Localization and analysis of nonpolar regions in onconase.

A detailed analysis of the composition and properties of hydrophobic nuclei and microclusters has been carried out for onconase. Two main hydrophobic nuclei in the onconase structure were detected. Their composition and shape were found to be very similar to those of RNase A, in accordance with the predictions made. The nuclei in onconase are more compact, the side-chain atoms of residues included in the nuclei in onconase form more contacts with the environment than in RNase A. The hydrophobic nuclei should be considered as individual structural units along with elements of the secondary structure. Differences in composition and conformation of exposed loops between onconase and RNase A were found. The additional hydrophobic clusters attached to the nuclei in onconase might be involved in the fixation of an appropriate conformation of site(s) for manifestation of the biological activity of onconase. A comparison of amphibian representatives of the RNase A superfamily was also made. The results obtained suggest that the availability of nonpolar residues in established key positions of amino acid sequences determines the characteristic fold of homologous proteins and the structure of the active site cleft.

Crystal structures and solution studies of oxime adducts of mitochondrial aspartate aminotransferase.

The interaction of mitochondrial aspartate aminotransferase with hydroxylamine and five derivatives (in which the hydroxyl hydrogen is replaced by the side chain of naturally occurring amino acids) was investigated by X-ray diffraction as well as by kinetic and spectral measurements with the enzyme in solution. The inhibitors react with pyridoxal 5'-phosphate in the enzyme active site, both in solution and in the crystalline state, in a reversible single-step reaction forming spectrally distinct oxime adducts. Dissociation constants determined in solution range from 10(-8) M to 10(-6) M depending on the nature of the side-chain group. The crystal structures of the adducts of mitochondrial aspartate aminotransferase with the monocarboxylic analogue of L-aspartate in the open and closed enzyme conformation were determined at 0.23-nm and 0.25-nm resolution, respectively. This inhibitor binds to both the open and closed crystal forms of the enzyme without disturbing the crystalline order. Small differences in the conformation of the cofactor pyridoxal phosphate were detected between the omega-carboxylate of the inhibitor and Arg292 of the neighbouring subunit is mainly responsible for the attainment of near-coplanarity of the aldimine bond with the pyridine ring in the oxime adducts. Studies with a fluorescent probe aimed to detect shifts in the open/closed conformational equilibrium of the enzyme in oxime complexes showed that the hydroxylamine-derived inhibitors, even those containing a carboxylate group, do not induce the 'domain closure' in solution. This is probably due to the absence of the alpha-carboxylate group in the monocarboxylic hydroxylamine-derived inhibitors, emphasizing that both carboxylates of the substrates L-Asp and L-Glu are essential for stabilizing the closed form of aspartate aminotransferase.

Complexes of aspartate aminotransferase with hydroxylamine derivatives: spectral studies in solution and in the crystalline state.

Hydroxylamine and its derivatives of general formula H2NOR react with aldehydes and aldimines to produce oximes. If R corresponds to the side chain of a natural amino acid, such compounds can be thought of as analogs of the corresponding amino acids, lacking the alpha-carboxylate group. Oximes formed between such compounds and pyridoxal phosphate in the active site of aspartate amino-transferase mimic external aldimine intermediates that occur during catalysis by this enzyme. The properties of oxime derivatives of mitochondrial aspartate aminotransferase with hydroxylamine and 6 compounds H2NOR were studied by absorption spectroscopy and circular dichroism in solution and by linear dichroism in crystals. Stable oximes, absorbing at lambda max congruent to 380 nm and exhibiting a negative Cotton effect, were obtained with the carboxylate-containing compounds. The oximes formed with carboxylate-free compounds showed somewhat different properties and stability. With H-Tyr a stable complex absorbing at lambda max congruent to 370 nm rather than at 380 nm, was obtained, H-Ala and H-Phe produced unstable oximes with the initial absorption band at lambda max congruent to 380 nm that was gradually replaced by a band at lambda max congruent to 340 nm. The species absorbing at 340 nm were shown to be coenzyme-inhibitor complexes which were gradually released from the enzyme. A similar 330-340 nm absorption band was observed upon reaction of the free coenzyme with all hydroxylamine inhibitors at neutral pH-values. The results of the circular dichroism experiments in solution and the linear dichroism studies in microcrystals of mAspAT indicate that the coenzyme conformation in these inhibitor/enzyme complexes is similar to that occurring in an external aldimine analogue, the 2-MeAsp/mAspAT complex. Co-crystallizations of the enzyme with the H2NOR compounds were also carried out. Triclinic crystals were obtained in all cases, suggesting that the "closed" structure cannot be stabilized by a single carboxylate group.

Guanyl-specific ribonuclease from the fungus Penicillium chrysogenum strain 152 and its complex with guanosine 3'-phosphate studied by nuclear magnetic resonance.

The method of proton magnetic resonance was used to obtain information on the active site of the guanyl-specific ribonuclease from Penicillium chrysogenum, strain 152A. Four pH-dependent signals in the aromatic region of the proton NMR spectrum of the enzyme were assigned to the C-2 and C-4 protons of the two histidine residues. To determine the pK values and the environment of the histidine residues the pH dependence of their chemical shifts was studied and experimental curves thus obtained were analyzed taking into account the effect of other dissociating groups of the enzyme. The pK values of the histidine residues were found to be equal to 7.92 +/- 0.04 and 7.86 +/- 0.09. The results of the calculations indicate that each histidine residue should interact with an acidic group (carboxylic) of the protein (pK 4.33 and 3.48) and the distance between two histidine residues does not exceed 0.85 nm. The rate constants for the quasi-first order reaction of deuterium exchange of the histidine residues (11.2 s-1 and 3.7 x-1) suggest that both residues are accessible, though to a different degree to solvent. Formation of a complex between the enzyme and guanosine 3'-phosphate (Guo3'P) is accompanied by the shift of the histidine pK toward the alkaline region by 0.5. The existence of the complex is controlled by dissociation of a histidine residue with pK 8.7 in alkaline medium and by protonation of the N-7 of Guo3'P (pK 2.4) in acid medium. Nuclear Overhauser effect measurements were used to determine the glycosidic torsion angle for the Guo3'P in the complex and to estimate the distances between the histidine residues of the enzyme and ribose ring of Guo-3'P. The results obtained suggest that the nucleotide in the complex has an anti conformation and the least exposed histidine is spaced not more than 0.5 nm from the C-1' proton of the nucleotide ribose ring. A model for the enzyme-nucleotide complex is presented.

The nature of the circular-dichoric spectra of complexes between ribonuclease A and nucleotides.

The circular-dichroism and proton-magnetic-resonance spectra of complexes of ribonuclease A with dihydrouridine 3'-phosphate, 2'- and 3'-CMP, arabinosyl-3'-CMP, 1-(2-hydroxyethyl)cytosine 2'-phosphate and 1-(3-hydroxypropyl)cytosine 3'-phosphate were studied. Comparison of the results shows that non-additivity of the circular-dichroic spectrum of an enzyme-nucleotide complex may be due to: (a), alteration of the circular dichroic spectrum of the nucleotide under the influence of the asymmetric protein matrix (induced dichroism), and (b) a change in the nucleotide conformation. The contribution of each of the two factors was estimated to calculate the circular-dichoroic spectra of 2'-CMP and 3'-CMP in complex with ribonuclease A. 3'-CMP in this complex was characterized by negative circular dichroism in the long-wavelength absorption band of the nucleotide, whereas 2'-CMP was characterized by positive circular dichroism. Since both nucleotides in the complex are known to be in an anti conformation, it follows that even small changes in the conformation considerably modify the circular-dichroic spectrum of the nucleotide in complex with the enzyme.

Spectral properties of phosphopyridoxyl-Lys-7(-41)-ribonuclease A.

We have studied the spectral properties of RNAase A containing a phosphopyridoxyl residue at the epsilon-NH2 group of Lys-7 or Lys-14. The overall conformations of the native and modified enzymes were shown to be rather similar. All three proteins have similar circular dichroism spectra within the 220-300-nm region, and similar thermal transition temperatures. All the changes in the RNAase A molecule modified are located in close proximity to the alkylated lysine residue. The phosphopyridoxyl group of (P-Pxy)-epsilon-Lys-41-RNAase A is situated directly at the enzyme active site and is 25% butied in the protein globule. The P-pyridoxyl group of (P-Pxy)-epsilon-Lys-7-RNAase A was shown to be located in the vicinity of the active site and to be more exposed to the solvent. In the pyridoxyl phosphate absorption band, optical activity is induced in both proteins. Study of the pH dependence of the changes occurring in the circular dichroism and absorption spectra has shown that in the modified proteins, the pyridoxyl phosphate chromophore is rather sensitive to the ionic state of the surrounding medium and serves as a "reporter" group when the relationship between structure and function of the RNAase A active site is being investigated.