effects exerted by Onconase. Among these genes, activating transcription factor 3 (ATF3) plays a central role in the key events triggered by Onconase in treated cancer cells that finally lead to apoptosis. This mechanism, mediated by ATF3, is cell-type independent. Up-regulation of ATF3 may also explain the antiviral properties of this ribonuclease because this factor is involved in halting viral genome replication, keeping virus latency or preventing viral oncogenesis. Finally, Onconase-regulated genes are different from those affected by nuclear-directed ribonucleases.

Native state dynamics affects the folding transition of porcine pancreatic phospholipase A2

Porcine pancreatic phospholipase A2, a small and disulfide rich protein, is extremely resistant against chemically or thermally induced unfolding. Despite this marked resistance, the protein displays broad unfolding transitions resulting in comparatively low apparent thermodynamic stability. Broad unfolding transitions may result from undetected folding intermediates, residual structures in the unfolded state or an inhomogeneity of the native state. Using circular dichroism, fluorescence, and NMR spectroscopy, we ruled out the existence of stably populated folding intermediates, whereas UV absorbance measurements hinted at stable residual structures in the unfolded state. These residual structures proved, however, to have no impact on the folding parameters. Studies by limited proteolysis, CD, and NMR spectroscopy under non-denaturing conditions suggested pronounced dynamics of the protein in the native state, which as long as unrestrained by acidic pH or bound Ca(2+) ions exert considerable influence on the unfolding transition.

Stability and folding of amphibian ribonuclease A superfamily members in comparison with mammalian homologues

Comparative studies on homologous proteins can provide knowledge on how limited changes in the primary structure find their expression in large effects on catalytic activity, stability or the folding behavior. For more than half a century, members of the ribonuclease A superfamily have been the subject of a myriad of studies on protein folding and stability. Both the unfolding and refolding kinetics as well as the structure of several folding intermediates of ribonuclease A have been characterized in detail. Moreover, the RNA-degrading activity of these enzymes provides a basis for their cytotoxicity, which renders them potential tumor therapeutics. Because amphibian ribonuclease A homologues evade the human ribonuclease inhibitor, they emerged as particularly promising candidates. Interestingly, the amphibian ribonuclease A homologues investigated to date are more stable than the mammalian homologues. Nevertheless, despite the generation of numerous genetically engineered variants, knowledge of the folding of amphibian ribonuclease A homologues remains rather limited. An exception is onconase, a ribonuclease A homologue from Rana pipiens, which has been characterized in detail. This review summarizes the data on the unfolding and refolding kinetics and pathways, as well on the stability of amphibian ribonuclease A homologues compared with those of ribonuclease A, the best known member of this superfamily.

Direct cell lysis for single-cell gene expression profiling

The interest to analyze single and few cell samples is rapidly increasing. Numerous extraction protocols to purify nucleic acids are available, but most of them compromise severely on yield to remove contaminants and are therefore not suitable for the analysis of samples containing small numbers of transcripts only. Here, we evaluate 17 direct cell lysis protocols for transcript yield and compatibility with downstream reverse transcription quantitative real-time PCR. Four endogenously expressed genes are assayed together with RNA and DNA spikes in the samples. We found bovine serum albumin (BSA) to be the best lysis agent, resulting in efficient cell lysis, high RNA stability, and enhanced reverse transcription efficiency. Furthermore, we found direct cell lysis with BSA superior to standard column based extraction methods, when analyzing from 1 up to 512 mammalian cells. In conclusion, direct cell lysis protocols based on BSA can be applied with most cell collection methods and are compatible with most analytical workflows to analyze single-cells as well as samples composed of small numbers of cells.

Consequences of proline-to-alanine substitutions for the stability and refolding of onconase

Peptidyl-prolyl isomerization reactions can make for rate-limiting steps in protein folding due to their high activation energy. Onconase, an unusually stable ribonuclease A homologue from the Northern leopard frog, contains four trans proline residues in its native state. During the refolding from its guanidine hydrochloride unfolded state, which includes the formation of a folding intermediate, the slowest of the three phases has earlier been attributed to a cis-to-trans peptidyl-prolyl isomerization reaction. We thus substituted all four proline residues individually by alanine and investigated the effect of the amino acid substitutions on the folding and stability of the onconase variants. All onconase variants proved to adopt a tertiary structure comparable with that of the wild-type protein. Although the slow phase was not eliminated for any of the variants, the P43A substitution resulted in an increase in the rate constant of the fast folding phase, i.e. a faster formation of the folding intermediate. This variant also exhibits a significant increase in thermodynamic stability. As residue 43 belongs to those residues that are protected from hydrogen exchange with the solvent in the folding intermediate, the increase in the rate constant and stability of the P43A variant emphasizes the importance of the intermediate for the folding of onconase.

Protein prosthesis: β-peptides as reverse-turn surrogates

The introduction of non-natural modules could provide unprecedented control over folding/unfolding behavior, conformational stability, and biological function of proteins. Success requires the interrogation of candidate modules in natural contexts. Here, expressed protein ligation is used to replace a reverse turn in bovine pancreatic ribonuclease (RNase A) with a synthetic β-dipeptide: β²-homoalanine-β³-homoalanine. This segment is known to adopt an unnatural reverse-turn conformation that contains a 10-membered ring hydrogen bond, but one with a donor-acceptor pattern opposite to that in the 10-membered rings of natural reverse turns. The RNase A variant has intact enzymatic activity, but unfolds more quickly and has diminished conformational stability relative to native RNase A. These data indicate that hydrogen-bonding pattern merits careful consideration in the selection of beneficial reverse-turn surrogates.

Arginine residues are more effective than lysine residues in eliciting the cellular uptake of onconase

Onconase is an amphibian member of the pancreatic ribonuclease family of enzymes that is in clinical trials for the treatment of cancer. Onconase, which has an abundance of lysine residues, is internalized by cancer cells through endocytosis in a mechanism similar to that of cell-penetrating peptides. Here, we compare the effect of lysine versus arginine residues on the biochemical attributes necessary for Onconase to elicit its cytotoxic activity. In the variant R-Onconase, 10 of the 12 lysine residues in Onconase are replaced with arginine, leaving only the two active-site lysines intact. Cytometric assays quantifying internalization showed a 3-fold increase in the internalization of R-Onconase compared with Onconase. R-Onconase also showed greater affinity for heparin and a 2-fold increase in ribonucleolytic activity. Nonetheless, arginine substitution endowed only a slight increase in toxicity toward human cancer cells. Analysis of denaturation induced with guanidine-HCl showed that R-Onconase has less conformational stability than does the wild-type enzyme; moreover, R-Onconase is more susceptible to proteolytic degradation. These data indicate that arginine residues are more effective than lysine in eliciting cellular internalization but can compromise other aspects of protein structure and function.

Three-state thermal unfolding of onconase

Onconase is a member of the ribonuclease A superfamily currently in phase IIIb clinical trials as a treatment for malign mesothelioma due to its cytotoxic activity selective against tumor-cells. In this work, we have studied the equilibrium thermal unfolding of onconase using a combination of several structural and biophysical techniques. Our results indicate that at least one significantly populated intermediate, which implies the exposure of hydrophobic surface and significant changes in the environment around Trp3, occurs during the equilibrium unfolding process of this protein. The intermediate begins to populate at about 30° below the global unfolding temperature, reaching a maximum population of nearly 60%, 10° below the global unfolding temperature.

A structure-based computational mutagenesis elucidates the spectrum of stability-activity relationships in proteins

Protein engineering experiments involving single amino acid substitutions are routinely implemented for the analysis of protein structure, stability, and function. The resulting change in just one of these characteristics relative to the native protein constitutes the focus of any single study, as is the case with predictive computational models developed for the same purpose. Other than investigations into stability-activity trade-offs specifically resulting from active site residue replacements in a few enzymes, a literature survey fails to reveal a comprehensive analysis of stability-activity relationships in proteins upon mutation. Here, we employ a computational mutagenesis for quantifying overall protein structural change upon mutation, which is applied to a dataset of 938 single residue replacements distributed at positions throughout twenty diverse proteins. These mutants are selected based on the availability of both experimental stability and activity change data, and their structural change data are used to characterize the full range of stability-activity relationships.

The (1)H, (13)C, (15)N resonance assignment, solution structure, and residue level stability of eosinophil cationic protein/RNase 3 determined by NMR spectroscopy

Eosinophil cationic protein (ECP)/human RNase 3, a member of the RNase A family, is a remarkably cytotoxic protein implicated in asthma and allergies. These activities are probably due to ECP's ability to interact with and disrupt membranes and depend on two Trp, 19 Arg, and possibly an extremely high conformational stability. Here, we have used NMR spectroscopy to assign essentially all (1)H, (15)N, and backbone (13)C resonances, to solve the 3D structure in aqueous solution and to quantify its residue-level stability. The NMR solution structure was determined on the basis of 2316 distance constraints and is well-defined (backbone RMSD = 0.81 A). The N-terminus and the loop composed of residues 114-123 are relatively well-ordered; in contrast, conformational diversity is observed for the loop segments 17-22, 65-68, and 92-95 and most exposed sidechains. The side chain NH groups of the two Trp and 19 Arg showed no significant protection against hydrogen/deuterium exchange. The most protected NH groups belong to the first and last two beta-strands, and curiously, the first alpha-helix. Analysis of their exchange rates reveals a strikingly high global stability of 11.8 kcal/mol. This value and other stability measurements are used to better quantify ECP's unfolding thermodynamics.

Ribonucleases as potential modalities in anticancer therapy

Antitumor ribonucleases are small (10-28 kDa) basic proteins. They were found among members of both, ribonuclease A and T1 superfamilies. Their cytotoxic properties are conferred by enzymatic activity, i.e., the ability to catalyze cleavages of phosphodiester bonds in RNA. They bind to negatively charged cell membrane, enter cells by endocytosis and translocate to cytosol where they evade mammalian protein ribonuclease inhibitor and degrade RNA. Here, we discuss structures, functions and mechanisms of antitumor activity of several cytotoxic ribonucleases with particular emphasis to the amphibian Onconase, the only enzyme of this class that reached clinical trials. Onconase is the smallest, very stable, less catalytically efficient and more cytotoxic than most RNase A homologues. Its cytostatic, cytotoxic and anticancer effects were extensively studied. It targets tRNA, rRNA, mRNA as well as the non-coding RNA (microRNAs). Numerous cancer lines are sensitive to Onconase; their treatment with 10-100 nM enzyme leads to suppression of cell cycle progression, predominantly through G(1), followed by apoptosis or cell senescence. Onconase also has anticancer properties in animal models. Many effects of this enzyme are consistent with the microRNAs, one of its critical targets. Onconase sensitizes cells to a variety of anticancer modalities and this property is of particular interest, suggesting its application as an adjunct to chemotherapy or radiotherapy in treatment of different tumors. Cytotoxic RNases as exemplified by Onconase represent a new class of antitumor agents, with an entirely different mechanism of action than the drugs currently used in the clinic. Further studies on animal models including human tumors grafted on severe combined immunodefficient (SCID) mice and clinical trials are needed to explore clinical potential of cytotoxic RNases.

The folding pathway of onconase is directed by a conserved intermediate

A promising approach to unravel the relationship between sequence information, tertiary structure, and folding mechanism of proteins is the analysis of the folding behavior of proteins with low sequence identity but comparable tertiary structures. Ribonuclease A (RNase A) and its homologues, forming the RNase A superfamily, provide an excellent model system for respective studies. RNase A has been used extensively as a model protein for folding studies. However, little is known about the folding of homologous RNases. Here, we analyze the folding pathway of onconase, a homologous protein from the Northern leopard frog with great potential as a tumor therapeutic, by high-resolution techniques. Although onconase and RNase A significantly differ in the primary structure (28% sequence identity) and in thermodynamic stability (DeltaDeltaG = 20 kJ mol(-1)), both enzymes possess very similar tertiary structures. The present folding studies on onconase by rapid mixing techniques in combination with fluorescence and NMR spectroscopy allow the structural assignment of the three kinetic phases observed in stopped-flow fluorescence spectroscopy. After a slow peptidyl-prolyl cis-to-trans isomerization reaction in the unfolded state, ONC folds via an on-pathway intermediate to the native state. By quenched-flow hydrogen/deuterium exchange experiments coupled with 2D NMR spectroscopy, 31 amino acid residues were identified to be involved in the structure formation of the intermediate. Twelve of these residues are identical in the RNase A sequence, which is a significantly higher percentage (39%) than the overall 28% sequence identity. Moreover, the structure of this intermediate closely resembles two of the intermediates that occur early during the refolding of RNase A. Obviously, in spite of considerable differences in their amino acid sequence the initial folding events of both proteins are comparable, guided by a limited number of conserved residues.

Pyroglutamate formation influences solubility and amyloidogenicity of amyloid peptides

N-Terminally truncated and pyroglutamate (pGlu) modified amyloid beta (Abeta) peptides are major constituents of amyloid deposits in sporadic and inherited Alzheimer's disease (AD). Formation of pGlu at the N-terminus confers resistance against cleavage by most aminopeptidases, increases toxicity of the peptides, and may seed Abeta aggregate formation. Similarly, the deposited amyloid peptides ABri and ADan, which cause a very similar histopathology in familial British dementia (FBD) and familial Danish dementia (FDD), are N-terminally blocked by pGlu. Triggered by the coincidence of pGlu-modified amyloid peptides and similar pathology in AD, FBD, and FDD, we investigated the impact of N-terminal pGlu on biochemical and biophysical properties of Abeta, ABri, and ADan. N-Terminal pGlu increases the hydrophobicity and changes the pH-dependent solubility profile, rendering the pGlu-modified peptides less soluble in the basic pH range. The pGlu residue increases the aggregation propensity of all amyloid peptides as evidenced by ThT fluorescence assays and dynamic light scattering. The far-UV CD spectroscopic analysis points toward an enhanced beta-sheet structure of the pGlu-Abeta. Importantly, changes in fibril morphology are clearly caused by the N-terminal pGlu, resulting in the formation of short fibers, which are frequently arranged in bundles. The effect of pGlu on the morphology is virtually indistinguishable between ABri, ADan, and Abeta. The data provide evidence for a comparable influence of the pGlu modification on the aggregation process of structurally different amyloid peptides, thus likely contributing to the molecularly distinct neurodegenerative diseases AD, FBD, and FDD. The main driving force for the aggregation is apparently an increase in the hydrophobicity and thus an accelerated seed formation.

Onconase cytotoxicity relies on the distribution of its positive charge

Onconase (ONC) is a member of the ribonuclease A superfamily that is toxic to cancer cells in vitro and in vivo. ONC is now in Phase IIIb clinical trials for the treatment of malignant mesothelioma. Internalization of ONC to the cytosol of cancer cells is essential for its cytotoxic activity, despite the apparent absence of a cell-surface receptor protein. Endocytosis and cytotoxicity do, however, appear to correlate with the net positive charge of ribonucleases. To dissect the contribution made by the endogenous arginine and lysine residues of ONC to its cytotoxicity, 22 variants were created in which cationic residues were replaced with alanine. Variants with the same net charge (+2 to +5) as well as equivalent catalytic activity and conformational stability were found to exhibit large (> 10-fold) differences in toxicity for the cells of a human leukemia line. In addition, a more cationic ONC variant could be either much more or much less cytotoxic than a less cationic variant, again depending on the distribution of its cationic residues. The endocytosis of variants with widely divergent cytotoxic activity was quantified by flow cytometry using a small-molecule fluorogenic label, and was found to vary by twofold or less. This small difference in endocytosis did not account for the large difference in cytotoxicity, implicating the distribution of cationic residues as being critical for lipid-bilayer translocation subsequent to endocytosis. This finding has fundamental implications for understanding the interaction of ribonucleases and other proteins with mammalian cells.

Ranpirnase as a potential antitumor ribonuclease treatment for mesothelioma and other malignancies

Ranpirnase, originally isolated from oocytes of the northern leopard frog (Rana pipiens), is a member of the pancreatic RNase A superfamily of ribonucleases. Ranpirnase exerts antiproliferative and cytotoxic effects in vitro and in vivo and has been shown to act synergistically with different cancer therapeutic agents. The cytotoxic and cytostatic effects of ranpirnase are the consequence of tRNA degradation that results in the disruption of protein translation and the induction of programmed cell death (apoptosis). Ranpirnase has been shown to target malignant cells both in human cancer cell lines and in animal models, and has demonstrated efficacy in the treatment of several human cancers in clinical studies. Most clinical studies have been conducted in patients with malignant mesothelioma, and a confirmatory Phase IIIb trial is currently underway for the treatment of this disease. Owing to its selective destruction of malignant cells and favorable toxicology profile, ranpirnase is a promising antitumor agent with ideal attributes that are generally lacking in conventional cytotoxic drugs.

Contribution of the C30/C75 disulfide bond to the biological properties of onconase

Onconase, a member of the pancreatic type ribonuclease family, is currently used as a chemotherapeutic agent for the treatment of different types of cancer. It is widely accepted that one of the properties that renders this enzyme cytotoxic is its ability to evade the cytosolic ribonuclease inhibitor (RI). In the present work, we produced and characterized an onconase variant that lacks the disulfide bond C30/C75. This variant mimics the stable unfolding intermediate des(30–75) produced in the reductive unfolding pathway of onconase. We found that the reduction of the C30/C75 disulfide bond does not significantly alter the cytotoxic properties of onconase, although the variant possesses a notably reduced conformational stability. Interestingly, both its catalytic activity and its ability to evade RI are comparable to wild-type onconase under mild reductive conditions in which the three disulfide containing intermediate des(30–75) is present. These results suggest that the C30/C75 disulfide bond could easily be reduced under physiological redox conditions.

The folding process of acylphosphatase from Escherichia coli is remarkably accelerated by the presence of a disulfide bond

The acylphosphatase from Escherichia coli (EcoAcP) is the first AcP so far studied with a disulfide bond. A mutational variant of the enzyme lacking the disulfide bond has been produced by substituting the two cysteine residues with alanine (EcoAcP mutational variant C5A/C49A, mutEcoAcP). The native states of the two protein variants are similar, as shown by far-UV and near-UV circular dichroism and dynamic light-scattering measurements. From unfolding experiments at equilibrium using intrinsic fluorescence and far-UV circular dichroism as probes, EcoAcP shows an increased conformational stability as compared with mutEcoAcP. The wild-type protein folds according to a two-state model with a very fast rate constant (k(F)(H2O)=72,600 s(-1)), while mutEcoAcP folds ca 1500-fold slower, via the accumulation of a partially folded species. The correlation between the hydrophobicity of the polypeptide chain and the folding rate, found previously in the AcP-like structural family, is maintained only when considering the mutant but not the wild-type protein, which folds much faster than expected from this correlation. Similarly, the correlation between the relative contact order and the folding rate holds only for mutEcoAcP. The correlation also holds for EcoAcP, provided the relative contact order value is recalculated by considering the disulfide bridge as an alternate path for the backbone to determine the shortest sequence separation between contacting residues. These results indicate that the presence of a disulfide bond in a protein is an important determinant of the folding rate and allows its contribution to be determined in quantitative terms.

Ribonucleases as novel chemotherapeutics : the ranpirnase example

Ranpirnase, a cytotoxic ribonuclease from the frog Rana pipiens, is the archetype of a novel class of cancer chemotherapeutic agents based on homologs and variants of bovine pancreatic ribonuclease (RNase A). Ranpirnase in combination with doxorubicin is in clinical trials for the treatment of unresectable malignant mesothelioma and other cancers. The putative mechanism for ranpirnase-mediated cytotoxicity involves binding to anionic components of the extracellular membrane, cytosolic internalization, and degradation of transfer RNA leading to apoptosis. The maintenance of ribonucleolytic activity in the presence of the cytosolic ribonuclease inhibitor protein is a key aspect of the cytotoxic activity of ranpirnase. The basis for its specific toxicity for cancer cells is not known. This review describes the development of ranpirnase as a cancer chemotherapeutic agent.

Structural basis for catalysis by onconase

Onconase (ONC) is a homolog of bovine pancreatic ribonuclease (RNase A) from the frog Rana pipiens. ONC displays antitumoral activity and is in advanced clinical trials for the treatment of cancer. Here, we report the first atomic structures of ONC-nucleic acid complexes: a T89N/E91A ONC-5'-AMP complex at 1.65 A resolution and a wild-type ONC-d(AUGA) complex at 1.90 A resolution. The latter structure and site-directed mutagenesis were used to reveal the atomic basis for substrate recognition and turnover by ONC. The residues in ONC that are proximal to the scissile phosphodiester bond (His10, Lys31, and His97) and uracil nucleobase (Thr35, Asp67, and Phe98) are conserved from RNase A and serve to generate a similar bell-shaped pH versus k(cat)/K(M) profile for RNA cleavage. Glu91 of ONC forms two hydrogen bonds with the guanine nucleobase in d(AUGA), and Thr89 is in close proximity to that nucleobase. Installing a neutral or cationic residue at position 91 or an asparagine residue at position 89 virtually eliminated the 10(2)-fold guanine:adenine preference of ONC. A variant that combined such substitutions, T89N/E91A ONC, actually preferred adenine over guanine. In contrast, installing an arginine residue at position 91 increased the guanine preference and afforded an ONC variant with the highest known k(cat)/K(M) value. These data indicate that ONC discriminates between guanine and adenine by using Coulombic interactions and a network of hydrogen bonds. The structure of the ONC-d(AUGA) complex was also used to probe other aspects of catalysis. For example, the T5R substitution, designed to create a favorable Coulombic interaction between ONC and a phosphoryl group in RNA, increased ribonucleolytic activity by twofold. No variant, however, was more toxic to human cancer cells than wild-type ONC. Together, these findings provide a cynosure for understanding catalysis of RNA cleavage in a system of high medicinal relevance.

Natural and engineered ribonucleases as potential cancer therapeutics

By reason of their cytotoxicity, ribonucleases (RNases) are potential anti-tumor drugs. Particularly members from the RNase A and RNase T1 superfamilies have shown promising results. Among these enzymes, Onconase, an RNase from the Northern Leopard frog, is furthest along in clinical trials. A general model for the mechanism of the cytotoxic action of RNases includes the interaction of the enzyme with the cellular membrane, internalization, translocation to the cytosol, and degradation of ribonucleic acid. The interplay of these processes as well as the role of the thermodynamic and proteolytic stability, the catalytic activity, and the capability of the RNase to evade the intracellular RNase inhibitor has not yet been fully elucidated. This paper discusses the various approaches to exploit RNases as cytotoxic agents.