Applications of pulse radiolysis to protein chemistry

Radiation-induced binding of methanol, ethanol and 1-butanol to haemoglobin

Aqueous solutions of haemoglobin were irradiated under N2 and under N2O in the presence of methanol, ethanol and 1-butanol, which were partly 14C-labelled. The amount of bound alcohol was measured after gel filtration on Sephadex G-100 and with sodium dodecyl sulphate on Sepharose 6B-CL. All alcohols became covalently linked to haemoglobin. After radiolysis under N2 the G values for the three different alcohols were very similar, but under N2O the yield of cross-linking of haemoglobin with 1-butanol is twice that of methanol or ethanol. The high degree of modification by butanol results in extensive dissociation to dimers. Possible mechanisms yielding covalent cross-links between alcohols and haemoglobin are discussed. The high G value for cross-linking with butanol suggests that butanol radicals can add very efficiently to double bonds.

Pulse radiolysis study of a yeast cytochrome c from Hansenula anomala

The reduction of Hansenula anomala yeast cytochrome c by e-aq and CO-.2 was investigated by pulse radiolysis, at a high reductant to protein concentration ratio. The reactivity of the radicals was studied by observing absorbance changes in the cytochrome c spectrum over the wavelength range 280-600 nm. At pH 7, over the time scale of the radical decays (i.e. 0-4 microseconds for e-aq; 0-40 microseconds for CO-.2s) and beyond, the hemoprotein was reduced without any spectrally detected intermediate between ferri-and ferro-forms. This conclusion was reached by simulation studies based on the direct reduction of the yeast cytochrome c from the ferri- to the ferro-form, yielding a correct fit between experimental and calculated absorbance curves. The reduction rate constants were determined to be 1.0 +/- 01 X 10(10) M-1 S-1 for e-aq and 0.7 +/- 0.05 X 10(9) M-1 S-1 for CO-.2 at 0.16 M ionic strength, pH 7.0 and 20 degrees C, thus not significantly different from other values reported for horse heart cytochrome c. However, in the 360-390 nm region the generation of an additional radical species was noticed. The present experimental data were compared with previously published reports.

Unpaired electron migration between aromatic and sulfur peptide units

Cysteine thiyl radicals (Cys/S.) were found capable of one-electron oxidation of tyrosine. Equilibration occurred, using Cys and Gly-Tyr, with an equilibrium constant of K5 = 20 +/- 4 at pH 9.15: Cys/S. + Tyr in equilibrium Cys + Tyr/O. (5) Hence the reduction potentials of these couples differ at pH 9.15 by E(Cys/S., Cys) - E(Tyr/O., Tyr) = 80 mV. Oxidation of Trp-Gly by Cys/S. was not detectable from pH 7 to 12. The methionyl radical cation (Met/S.N), formed via .OH-attack on Met-Gly, reacts with Trp-Gly to generate the indolyl radical (Trp/N.). New results on intramolecular Trp/N.----Tyr/O. transitions indicate that the reaction requires direct contact between the two redox centers. Various possible pathways for migration of unpaired electrons between peptide units are compiled in a scheme.

Intramolecular electron and proton transfer in proteins: CO2- reduction of riboflavin binding protein and ribonuclease A

The formate radical (CO2-) reacts with ribonuclease A to form the cystine disulfide radical as one of the products. CO2- reacts with the riboflavin binding protein of chicken egg white with the ultimate product being the neutral flavin semiquinone. Formation of the disulfide radical in ribonuclease is slower than the reaction between protein and CO2-; formation of the flavin semiquinone in the riboflavin binding protein is slower than the protein-CO2- reaction. We conclude for both proteins that CO2- must reduce an as yet unidentified group or groups, which in turn reduce(s) the disulfide of RNase or the flavin of riboflavin binding protein. This conclusion is supported in the case of ribonuclease by the observation of a transient, broad absorption band centered between 350 and 370 nm. The CO2--initiated reductions of the disulfide in ribonuclease and the flavin in the riboflavin binding protein are mixed first- and second-order processes. We propose that the transfer of an electron from the unknown intermediate(s) to the final product involves both inter- and intramolecular paths between groups that may not be in van der Waals contact. With the hydrated electron, in contrast to CO2-, as reductant of the riboflavin binding protein, the anionic semiquinone is observed as an intermediate. The anionic semiquinone is then rapidly protonated, yielding the stable neutral semiquinone. From the reaction kinetics and protein concentration dependence, we conclude that a group or groups on the protein donate(s) a proton to the anionic semiquinone by both inter- and intramolecular paths.

The reduction of flavocytochrome b2 by carboxylate radicals. A pulse radiolysis study

The reaction of flavocytochrome b2 with carboxylate radicals has been studied by the pulse-radiolysis technique. The absorbance difference spectra observed 2, 10 and 90 mus after the pulse were very similar to the (reduced-minus-oxidized) difference spectrum of the cytochrome b2 core, a deflavoderivative prepared from flavocytochrome b2. The heme b2 group was reduced in a bimolecular reaction with a rate constant of (2.1 +/- 0.2) X 10(8) M-1 X s-1. Simulation data allowed us to assign this reduction mainly to a direct reaction of CO-2 on heme b2 without flavin involvement. However, this heme b2 reduction was accelerated in a flavocytochrome b2 sample of low molar activity. This observation seemed to reflect modifications of the heme b2 environment, in particular a closer contact with the solvent. Moreover, in such samples the flavin became reducible to the semiquinone state as deduced from absorbance difference spectra in the 440-490 nm region. The agreement between experimental and computed time-courses allowed to estimate the reaction rate of bound flavin to be equal to 2 X 10(8) M-1 X s-1 in this studied sample. Thus the reactivity of bound heme b2 and flavin within flavocytochrome b2 with CO-2 seems to be sensitive to the physical alterations of the polypeptide chain.

Pulse radiolysis kinetics of the reaction of hydrated electrons with ferric-, ferrous-, protoporphyrin IX- and apo-myoglobin

The kinetics of the reaction of hydrated electron (e-aq) with ferric-, ferrous-, metal-free protoporphyrin IX-and apo-myoglobin have been studied by pulse radiolysis so that a direct kinetic measure of the relative reactivities of the heme and the protein part of myoglobin can be made. The second-order association rate constant with ferric Mb is about 3-times that for ApoMb, while ferric Mb, ferrous Mb and protoporphyrin IX-Mb all react at about the same rate, indicating that it is mainly the porphyrin that is the electron-attracting site. The magnitude of the rate constants (8-25 nM-1 X S-1) indicates that the encounter of e-aq with the protein is almost certainly diffusion-controlled. The initial encounter is probably followed by electron migration along parallel paths to the heme and most likely several of the 12 histidine residues. The heme competes very effectively (approx. 70%) with these other sites. The kinetically measured reduction yield of heme is consistent with that found spectrally, indicating that a histidine radical on the protein does not effectively transfer an electron intramolecularly to the heme. The spectral changes found upon the completion of the fast reaction (approx. 40 microseconds) for protoporphyrin IX-Mb and ferrous Mb are consistent with the formation of a porphyrin anion radical. For ApoMb the spectral changes are consistent with the formation of a histidine free radical.

Radical scavenging and electron-transfer reactions in Polyporus versicolor laccase a pulse radiolysis study

The interaction of the radicals OH, t-BuO, Eaq, CO2 and O2 with the copper oxidase, laccase, from Polyporus, has been studied by the pulse-radiolysis technique. Each of these radicals formed transient adducts with a broad absorption maximum around 310 nm. Analysis of the optical properties and of the very fast rates of formation of these compounds shows that each radical interacts with a limited number of sites on the polypeptide part of the protein amongst R-S-S-R, histidine and aromatic residues. Interaction with the carbonyl group of some of the peptides bonds is also possible. The few target sites are probably hit simultaneously and electron transfer between these sites may also occur. In all cases, ina subsequent step, intramolecular electron transfer from the polypeptide radical adducts leads to a partial reduction of the blue type-1 Cu2+ with rates varying between 10(3) adn 10(4) s-1. Further reduction of the type-1 CU2+ occurs through a slow intermolecular reaction between two laccase radical transient adducts. In the case of CO2 and O2, this slow reduction could alternatively be due to an intermolecular reaction between laccase and CO2 or O2. The oxidation radicals OH, Br2 and (SCN)2, which formed radical adducts with fully ascorbate-reduced laccase, did not induce any type-1 copper reoxidation.