Catalytic acitivity of Ntau-carboxymethylhistidine-12 ribonuclease

The structural and functional studies of His119 and His12 in RNase A via chemical modification

The histidyl residues of bovine pancreatic ribonuclease A (RNase A) play a crucial role in enzymatic activity. Diethylpyrocarbonate (DEPC) is a potent inhibitor of RNase A, and its precise sites of action on the imidazole rings of the four histidyl residues of RNase A are not clearly defined. We have used a multidisciplinary approach including enzyme assay, calculation of accessible surface area (ASA), isoelectric pH gradient technique, fluorescence investigations, circular dichroism spectroscopy, differential scanning calorimetry, and 1H NMR analysis to study the sites of DEPC interaction with the imidazole rings of the four histidyl residues. Our results demonstrate that among the histidyl residues of RNase A, His48 is not accessible to react with DEPC. However, the sequential carbethoxylation of the imidazole rings of His119, His105, and His12 occurs on the nitrogen atoms of Ndelta, Nepsilon, and Nepsilon, respectively. Carbethoxylation of His119 was followed by conversion of the A conformation to the B conformation in the active site. However, the carbethoxylation of His12 was accompanied by a second spatial rotation of the corresponding imidazole ring in the active site to adopt a new conformation. These conformation changes are accompanied by subsequent decrements in the thermal stability of the protein. Therefore, these findings reinforce the important structural roles of the spatial positions for His119 and His12 in the active site of RNase A.

Computer modelling studies on the mechanism of action of ribonuclease T1

The mechanism of action of ribonuclease (RNase) T1 is still a matter of considerable debate as the results of x-ray, 2-D nmr and site-directed mutagenesis studies disagree regarding the role of the catalytically important residues. Hence computer modelling studies were carried out by energy minimisation of the complexes of RNase T1 and some of its mutants (His40Ala, His40Lys, and Glu58Ala) with the substrate guanyl cytosine (GpC), and of native RNase T1 with the reaction intermediate guanosine 2',3'-cyclic phosphate (G greater than p). The puckering of the guanosine ribose moiety in the minimum energy conformer of the RNase T1-GpC (substrate) complex was found to be O4'-endo and not C3'-endo as in the RNase T1-3'-guanylic acid (inhibitor/product) complex. A possible scheme for the mechanism of action of RNase T1 has been proposed on the basis of the arrangement of the catalytically important amino acid residues His40, Glu58, Arg77, and His92 around the guanosine ribose and the phosphate moiety in the RNase T1-GpC and RNase T1-G greater than p complexes. In this scheme, Glu58 serves as the general base group and His92 as the general acid group in the transphosphorylation step. His40 may be essential for stabilising the negatively charged phosphate moiety in the enzyme-transition state complex.

Dissociation and reconstitution of bovine seminal RNAase: construction of a hyperactive hybrid dimer

The quaternary structure of bovine seminal ribonuclease, the only dimeric protein in the superfamily of ribonucleases, is maintained both by noncovalent forces and by two intersubunit disulfides. The available monomeric derivatives of the enzyme may not be reassembled into dimers. They are catalytically active, but do not retain certain properties of the dimeric enzyme, such as: (i) the ability to respond cooperatively to increasing substrate concentrations in the rate-limiting reaction step; and (ii) the antitumor and immunosuppressive actions. In this report we described the preparation of stable monomers of seminal ribonuclease which can be reassociated into covalent dimers indistinguishable from the native protein. With this procedure a hybrid dimer was constructed, made up of a native subunit associated to a subunit catalytically inactivated by selective alkylation of the active site His-119. This dimer was found to have enzymic properties typical of monomeric ribonucleases, such as a hyperbolic saturation curve in the hydrolytic rate-limiting step of the reaction. However, the hybrid dimer was one order-of-magnitude more active than the dimeric enzyme.

The pentaammineruthenium(III)-histidine complex in ribonuclease A as an optical probe of conformational change

The formation of stable transition metal complexes between pentaammineruthenium(III) and the imidazole moiety of histidine residues in ribonuclease A is described. Studies of the kinetics of incorporation suggest that three of the four histidines are reactive and ion-exchange chromatography affords the isolation of three derivatives containing a single ruthenium-histidine complex. Equilibrium studies of the thermal- and urea-induced unfolding of one of these derivatives show that the ligand-to-metal change-transfer absorptions above 300 nm can be used to follow the conformational changes in the vicinity of the complex. For the urea-induced unfolding of this derivative, the coincidence of the transition curves obtained from the change-transfer absorptions and from difference ultraviolet spectroscopy at 286 nm, which measures the exposure of buried tyrosine residues to solvent, suggest that only two conformations, native and unfolded, are present at equilibrium in significant concentrations.

Catalytic activity of Ntau-carboxymethylhistidine-12 ribonuclease: pH dependence

The pH-dependence of RNAase A and of Ntau-carboxymethylhistidine-12-RNAase (ribonucleate 3'-pyrimidino-oligonucleotidohydrolase) catalysis was studied. Apparent acid dissociation constants were obtained by least squares analysis of the kinetics data. These dissociation constants were compared with pKa values of model imidazole compounds, and with pKa values of histidine residues 12 and 119 on the protein. The shapes of the kcat versus pH profiles for RNAase A and its carboxymethyl derivative are very similar, from which it is concluded that the mechanism of catalysis is closely similar in the two proteins. Apparent pKa values obtained from the kinetic data are higher for the carboxymethylated protein than for RNAase A, as are the pKa values of residues 12 and 119. The similar shifts are consistent with the conclusions that both these residues are functionally significant in native and modified enzyme, and that an unblocked tau-nitrogen on histidine-12 is not essential for activity. From the enzyme's catalytic dependence on pH, and the NMR determined pKa values we propose that histidine 12 and 119 function catalytically in their basic and acidic forms respectively.