Chemical modification and nuclear magnetic resonance studies on human plasminogen kringle 4. Assignment of tyrosine and histidine resonances to specific residues in the sequence

Modification of kringle 4 with tetranitromethane leads to the selective nitration of tyrosine 40 but on prolonged incubation with reagent, reaction of tyrosine 49 is also observed. Nitration of tyrosines 40 and 49 had no influence on the lysine-Sepharose affinity of kringle 4, indicating that these residues are not important for the functional integrity of the ligand-binding site. Comparison of the NMR spectra of native kringle 4 with those of kringle 4 in which tyrosine 40 or tyrosines 40 and 49 are nitrated permitted the identification of the resonances of these residues. These NMR studies also showed that the chemical modifications caused little perturbation of the three-dimensional structure of the protein. Cross-linking of lysine 35 and tyrosine 40 with 1,3-difluoro-4,6-dinitrobenzene demonstrates that in the kringle-fold the reactive epsilon-amino and phenolic groups of these residues can approach each other to a distance of 0.5 nm. NMR spectra of this kringle 4 species also confirmed the assignment of the resonances to tyrosine 40. NMR spectra of a kringle 4 derivative in which the disulphide bridge between cysteines 1 and 79 has been broken by selective reduction and alkylation showed that the core structure of the kringle-fold and the lysine-binding site are unaltered by this modification. This observation is in agreement with earlier results which showed that the lysine-Sepharose affinity of kringle 4 is not affected by reduction and alkylation of this disulphide bridge. Comparison of the NMR spectra of native and disulphide-cleaved kringle 4 aided in the assignment of resonances to residues adjacent to the site of modification (tyrosine 2 and histidine 3) and permitted the tentative assignment of the resonances of tyrosines 9 and 73.     

1H-nuclear magnetic resonance studies of the neuropeptide head activator

The 1H-NMR spectrum of the neuropeptide head activator in aqueous solution has been completely assigned by two-dimensional NMR spectroscopy and selective deuteration. The apparent pseudo-first-order exchange rate, kex, of the backbone amide protons and the correspondent activation enthalpies, delta H not equal to, were determined. The exchange rates decrease and the activation enthalpies increase from the N-terminal to the C-terminal part of the peptide. The exchange rates vary from 21 to 0.3 s-1 at 274 K, the activation enthalpies from 60 to 75 kJ.mol-1. The pK values of the terminal carboxyl group and of the lysine amino group have been estimated as 3.3 and 10.3, respectively. The NMR results are in line with a dimeric structure in an antisymmetric arrangement of the subunits, forming an antiparallel beta-pleated sheet between C-terminal segments. The peptide bonds between pGlu-1, Pro-2 and Pro-3 are predominantly in trans-configuration, in fact no cis-isomers can be observed spectroscopically. The structure appears to be very stable; in the temperature and pH range studied, i.e., from 274 to 338 K and from pH 0.8 to pH 11.6, there are no spectroscopic indications for a global structural change.     

Chemical modification of the two histidine and single cysteine residues in the channel-forming domain of colicin E1

Summary The two histidine residues of COOH-terminal channel-forming peptides of colicin E1 were modified by addition of a carbethoxy group through pretreatment with diethylpyrocarbonate. The consequences of the modification were examined by the action of the altered product on both phospholipid vesicles and planar membranes. At pH 6, where activity is low, histidine modification resulted in a decrease of the single channel conductance from 20 pS to approximately 9 pS and a decrease in the selectivity for sodium relative to chloride, showing that histidine modification affected the permeability properties of the channel. At pH 4, where activity is high, the single channel conductance and ion selectivity were not significantly altered by histidine modification. The histidine modification assayed at pH 4 resulted in a threefold increase in the rate of Cl^– efflux from asolectin vesicles, and a similar increase in conductance assayed with planar membranes. This conductance increase was inferred to arise from an increase in the fraction of bound histidine-modified colicin molecules forming channels at pH 4, since the increase in activity was not due to (i) an increase in binding of the modified peptide, (ii) a change in ion selectivity, (iii) a change of single channel conductance, or (iv) a change in the pH dependence of binding. The sole cysteine in the colicin molecule was modified in 6m urea with 5,5-dithiobis(2-nitrobenzoic acid). The activities of the colicin and its COOH-terminal tryptic peptide were found to be unaffected by cysteine modification, arguing against a role of (-SH) groups in protein insertion and/or channel formation.     

Probing the role of threonine and serine residues of E. coli asparaginase II by site-specific mutagenesis.

Site-specific mutagenesis has been used to probe amino acid residues proposed to be critical in catalysis by Escherichia coli asparaginase II. Thr12 is conserved in all known asparaginases. The catalytic constant of a T12A mutant towards L-aspartic acid beta-hydroxamate was reduced to 0.04% of wild type activity, while its Km and stability against urea denaturation were unchanged. The mutant enzyme T12S exhibited almost normal activity but altered substrate specificity. Replacement of Thr119 with Ala led to a 90% decrease of activity without markedly affecting substrate binding. The mutant enzyme S122A showed normal catalytic function but impaired stability in urea solutions. These data indicate that the hydroxyl group of Thr12 is directly involved in catalysis, probably by favorably interacting with a transition state or intermediate. By contrast, Thr119 and Ser122, both putative target sites of the inactivator DONV, are functionally less important.     

1H nuclear magnetic resonance study of the histidine residues of insulin

Insulin has proved difficult to study by nuclear magnetic resonance spectroscopy because of its complex aggregation behaviour in solution and its insolubility between pH 4 and 7. Now for the first time it has been possible to assign the ¹H nuclear magnetic resonances of the H-2 histidine protons of residues B5 and B10 of bovine 2 Zn insulin and Zn-free insulin, and the B5 and A8 residues of hagfish insulin. As expected, the addition of Zn to Zn-free insulin causes virtually no change in the chemical shift or the rate of H-D exchange of the H-2 proton of histidine B5, which is not involved in Zn binding in the 2 Zn insulin hexamer. The rate of H-D exchange of the H-2 proton of histidine B10 is decreased markedly on Zn binding at this residue, but the chemical shift of the resonance remains virtually constant owing to the balancing of an upfield ring current shift of the ordered histidine residues by a downfield shift due to electron withdrawal from the ring nitrogen by the Zn binding.     

Chemical modification of histidine residues in rabbit liver glutathione reductase

The role of histidine residues of glutathione reductase from rabbit liver was investigated by chemical modification with both ethoxyformic anhydride and dansyl chloride. At least four histidine residues were concomitantly modified by ethoxyformic anhydride at pH 6; both the GSSG reductase and the transhydrogenase activities were inhibited to the same extent. Dansyl chloride inactivated the enzyme showing pH-independence in the range 7-9. About 2.6 moles dansyl were incorporated in the protein 80% inactivated at pH 8, whereas at pH 7 a lower amount of labelling was found. Nearly complete reactivation of the inactivated enzyme could be obtained by incubation with hydroxylamine, which released all the acid-labile bound dansyl. Of the two histidine residues modified, only the slower reacting residue seems essential for activity. The modification with dansyl chloride will allow the identification of the histidine residues modified, in the sequence of the protein.     

Chemical modification of essential histidine residues in aspartase with diethylpyrocarbonate

Aspartase purified from Escherichia coli W cells was inactivated by diethylpyrocarbonate following pseudo-first order kinetics. Upon treatment of the inactivated enzyme with NH2OH, the enzyme activity was completely restored. The difference absorption spectrum of the modified vs. native enzyme preparations exhibited a prominent peak around 240 nm. The pH-dependence of the inactivation rate suggested that an amino acid residue having a pK value of 6.6 was involved in the inactivation. These results indicate that the inactivation was due to the modification of histidine residues. L-Aspartate and fumarate, substrates for the enzyme, and the Cl- ion, an inhibitor, protected the enzyme against the inactivation. Inspection of the spectral change at 240 nm associated with the inactivation in the presence and absence of the Cl- ion revealed that the number of histidine residues essential for the enzyme activity was less than two. Partial inactivation did not result in an appreciable change in the substrate saturation profiles. These results suggest that one or two histidine residues are located at the active site of aspartase and participate in an essential step in the catalytic reaction.     

Site-specific mutagenesis of Escherichia coli asparaginase II. None of the three histidine residues is required for catalysis.

Site-specific mutagenesis was used to replace the three histidine residues of Escherichia coli asparaginase II (EcA2) with other amino acids. The following enzyme variants were studied: [H87A]EcA2, [H87L]EcA2, [H87K]EcA2, [H183L]EcA2 and [H197L]EcA2. None of the mutations substantially affected the Km for L-aspartic acid beta-hydroxamate or impaired aspartate binding. The relative activities towards L-Asn, L-Gln, and l-aspartic acid beta-hydroxamate were reduced to the same extent, with residual activities exceeding 10% of the wild-type values. These data do not support a number of previous reports suggesting that histidine residues are essential for catalysis. Spectroscopic characterization of the modified enzymes allowed the unequivocal assignment of the histidine resonances in 1H-NMR spectra of asparaginase II. A histidine signal previously shown to disappear upon aspartate binding is due to His183, not to the highly conserved His87. The fact that [H183L]EcA2 has normal activity but greatly reduced stability in the presence of urea suggests that His183 is important for the stabilization of the native asparaginase tetramer. 1H-NMR and fluorescence spectroscopy indicate that His87 is located in the interior of the protein, possibly adjacent to the active site.     

Chemical modification of histidine, tyrosine, tryptophan and cysteine residues in carp (Cyprinus carpio) muscle enolase

Enolase from carp (Cyprinus Carpio) muscle was modified by diethylpyrocarbonate, tetranitromethane, N-bromosuccinimide and 5,5'-dithiobis(2-nitrobenzoic acid). The extent and rate of modification and its effect on the enzyme activity were determined. Modification of histidine, tyrosine and tryptophan residues caused complete inactivation of the enzyme; Mg2+ as well as 2-phosphoglycerate markedly altered the rates of modification and inactivation. The above-mentioned amino acid residues seem to be essential for the functioning of muscle enolases. Modification of cysteine residues had no effect on the enolase activity.     

13C- and 1H-nuclear magnetic resonance spectroscopy of permethylated disaccharides

The equilibrium composition of D-psicose in water, as determined from its ¹³C n.m.r. spectrum, is 7:2:5:5 α-furanose:β-furanose:α-pyranose:β-pyranose. These data, which are discussed in relation to the anomeric and ring-form equilibria of the other 2-hexuloses, are in general agreement with expectations based on conformational analysis. However, although the ¹³C chemical-shift pattern of the β-pyranose is closely consistent with the 1C(D) conformation predicted for this anomer, that of the α-pyranose is less readily reconciled with its predicted C1(D) conformation. Usually, carbon-13 nuclei of the furanose rings are substantially less shielded than those of their pyranose counterparts; for 2-hexulopyranoses in general, overall ¹³C shielding is close to that of those aldopyranoses expected to have similar conformational free-energies. Spectral data are also reported for several derivatives (glycosides, ethers, and selectively deuterated compounds) that were utilized in analysis of the D-psicose spectrum.     

Chemical modification of liquefying alpha-amylase: role of tyrosine residues at its active center

Liquefying alpha-amylase from Bacillus amyloliquefaciens was inactivated by treatment with tetranitromethane and N-acetylimidazole. The loss of activity occurred with modification of five tyrosine residues. Preincubation of the enzyme with either the substrate or the competitive inhibitor at saturating levels provided complete protection against inactivation. However, the presence of substrate/inhibitor in the reaction mixture protected only two of the five modifiable tyrosine residues, suggesting the involvement of only two tyrosine residues at the active center. This was confirmed when hydroxylamine treatment of the acetylated enzyme fully restored the enzymatic activity. Both nitration and acetylation increased the apparent Km of the enzyme for soluble starch, which indicated that the tyrosine residues are involved in substrate binding. Reduction of nitrotyrosine residues to aminotyrosine residues failed to restore the enzymatic activity. So, the loss of activity on modification of tyrosine residues was ascribed to conformational perturbances and not simply to the changes in the ionic character of tyrosine residues.