Functional and structural roles of constituent amino acid residues of bovine pancreatic ribonuclease A

Structural basis for the toxic activity of MafB2 from maf genomic island 2 (MGI-2) in N. meningitidis B16B6

The Maf polymorphic toxin system is involved in conflict between strains found in pathogenic Neisseria species such as Neisseria meningitidis and Neisseria gonorrhoeae. The genes encoding the Maf polymorphic toxin system are found in specific genomic islands called maf genomic islands (MGIs). In the MGIs, the MafB and MafI encode toxin and immunity proteins, respectively. Although the C-terminal region of MafB (MafB-CT) is specific for toxic activity, the underlying enzymatic activity that renders MafB-CT toxic is unknown in many MafB proteins due to lack of homology with domain of known function. Here we present the crystal structure of the MafB2-CT_MGI-2B16B6/MafI2_MGI-2B16B6 complex from N. meningitidis B16B6. MafB2-CT_MGI-2B16B6 displays an RNase A fold similar to mouse RNase 1, although the sequence identity is only ~ 14.0%. MafB2-CT_MGI-2B16B6 forms a 1:1 complex with MafI2_MGI-2B16B6 with a Kd value of ~ 40 nM. The complementary charge interaction of MafI2_MGI-2B16B6 with the substrate binding surface of MafB2-CT_MGI-2B16B6 suggests that MafI2_MGI-2B16B6 inhibits MafB2-CT_MGI-2B16B6 by blocking access of RNA to the catalytic site. An in vitro enzymatic assay showed that MafB2-CT_MGI-2B16B6 has ribonuclease activity. Mutagenesis and cell toxicity assays demonstrated that His335, His402 and His409 are important for the toxic activity of MafB2-CT_MGI-2B16B6, suggesting that these residues are critical for its ribonuclease activity. These data provide structural and biochemical evidence that the origin of the toxic activity of MafB2_MGI-2B16B6 is the enzymatic activity degrading ribonucleotides.

Nucleotide substrate binding characterization in human pancreatic-type ribonucleases

Human genome contains a group of more than a dozen similar genes with diverse biological functions including antiviral, antibacterial and angiogenesis activities. The characterized gene products of this group show significant sequence similarity and a common structural fold associated with binding and cleavage of ribonucleic acid (RNA) substrates. Therefore, these proteins have been categorized as members of human pancreatic-type ribonucleases (hRNases). hRNases differ in cell/tissue localization and display distinct substrate binding preferences and a wide range of ribonucleolytic catalytic efficiencies. Limited information is available about structural and dynamical properties that influence this diversity among these homologous RNases. Here, we use computer simulations to characterize substrate interactions, electrostatics and dynamical properties of hRNases 1-7 associated with binding to two nucleotide substrates (ACAC and AUAU). Results indicate that even with complete conservation of active-site catalytic triad associated with ribonucleolytic activity, these enzymes show significant differences in substrate interactions. Detailed characterization suggests that in addition to binding site electrostatic and van der Waals interactions, dynamics of distal regions may also play a role in binding. Another key insight is that a small difference in temperature of 300 K (used in experimental studies) and 310 K (physiological temperature) shows significant changes in enzyme-substrate interactions.

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.

The CDI toxin of Yersinia kristensenii is a novel bacterial member of the RNase A superfamily

Contact-dependent growth inhibition (CDI) is an important mechanism of inter-bacterial competition found in many Gram-negative pathogens. CDI⁺ cells express cell-surface CdiA proteins that bind neighboring bacteria and deliver C-terminal toxin domains (CdiA-CT) to inhibit target-cell growth. CDI⁺ bacteria also produce CdiI immunity proteins, which specifically neutralize cognate CdiA-CT toxins to prevent self-inhibition. Here, we present the crystal structure of the CdiA-CT/CdiI^Ykris complex from Yersinia kristensenii ATCC 33638. CdiA-CT^Ykris adopts the same fold as angiogenin and other RNase A paralogs, but the toxin does not share sequence similarity with these nucleases and lacks the characteristic disulfide bonds of the superfamily. Consistent with the structural homology, CdiA-CT^Ykris has potent RNase activity in vitro and in vivo. Structure-guided mutagenesis reveals that His175, Arg186, Thr276 and Tyr278 contribute to CdiA-CT^Ykris activity, suggesting that these residues participate in substrate binding and/or catalysis. CdiI^Ykris binds directly over the putative active site and likely neutralizes toxicity by blocking access to RNA substrates. Significantly, CdiA-CT^Ykris is the first non-vertebrate protein found to possess the RNase A superfamily fold, and homologs of this toxin are associated with secretion systems in many Gram-negative and Gram-positive bacteria. These observations suggest that RNase A-like toxins are commonly deployed in inter-bacterial competition.

Conserved amino acid networks modulate discrete functional properties in an enzyme superfamily

In this work, we applied the sequence-based statistical coupling analysis approach to characterize conserved amino acid networks important for biochemical function in the pancreatic-type ribonuclease (ptRNase) superfamily. This superfamily-wide analysis indicates a decomposition of the RNase tertiary structure into spatially distributed yet physically connected networks of co-evolving amino acids, termed sectors. Comparison of this statistics-based description with new NMR experiments data shows that discrete amino acid networks, termed sectors, control the tuning of distinct functional properties in different enzyme homologs. Further, experimental characterization of evolutionarily distant sequences reveals that sequence variation at sector positions can distinguish homologs with a conserved dynamic pattern and optimal catalytic activity from those with altered dynamics and diminished catalytic activities. Taken together, these results provide important insights into the mechanistic design of the ptRNase superfamily, and presents a structural basis for evolutionary tuning of function in functionally diverse enzyme homologs.

The first crystal structure of human RNase 6 reveals a novel substrate-binding and cleavage site arrangement

We describe the first human RNase 6 crystal structure in complex with sulfate anions. Kinetic analysis, site-directed mutagenesis and molecular dynamics simulations identified novel substrate recognition and cleavage sites.

Human RNase 6 is a cationic secreted protein that belongs to the RNase A superfamily. Its expression is induced in neutrophils and monocytes upon bacterial infection, suggesting a role in host defence. We present here the crystal structure of RNase 6 obtained at 1.72 Å (1 Å=0.1 nm) resolution, which is the first report for the protein 3D structure and thereby setting the basis for functional studies. The structure shows an overall kidney-shaped globular fold shared with the other known family members. Three sulfate anions bound to RNase 6 were found, interacting with residues at the main active site (His¹⁵, His¹²² and Gln¹⁴) and cationic surface-exposed residues (His³⁶, His³⁹, Arg⁶⁶ and His⁶⁷). Kinetic characterization, together with prediction of protein–nucleotide complexes by molecular dynamics, was applied to analyse the RNase 6 substrate nitrogenous base and phosphate selectivity. Our results reveal that, although RNase 6 is a moderate catalyst in comparison with the pancreatic RNase type, its structure includes lineage-specific features that facilitate its activity towards polymeric nucleotide substrates. In particular, enzyme interactions at the substrate 5′ end can provide an endonuclease-type cleavage pattern. Interestingly, the RNase 6 crystal structure revealed a novel secondary active site conformed by the His³⁶–His³⁹ dyad that facilitates the polynucleotide substrate catalysis.

Eosinophil-associated ribonuclease 11 is a macrophage chemoattractant

RNase A is the prototype of an extensive family of divergent proteins whose members share a unique disulfide-bonded tertiary structure, conserved catalytic motifs, and the ability to hydrolyze polymeric RNA. Several members of this family maintain independent roles as ribonucleases and modulators of innate immunity. Here we characterize mouse eosinophil-associated RNase (Ear) 11, a divergent member of the eosinophil ribonuclease cluster, and the only known RNase A ribonuclease expressed specifically in response to Th2 cytokine stimulation. Mouse Ear 11 is differentially expressed in somatic tissues at baseline (brain ≪ liver < lung < spleen); systemic stimulation with IL-33 results in 10-5000-fold increased expression in lung and spleen, respectively. Ear 11 is also expressed in response to protective priming of the respiratory mucosa with Lactobacillus plantarum; transcripts are detected both locally in lung as well as systemically in bone marrow and spleen. Mouse Ear 11 is enzymatically active, although substantially less so than mEar 1 and mEar 2; the relative catalytic efficiency (kcat/Km) of mEar 11 is diminished ∼1000-1500-fold. However, in contrast to RNase 2/EDN and mEar 2, which have been characterized as selective chemoattractants for CD11c(+) dendritic cells, mEar 11 has prominent chemoattractant activity for F4/80(+)CD11c(-) tissue macrophages. Chemoattractant activity is not dependent on full enzymatic activity, and requires no interaction with the pattern recognition receptor, Toll-like receptor 2 (TLR2). Taken together, this work characterizes a divergent RNase A ribonuclease with a unique expression pattern and function, and highlights the versatility of this family in promoting innate immunity.

Adsorption-induced changes in ribonuclease A structure and enzymatic activity on solid surfaces

Ribonuclease A (RNase A) is a small globular enzyme that lyses RNA. The remarkable solution stability of its structure and enzymatic activity has led to its investigation to develop a new class of drugs for cancer chemotherapeutics. However, the successful clinical application of RNase A has been reported to be limited by insufficient stability and loss of enzymatic activity when it was coupled with a biomaterial carrier for drug delivery. The objective of this study was to characterize the structural stability and enzymatic activity of RNase A when it was adsorbed on different surface chemistries (represented by fused silica glass, high-density polyethylene, and poly(methyl-methacrylate)). Changes in protein structure were measured by circular dichroism, amino acid labeling with mass spectrometry, and in vitro assays of its enzymatic activity. Our results indicated that the process of adsorption caused RNase A to undergo a substantial degree of unfolding with significant differences in its adsorbed structure on each material surface. Adsorption caused RNase A to lose about 60% of its native-state enzymatic activity independent of the material on which it was adsorbed. These results indicate that the native-state structure of RNase A is greatly altered when it is adsorbed on a wide range of surface chemistries, especially at the catalytic site. Therefore, drug delivery systems must focus on retaining the native structure of RNase A in order to maintain a high level of enzymatic activity for applications such as antitumor chemotherapy.

Structural and functional importance of local and global conformational fluctuations in the RNase A superfamily

Understanding the relationship between protein structure and flexibility is of utmost importance for deciphering the tremendous rates of reactions catalyzed by enzyme biocatalysts. It has been postulated that protein homologs have evolved similar dynamic fluctuations to promote catalytic function, a property that would presumably be encoded in their structural fold. Using one of the best-characterized enzyme systems of the past century, we explore this hypothesis by comparing the numerous and diverse flexibility reports available for a number of structural and functional homologs of the pancreatic-like RNase A superfamily. Using examples from the literature and from our own work, we cover recent and historical evidence pertaining to the highly dynamic nature of this important structural fold, as well as the presumed importance of local and global concerted motions on the ribonucleolytic function. This minireview does not pretend to cover the overwhelming RNase A literature in a comprehensive manner; rather, efforts have been made to focus on the characterization of multiple timescale motions observed in the free and/or ligand-bound structural homologs as they proceed along the reaction coordinates. Although each characterized enzyme of this architectural fold shows unique motional features on a local scale, accumulating evidence from X-ray crystallography, NMR spectroscopy and molecular dynamics simulations suggests that global dynamic fluctuations, such as the functionally relevant hinge-bending motion observed in the prototypical RNase A, are shared between homologs of the pancreatic-like RNase superfamily. These observations support the hypothesis that analogous dynamic residue clusters are evolutionarily conserved among structural and functional homologs catalyzing similar enzymatic reactions.

Organosoluble enzyme conjugates with poly(2-oxazoline)s via pyromellitic acid dianhydride

The use of enzymes in organic solvents offers a great opportunity for the synthesis of complex organic compounds and is therefore in focus of current research. In this work we describe the synthesis of poly(2-methyl-1,3-oxazoline) (PMOx) and poly(2-ethyl-1,3-oxazoline) (PEtOx) enzyme conjugates with hen-egg white lysozyme, RNase A and α-chymotrypsin using a new coupling technique. The POXylation was carried out reacting pyromellitic acid dianhydride subsequently with ethylenediamine terminated POx and then with the NH₂-groups of the respective enzymes. Upon conjugation with the polymers, RNase A and lysozyme became fully soluble in DMF (1.4 mg/ml). These are the first examples of fully POXylated proteins, which become organosoluble. The synthesized enzyme conjugates were characterized by SDS-PAGE, isoelectric focusing, dynamic light scattering and size exclusion chromatography, which all indicated the full POXylation of the enzymes. The modified enzymes even partly retained their activity in water. With α-chymotrypsin as example we could demonstrate that the molecular weight of the attached polymer significantly influences the activity.

X-ray crystallographic studies of RNase A variants engineered at the most destabilizing positions of the main hydrophobic core: further insight into protein stability

To investigate the structural origin of decreased pressure and temperature stability, the crystal structure of bovine pancreatic ribonuclease A variants V47A, V54A, V57A, I81A, I106A, and V108A was solved at 1.4-2.0 A resolution and compared with the structure of wild-type protein. The introduced mutations had only minor influence on the global structure of ribonuclease A. The structural changes had individual character that depends on the localization of mutated residue, however, they seemed to expand from mutation site to the rest of the structure. Several different parameters have been evaluated to find correlation with decrease of free energy of unfolding DeltaDeltaG(T), and the most significant correlation was found for main cavity volume change. Analysis of the difference distance matrices revealed that the ribonuclease A molecule is organized into five relatively rigid subdomains with individual response to mutation. This behavior consistent with results of unfolding experiments is an intrinsic feature of ribonuclease A that might be surviving remnants of folding intermediates and reflects the dynamic nature of the molecule.

Elucidating the exact role of engineered CRABPII residues for the formation of a retinal protonated Schiff base

Cellular Retinoic Acid Binding Protein II (CRABPII) has been reengineered to specifically bind and react with all-trans-retinal to form a protonated Schiff base. Each step of this process has been dissected and four residues (Lys132, Tyr134, Arg111, and Glu121) within the CRABPII binding site have been identified as crucial for imine formation and/or protonation. The precise role of each residue has been examined through site directed mutagenesis and crystallographic studies. The crystal structure of the R132K:L121E-CRABPII (PDB-3I17) double mutant suggests a direct interaction between engineered Glu121 and the native Arg111, which is critical for both Schiff base formation and protonation.

Refolding of ribonuclease A monitored by real-time photo-CIDNP NMR spectroscopy

Photo-CIDNP NMR spectroscopy is a powerful method for investigating the solvent accessibility of histidine, tyrosine and tryptophan residues in a protein. When coupled to real-time NMR, this technique allows changes in the environments of these residues to be used as a probe of protein folding. In this paper we describe experiments performed to monitor the refolding of ribonuclease A following dilution from a high concentration of chemical denaturant. These experiments provide a good example of the utility of this technique which provides information that is difficult to obtain by other biophysical methods. Real-time photo-CIDNP measurements yield residue-specific kinetic data pertaining to the folding reaction, interpreted in terms of current knowledge of the folding of bovine pancreatic ribonuclease A.

Direct observation of conformational folding coupled with disulphide rearrangement by using a water-soluble selenoxide reagent--a case of oxidative regeneration of ribonuclease A under weakly basic conditions

Oxidative regeneration pathways of bovine pancreatic ribonuclease A (RNase A), which has four SS linkages, were studied at 25 degrees C and pH 8.0 by using trans-3,4-dihydroxy-1-selenolane oxide (DHS(ox)), a new selenoxide reagent with strong oxidation power. The short-term folding study using a quench-flow instrument ( approximately 1 min) revealed that early intermediates (1S, 2S, 3S and 4S) are formed stochastically and irreversibly from the reduced protein (R) and do not have any stable structures. In the long-term folding study ( approximately 300 min), on the other hand, slow generation of the key intermediates (des[65-72] and des[40-95]) through SS rearrangement from the 3S intermediate ensemble was observed, followed by slight formation of native RNase A (N). The parallel UV and CD measurements demonstrated that formation of the key intermediates is accompanied with the formation of the native-like structures. Thus, DHS(ox) allowed facile identification of the conformational folding steps coupled with SS rearrangement on the major oxidative folding pathways.

Structural and functional implications of positive selection at the primate angiogenin gene

BACKGROUND

Angiogenesis, the formation of new blood vessels, is a primordial process in development and its dysregulation has a central role in the pathogenesis of many diseases. Angiogenin (ANG), a peculiar member of the RNase A superfamily, is a potent inducer of angiogenesis involved in many different types of cancer, amyotrophic lateral sclerosis and also with a possible role in the innate immune defense. The evolutionary path of this family has been a highly dynamic one, where positive selection has played a strong role. In this work we used a combined gene and protein level approach to determine the main sites under diversifying selection on the primate ANG gene and analyze its structural and functional implications.

RESULTS

We obtained evidence for positive selection in the primate ANG gene. Site specific analysis pointed out 15 sites under positive selection, most of which also exhibited drastic changes in amino acid properties. The mapping of these sites in the ANG 3D-structure described five clusters, four of which were located in functional regions: two in the active site region, one in the nucleolar location signal and one in the cell-binding site. Eight of the 15 sites under selection in the primate ANG gene were highly or moderately conserved in the RNase A family, suggesting a directed event and not a simple consequence of local structural or functional permissiveness. Moreover, 11 sites were exposed to the surface of the protein indicating that they may influence the interactions performed by ANG.

CONCLUSION

Using a maximum likelihood gene level analysis we identified 15 sites under positive selection in the primate ANG genes, that were further corroborated through a protein level analysis of radical changes in amino acid properties. These sites mapped onto the main functional regions of the ANG protein. The fact that evidence for positive selection is present in all ANG regions required for angiogenesis may be a good indication that angiogenesis is the process under selection. However, other possibilities to be considered arise from the possible involvement of ANG in innate immunity and the potential influence or co-evolution with its interacting proteins and ligands.

Genetic selection for critical residues in ribonucleases

Homologous mammalian proteins were subjected to an exhaustive search for residues that are critical to their structure/function. Error-prone polymerase chain reactions were used to generate random mutations in the genes of bovine pancreatic ribonuclease (RNase A) and human angiogenin, and a genetic selection based on the intrinsic cytotoxicity of ribonucleolytic activity was used to isolate inactive variants. Twenty-three of the 124 residues in RNase A were found to be intolerant to substitution with at least one particular amino acid. Twenty-nine of the 123 residues in angiogenin were likewise intolerant. In both RNase A and angiogenin, only six residues appeared to be wholly intolerant to substitution: two histidine residues involved in general acid/base catalysis and four cysteine residues that form two disulfide bonds. With few exceptions, the remaining critical residues were buried in the hydrophobic core of the proteins. Most of these residues were found to tolerate only conservative substitutions. The importance of a particular residue as revealed by this genetic selection correlated with its sequence conservation, though several non-conserved residues were found to be critical for protein structure/function. Despite voluminous research on RNase A, the importance of many residues identified herein was unknown, and those can now serve as targets for future work. Moreover, a comparison of the critical residues in RNase A and human angiogenin, which share only 35% amino acid sequence identity, provides a unique perspective on the molecular evolution of the RNase A superfamily, as well as an impetus for applying this methodology to other ribonucleases.

Pressure as a tool to study protein-unfolding/refolding processes: The case of ribonuclease A

This paper gives an overview of the application of high-pressure to study the folding/unfolding processes of proteins using Ribonuclease A as a model protein. A particular focus is the study of pressure-equilibrium unfolding and folding kinetics using variants and the information obtained by comparing these with the wild-type enzyme.

Probing the unfolding region of ribonuclease A by site-directed mutagenesis

Ribonuclease A contains two exposed loop regions, around Ala20 and Asn34. Only the loop around Ala20 is sufficiently flexible even under native conditions to allow cleavage by nonspecific proteases. In contrast, the loop around Asn34 (together with the adjacent beta-sheet around Thr45) is the first region of the ribonuclease A molecule that becomes susceptible to thermolysin and trypsin under unfolding conditions. This second region therefore has been suggested to be involved in early steps of unfolding and was designated as the unfolding region of the ribonuclease A molecule. Consequently, modifications in this region should have a great impact on the unfolding and, thus, on the thermodynamic stability. Also, if the Ala20 loop contributes to the stability of the ribonuclease A molecule, rigidification of this flexible region should stabilize the entire protein molecule. We substituted several residues in both regions without any dramatic effects on the native conformation and catalytic activity. As a result of their remarkably differing stability, the variants fell into two groups carrying the mutations: (a) A20P, S21P, A20P/S21P, S21L, or N34D; (b) L35S, L35A, F46Y, K31A/R33S, L35S/F46Y, L35A/F46Y, or K31A/R33S/F46Y. The first group showed a thermodynamic and kinetic stability similar to wild-type ribonuclease A, whereas both stabilities of the variants in the second group were greatly decreased, suggesting that the decrease in DeltaG can be mainly attributed to an increased unfolding rate. Although rigidification of the Ala20 loop by introduction of proline did not result in stabilization, disturbance of the network of hydrogen bonds and hydrophobic interactions that interlock the proposed unfolding region dramatically destabilized the ribonuclease A molecule.

Temperature-Induced Unfolding of Ribonuclease A Embedded in Spherical Polyelectrolyte Brushes

We use Fourier Transform infrared spectroscopy (FT-IR) spectroscopy to study the thermal unfolding and refolding behavior of ribonuclease (RNase A) adsorbed to spherical polyelectrolyte brushes (SPB). The SPB consist of a solid poly(styrene) core of ca. 100 nm diameter onto which long chains of poly(styrene sulfonic acid), PSS have been densely attached. The particles bearing the adsorbed protein are dispersed in aqueous buffer solution at a pH close to the isoelectric point (9.6) of the protein. The secondary structure of the protein was analyzed by FT-IR spectroscopy and compared to the structure of the native protein before adsorption. The unfolding of the free RNase A in solution was found to be fully reversible with an unfolding temperature of 65 degrees C, in accordance to previous studies. However, after adsorption to the SPB, the unfolding temperature of the protein molecule is lowered by 10 degrees C and the Van't Hoff enthalpy of the unfolding process is significantly reduced. Moreover the unfolding of the adsorbed protein is irreversible. The phenomenon may be explained by an increase in binding sites due to unfolding of the globular structure. Protein adsorption to a spherical polyelectrolyte brush.

Individual tyrosine side-chain contributions to circular dichroism of ribonuclease

Experimental and theoretical studies using site-directed mutants of ribonuclease A (RNase A) offer more extensive information on the tyrosine side-chain contributions to the circular dichroism (CD) of the enzyme. Bovine pancreatic RNase A has three exposed tyrosine residues (Tyr73, Tyr76, and Tyr115) and three buried tyrosine residues (Tyr25, Tyr92 and Tyr97). The difference CD spectra between the wild type and the mutants at pH 7.0 (Deltaepsilon(277,wt) - Deltaepsilon(277,mut)) show bands with more negative DeltaDeltaepsilon(277) values for Y73F and Y115F than those for Y25F and Y92F and bands with positive DeltaDeltaepsilon(277) values for Y76F and Y97F. The theoretical calculations are in good semiquantitative agreement for all the mutants. The pH difference spectrum (pH 11.3-7.0) for the wild type shows a negative band at 295 nm and an enhanced positive band at 245 nm. The three mutants at buried tyrosine sites and one mutant at an exposed tyrosine site (Y76F) exhibit pH-difference spectra that are similar to that of the wild type. In contrast, two mutants at exposed tyrosine sites (Y73F and Y115F) exhibit diminished 295-nm negative bands and, instead of positive bands at 245 nm, negative bands are observed. Our results indicate that Tyr73 and Tyr115, two of the exposed tyrosine residues, are the largest contributors to the 277- and 245-nm CD bands of RNaseA, but the buried tyrosine residues and the one remaining exposed residue also contribute to these bands. Disulfide contributions to the 277- and 240-nm bands and the peptide contribution to the 240-nm band are confirmed theoretically.