Crystal structure disposition of thymidylic acid tetramer in complex with ribonuclease A

Dynamics of non-covalent interactions during the P-O bond cleavage reaction by ribonuclease A

In this work, an atomistic-scale investigation of the phosphodiester P-O bond cleavage reaction by the enzyme ribonuclease A was carried out using computer simulation techniques. It is shown that during the reaction the network of non-covalent interactions in the active center of the ribonuclease changes significantly, while the role of these non-covalent interactions is different: coordination of the corresponding groups, electron density transfer, and ligand holding in the active center. It is shown that the process of proton transfer from Asp121 to His119 is the first stage of this reaction; at the same time, the hydrogen bond between the phosphate ligand and the imino group of Arg39 is broken, which, although keeping the ligand in the active center, does not allow the ligand to orient itself more conveniently for subsequent proton transfers. Furthermore, the key step of this reaction occurs: proton transfer with the participation of imidazole rings His12 and His119, in which the guiding role is played by several hydrogen bonds with the participation of Phe120, and the role of an electron density carrier is played by the pnictogen bond between the oxygen of the phosphate ligand and the pyridine-like nitrogen of the imidazole ring His119, which was detected for the first time.

Characterization of an RNase with two catalytic centers. Human RNase6 catalytic and phosphate-binding site arrangement favors the endonuclease cleavage of polymeric substrates

BACKGROUND

Human RNase6 is a small cationic antimicrobial protein that belongs to the vertebrate RNaseA superfamily. All members share a common catalytic mechanism, which involves a conserved catalytic triad, constituted by two histidines and a lysine (His15/His122/Lys38 in RNase6 corresponding to His12/His119/Lys41 in RNaseA). Recently, our first crystal structure of human RNase6 identified an additional His pair (His36/His39) and suggested the presence of a secondary active site.

METHODS

In this work we have explored RNase6 and RNaseA subsite architecture by X-ray crystallography, site-directed mutagenesis and kinetic characterization.

RESULTS

The analysis of two novel crystal structures of RNase6 in complex with phosphate anions at atomic resolution locates a total of nine binding sites and reveals the contribution of Lys87 to phosphate-binding at the secondary active center. Contribution of the second catalytic triad residues to the enzyme activity is confirmed by mutagenesis. RNase6 catalytic site architecture has been compared with an RNaseA engineered variant where a phosphate-binding subsite is converted into a secondary catalytic center (RNaseA-K7H/R10H).

CONCLUSIONS

We have identified the residues that participate in RNase6 second catalytic triad (His36/His39/Lys87) and secondary phosphate-binding sites. To note, residues His39 and Lys87 are unique within higher primates. The RNaseA/RNase6 side-by-side comparison correlates the presence of a dual active site in RNase6 with a favored endonuclease-type cleavage pattern.

GENERAL SIGNIFICANCE

An RNase dual catalytic and extended binding site arrangement facilitates the cleavage of polymeric substrates. This is the first report of the presence of two catalytic centers in a single monomer within the RNaseA superfamily.

Preliminary investigation of deoxyoligonucleotide binding to ribonuclease A using mass spectrometry: An attempt to develop a lab experience for undergraduates

Deoxyoligonucleotide binding to bovine pancreatic ribonuclease A (RNase A) was investigated using electrospray ionization ion-trap mass spectrometry (ESI-IT-MS). Deoxyoligonucleotides included CCCCC (dC ₅) and CCACC (dC ₂AC ₂).  This work was an attempt to develop a biochemistry lab experience that would introduce undergraduates to the use of mass spectrometry for the analysis of protein-ligand interactions.  Titration experiments were performed using a fixed RNase A concentration and variable deoxyoligonucleotide concentrations.  Samples at equilibrium were infused directly into the mass spectrometer under native conditions.  For each deoxyoligonucleotide, mass spectra showed one-to-one binding stoichiometry, with marked increases in the total ion abundance of ligand-bound RNase A complexes as a function of concentration, but the accurate determination of dC ₅ and dC ₂AC ₂ dissociation constants was problematic.

Imaging of protein crystals with two-photon microscopy

Second-order nonlinear optical imaging of chiral crystals (SONICC), which portrays second-harmonic generation (SHG) by noncentrosymmetric crystals, is emerging as a powerful imaging technique for protein crystals in media opaque to visible light because of its high signal-to-noise ratio. Here we report the incorporation of both SONICC and two-photon excited fluorescence (TPEF) into one imaging system that allows visualization of crystals as small as ~10 μm in their longest dimension. Using this system, we then documented an inverse correlation between the level of symmetry in examined crystals and the intensity of their SHG. Moreover, because of blue-green TPEF exhibited by most tested protein crystals, we also could identify and image SHG-silent protein crystals. Our experimental data suggest that the TPEF in protein crystals is mainly caused by the oxidation of tryptophan residues. Additionally, we found that unspecific fluorescent dyes are able to bind to lysozyme crystals and enhance their detection by TPEF. We finally confirmed that the observed fluorescence was generated by a two-photon rather than a three-photon process. The capability for imaging small protein crystals in turbid or opaque media with nondamaging infrared light in a single system makes the combination of SHG and intrinsic visible TPEF a powerful tool for nondestructive protein crystal identification and characterization during crystallization trials.

Site-selective artificial ribonucleases: oligonucleotide conjugates containing multiple imidazole residues in the catalytic domain

Design of site-selective artificial ribonucleases (aRNases) is one of the most challenging tasks in RNA targeting. Here, we designed and studied oligonucleotide-based aRNases containing multiple imidazole residues in the catalytic part and systematically varied structure of cleaving constructs. We demonstrated that the ribonuclease activity of the conjugates is strongly affected by the number of imidazole residues in the catalytic part, the length of a linker between the catalytic imidazole groups of the construct and the oligonucleotide, and the type of anchor group, connecting linker structure and the oligonucleotide. Molecular modeling of the most active aRNases showed that preferable orientation(s) of cleaving constructs strongly depend on the structure of the anchor group and length of the linker. The inclusion of deoxyribothymidine anchor group significantly reduced the probability of cleaving groups to locate near the cleavage site, presumably due to a stacking interaction with the neighbouring nucleotide residue. Altogether the obtained results show that dynamics factors play an important role in site-specific RNA cleavage. Remarkably high cleavage activity was displayed by the conjugates with the most flexible and extended cleaving construct, which presumably provides a better opportunity for imidazole residues to be correctly positioned in the vicinity of scissile phosphodiester bond.

The crystal structure of ribonuclease A in complex with thymidine-3'-monophosphate provides further insight into ligand binding

Thymidine-3'-monophosphate (3'-TMP) is a competitive inhibitor analogue of the 3'-CMP and 3'-UMP natural product inhibitors of bovine pancreatic ribonuclease A (RNase A). Isothermal titration calorimetry experiments show that 3'-TMP binds the enzyme with a dissociation constant (K(d)) of 15 microM making it one of the strongest binding members of the five natural bases found in nucleic acids (A, C, G, T, and U). To further investigate the molecular properties of this potent natural affinity, we have determined the crystal structure of bovine pancreatic RNase A in complex with 3'-TMP at 1.55 A resolution and we have performed NMR binding experiments with 3'-CMP and 3'-TMP. Our results show that binding of 3'-TMP is very similar to other natural and non-natural pyrimidine ligands, demonstrating that single nucleotide affinity is independent of the presence or absence of a 2'-hydroxyl on the ribose moiety of pyrimidines and suggesting that the pyrimidine binding subsite of RNase A is not a significant contributor of inhibitor discrimination. Accumulating evidence suggests that very subtle structural, chemical, and potentially motional variations contribute to ligand discrimination in this enzyme.

Structure of bovine pancreatic ribonuclease complexed with uridine 5'-monophosphate at 1.60 A resolution

Bovine pancreatic ribonuclease A (RNase A) was crystallized from a mixture of small molecules containing basic fuchsin, tobramycin and uridine 5'-monophosphate (U5P). Solution of the crystal structure revealed that the enzyme was selectively bound to U5P, with the pyrimidine ring of U5P residing in the pyrimidine-binding site at Thr45. The structure was refined to an R factor of 0.197 and an R(free) of 0.253.

Catalysis by ribonuclease A is limited by the rate of substrate association

The value of k(cat)/K(M) for catalysis of RNA cleavage by ribonuclease (RNase) A can exceed 10(9) M(-1) s(-1) in a solution of low salt concentration. This value approaches that expected for the diffusional encounter of the enzyme and its substrate. To reveal the physicochemical constraints upon catalysis by RNase A, the effects of salt concentration, pH, solvent isotope, and solvent viscosity on catalysis were determined with synthetic substrates that bind to all of the enzymic subsites and thereby enable a meaningful analysis. The pK(a) values determined from pH-k(cat)/K(M) profiles at 0.010, 0.20, and 1.0 M NaCl are inconsistent with the known macroscopic pK(a) values of RNase A. This incongruity indicates that catalysis of RNA cleavage by RNase A is limited by the rate of substrate association, even at 1.0 M NaCl. The effect of solvent isotope and solvent viscosity on catalysis support this conclusion. The data are consistent with a mechanism in which RNase A associates with RNA in an intermediate complex, which is stabilized by Coulombic interactions, prior to the formation of a Michaelis complex. Thus, RNase A has evolved to become an enzyme limited by physics rather than chemistry, a requisite attribute of a perfect catalyst.

Crystallographic and functional studies of a modified form of eosinophil-derived neurotoxin (EDN) with novel biological activities

The crystal structure of a post-translationally modified form of eosinophil-derived neurotoxin (EDN) with four extra residues on its N terminus ((-4)EDN) has been solved and refined at atomic resolution (1 A). Two of the extra residues can be placed unambiguously, while the density corresponding to two others is poor. The modified N terminus appears to influence the position of the catalytically important His129, possibly explaining the diminished catalytic activity of this variant. However, (-4)EDN has been shown to be cytotoxic to a Kaposi's sarcoma tumor cell line and other endothelial cell lines. Analysis of the structure and function suggests that the reason for cytotoxicity is most likely due to cellular recognition by the N-terminal extension, since the intrinsic activity of the enzyme is not sufficient for cytotoxicity and the N-terminal extension does not affect the conformation of EDN.

Structures of the two 3D domain-swapped RNase A trimers

When concentrated in mildly acidic solutions, bovine pancreatic ribonuclease (RNase A) forms long-lived oligomers including two types of dimer, two types of trimer, and higher oligomers. In previous crystallographic work, we found that the major dimeric component forms by a swapping of the C-terminal beta-strands between the monomers, and that the minor dimeric component forms by swapping the N-terminal alpha-helices of the monomers. On the basis of these structures, we proposed that a linear RNase A trimer can form from a central molecule that simultaneously swaps its N-terminal helix with a second RNase A molecule and its C-terminal strand with a third molecule. Studies by dissociation are consistent with this model for the major trimeric component: the major trimer dissociates into both the major and the minor dimers, as well as monomers. In contrast, the minor trimer component dissociates into the monomer and the major dimer. This suggests that the minor trimer is cyclic, formed from three monomers that swap their C-terminal beta-strands into identical molecules. These conclusions are supported by cross-linking of lysyl residues, showing that the major trimer swaps its N-terminal helix, and the minor trimer does not. We verified by X-ray crystallography the proposed cyclic structure for the minor trimer, with swapping of the C-terminal beta-strands. This study thus expands the variety of domain-swapped oligomers by revealing the first example of a protein that can form both a linear and a cyclic domain-swapped oligomer. These structures permit interpretation of the enzymatic activities of the RNase A oligomers on double-stranded RNA.

Mapping the ribonucleolytic active site of eosinophil-derived neurotoxin (EDN). High resolution crystal structures of EDN complexes with adenylic nucleotide inhibitors

Eosinophil-derived neurotoxin (EDN), a basic ribonuclease found in the large specific granules of eosinophils, belongs to the pancreatic RNase A family. Although its physiological function is still unclear, it has been shown that EDN is a neurotoxin capable of inducing the Gordon phenomenon in rabbits. EDN is also a potent helminthotoxin and can mediate antiviral activity of eosinophils against isolated virions of the respiratory syncytial virus. EDN is a catalytically efficient RNase sharing similar substrate specificity with pancreatic RNase A with its ribonucleolytic activity being absolutely essential for its neurotoxic, helminthotoxic, and antiviral activities. The crystal structure of recombinant human EDN in the unliganded form has been determined previously (Mosimann, S. C., Newton, D. L., Youle, R. J., and James, M. N. G. (1996) J. Mol. Biol. 260, 540-552). We have now determined high resolution (1.8 A) crystal structures for EDN in complex with adenosine-3',5'-diphosphate (3',5'-ADP), adenosine-2',5'-di-phosphate (2',5'-ADP), adenosine-5'-diphosphate (5'-ADP) as well as for a native structure in the presence of sulfate refined at 1.6 A. The inhibition constant of these mononucleotides for EDN has been determined. The structures present the first detailed picture of differences between EDN and RNase A in substrate recognition at the ribonucleolytic active site. They also provide a starting point for the design of tight-binding inhibitors, which may be used to restrain the RNase activity of EDN.

Three-dimensional crystal structure of human eosinophil cationic protein (RNase 3) at 1.75 A resolution

Eosinophil cationic protein (ECP; RNase 3) is a human ribonuclease found only in eosinophil leukocytes that belongs to the RNase A superfamily. This enzyme is bactericidal, helminthotoxic and cytotoxic to mammalian cells and tissues. The protein has been cloned, heterologously overexpressed, purified and crystallized. Its crystal structure has been determined and refined using data up to 1. 75 A resolution. The molecule displays the alpha+beta folding topology typical for members of the ribonuclease A superfamily. The catalytic active site residues are conserved with respect to other ribonucleases of the superfamily but some differences appear at substrate recognition subsites, which may account, in part, for the low catalytic activity. Most strikingly, 19 surface-located arginine residues confer a strong basic character to the protein. The high concentration of positive charges and the particular orientation of the side-chains of these residues may also be related to the low activity of ECP as a ribonuclease and provides an explanation for its unique cytotoxic role through cell membrane disruption.

Factors determining the selectivity of protein tyrosine nitration

Tyrosine nitration is a covalent posttranslational protein modification derived from the reaction of proteins with nitrating agents. Protein nitration appears to be a selective process since not all tyrosine residues in proteins or all proteins are nitrated in vivo. To investigate factors that may determine the biological selectivity of protein tyrosine nitration, we developed an in vitro model consisting of three proteins with similar size but different three-dimensional structure and tyrosine content. Exposure of ribonuclease A to putative in vivo nitrating agents revealed preferential nitration of tyrosine residue Y(115). Tyrosine residue Y(23) and to a lesser extent residue Y(20) were preferentially nitrated in lysozyme, whereas tyrosine Y(102) was the only residue modified by nitration in phospholipase A(2). Tyrosine Y(115) was the residue modified by nitration after exposure of ribonuclease A to different nitrating agents: chemically synthesized peroxynitrite, nitric oxide, and superoxide generated by SIN-1 or myeloperoxidase (MPO)/H(2)O(2) plus nitrite (NO(-2)) in the presence of bicarbonate/CO(2). The nature of the nitrating agent determined in part the protein that would be predominantly modified by nitration in a mixture of all three proteins. Ribonuclease A was preferentially nitrated upon exposure to MPO/H(2)O(2)/NO(-2), whereas phospholipase A(2) was the primary target for nitration upon exposure to peroxynitrite. The data also suggest that the exposure of the aromatic ring to the surface of the protein, the location of the tyrosine on a loop structure, and its association with a neighboring negative charge are some of the factors determining the selectivity of tyrosine nitration in proteins.

Kinetic and product distribution analysis of human eosinophil cationic protein indicates a subsite arrangement that favors exonuclease-type activity

With the use of a high yield prokaryotic expression system, large amounts of human eosinophil cationic protein (ECP) have been obtained. This has allowed a thorough kinetic study of the ribonuclease activity of this protein. The catalytic efficiencies for oligouridylic acids of the type (Up)nU>p, mononucleotides U>p and C>p, and dinucleoside monophosphates CpA, UpA, and UpG have been interpreted by the specific subsites distribution in ECP. The distribution of products derived from digestion of high molecular mass substrates, such as poly(U) and poly(C), by ECP was compared with that of RNase A. The characteristic cleavage pattern of polynucleotides by ECP suggests that an exonuclease-like mechanism is predominantly favored in comparison to the endonuclease catalytic mechanism of RNase A. Comparative molecular modeling with bovine pancreatic RNase A-substrate analog crystal complexes revealed important differences in the subsite structure, whereas the secondary phosphate-binding site (p2) is lacking, the secondary base subsite (B2) is severely impaired, and there are new interactions at the po, Bo, and p-1 sites, located upstream of the P-O-5' cleavable phosphodiester bond, that are not found in RNase A. The differences in the multisubsites structure could explain the reduced catalytic efficiency of ECP and the shift from an endonuclease to an exonuclease-type mechanism.

Coulombic forces in protein-RNA interactions: binding and cleavage by ribonuclease A and variants at Lys7, Arg10, and Lys66

The interactions between bovine pancreatic ribonuclease A (RNase A) and its RNA substrate extend beyond the scissile P-O5' bond. Enzymic subsites interact with the bases and phosphoryl groups of the bound substrate. Those residues interacting with the phosphoryl group comprise the P0, P1, and P2 subsites, with the scissile bond residing in the P1 subsite. Here, the function of the P0 and P2 subsites of RNase A is characterized in detail. Lys66 (P0 subsite) and Lys7 and Arg10 (P2 subsite) were replaced with alanine residues. Wild-type RNase A and the K66A, K7A/R10A, and K7A/R10A/K66A variants were evaluated as catalysts for the cleavage of poly(cytidylic acid) [poly(C)] and for their abilities to bind to single-stranded DNA, a substrate analogue. The values of kcat and Km for poly(C) cleavage were affected by altering the P0 and P2 subsites. The kcat/Km values for poly(C) cleavage by the K66A, K7A/R10A, and K7A/R10A/K66A variants were 3-fold, 60-fold, and 300-fold lower, respectively, than that of wild-type RNase A. These values indicate that the P0 and P2 subsites contribute 0.70 and 2.46 kcal/mol, respectively, to transition-state binding. Binding experiments indicate that the P0 and P2 subsites contribute 0.92 and 1.21 kcal/mol, respectively, to ground-state binding. Thus, the P0 subsite makes a uniform contribution toward binding the ground state and the transition state, whereas the P2 subsite differentiates, binding more tightly to the transition state than to the ground state. In addition, nucleic acid binding to wild-type RNase A is strongly dependent on NaCl concentration, but this dependence is diminished upon alteration of the P0 or P2 subsite. The logarithm of Kd is a linear function of the logarithm of [Na+] over the range 0.018 M </= [Na+] </= 0.14 M, with partial differential log Kd/ partial differential log [Na+] = 2.3 +/- 0.1, 1.8 +/- 0.1, 1.4 +/- 0.1, and 0.9 +/- 0.2 for nucleic acid binding to wild-type RNase A and the K66A, K7A/R10A, and K7A/R10A/K66A variants, respectively. Similar experiments with NaF and the wild-type enzyme yield partial differential log Kd/ partial differential log [Na+] = 2.0 +/- 0.2, indicating that the anion makes only a small contribution to nucleic acid binding. Together these data provide a detailed picture of the contributions of Coulombic interactions to binding and catalysis by RNase A, and illuminate the general role of Coulombic forces between proteins and nucleic acids.

The crystal structure of a 3D domain-swapped dimer of RNase A at a 2.1-A resolution

The dimer of bovine pancreatic ribonuclease A (RNase A) discovered by Crestfield, Stein, and Moore in 1962 has been crystallized and its structure determined and refined to a 2.1-A resolution. The dimer is 3D domain-swapped. The N-terminal helix (residues 1-15) of each subunit is swapped into the major domain (residues 23-124) of the other subunit. The dimer of bull seminal ribonuclease (BS-RNase) is also known to be domain-swapped, but the relationship of the subunits within the two dimers is strikingly different. In the RNase A dimer, the 3-stranded beta sheets of the two subunits are hydrogen-bonded at their edges to form a continuous 6-stranded sheet across the dimer interface; in the BS-RNase dimer, it is instead the two helices that abut. Whereas the BS-RNase dimer has 2-fold molecular symmetry, the two subunits of the RNase A dimer are related by a rotation of approximately 160 degrees. Taken together, these structures show that intersubunit adhesion comes mainly from the swapped helical domain binding to the other subunit in the "closed interface" but that the overall architecture of the domain-swapped oligomer depends on the interactions in the second type of interface, the "open interface." The RNase A dimer crystals take up the dye Congo Red, but the structure of a Congo Red-stained crystal reveals no bound dye molecule. Dimer formation is inhibited by excess amounts of the swapped helical domain. The possible implications for amyloid formation are discussed.

Mechanism of ribonuclease inhibition by ribonuclease inhibitor protein based on the crystal structure of its complex with ribonuclease A

We describe the mechanism of ribonuclease inhibition by ribonuclease inhibitor, a protein built of leucine-rich repeats, based on the crystal structure of the complex between the inhibitor and ribonuclease A. The structure was determined by molecular replacement and refined to an Rcryst of 19.4% at 2.5 A resolution. Ribonuclease A binds to the concave region of the inhibitor protein comprising its parallel beta-sheet and loops. The inhibitor covers the ribonuclease active site and directly contacts several active-site residues. The inhibitor only partially mimics the RNase-nucleotide interaction and does not utilize the p1 phosphate-binding pocket of ribonuclease A, where a sulfate ion remains bound. The 2550 A2 of accessible surface area buried upon complex formation may be one of the major contributors to the extremely tight association (Ki = 5.9 x 10(-14) M). The interaction is predominantly electrostatic; there is a high chemical complementarity with 18 putative hydrogen bonds and salt links, but the shape complementarity is lower than in most other protein-protein complexes. Ribonuclease inhibitor changes its conformation upon complex formation; the conformational change is unusual in that it is a plastic reorganization of the entire structure without any obvious hinge and reflects the conformational flexibility of the structure of the inhibitor. There is a good agreement between the crystal structure and other biochemical studies of the interaction. The structure suggests that the conformational flexibility of RI and an unusually large contact area that compensates for a lower degree of complementarity may be the principal reasons for the ability of RI to potently inhibit diverse ribonucleases. However, the inhibition is lost with amphibian ribonucleases that have substituted most residues corresponding to inhibitor-binding residues in RNase A, and with bovine seminal ribonuclease that prevents inhibitor binding by forming a dimer.

Bovine pancreatic ribonuclease A as a model of an enzyme with multiple substrate binding sites

Bovine pancreatic ribonuclease A is an enzyme that catalyses the depolymerization of RNA. This process involves the interaction of the enzyme with the polymeric substrate in the active site and its correct alignment on the surface of the enzyme through multiple binding subsites that essentially recognize the negatively charged phosphate groups of the substrate. The enzyme shows a strong specificity for pyrimidine bases at the 3'-position of the phosphodiester bond that is cleaved and a preference for purine bases at the 5'-position and, probably, for guanine at the next position. On the other hand, the enzyme shows a clear preference for polynucleotide substrates over oligonucleotides. In this review the contributions to the catalytic mechanism of some amino-acid residues that are located at non catalytic binding subsites are analysed.

A structural basis of the interactions between leucine-rich repeats and protein ligands

The leucine-rich repeat is a recently characterized structural motif used in molecular recognition processes as diverse as signal transduction, cell adhesion, cell development, DNA repair and RNA processing. We present here the crystal structure at 2.5 A resolution of the complex between ribonuclease A and ribonuclease inhibitor, a protein built entirely of leucine-rich repeats. The unusual non-globular structure of ribonuclease inhibitor, its solvent-exposed parallel beta-sheet and the conformational flexibility of the structure are used in the interaction; they appear to be the principal reasons for the effectiveness of leucine-rich repeats as protein-binding motifs. The structure can serve as a model for the interactions of other proteins containing leucine-rich repeats with their ligands.