Cytochrome c

Reduction of ferricytochrome c by some free radical agents

Fast pulse radiolysis and kinetic spectroscopy were used to rapidly generate a variety of free radicals in situ and study their reactions with ferricytochrome c in the time range 10(-6) to 1 second. The radicals included t-butanol, which is inert to ferricytochrome c; malate, lactate, and ethanol, which react with it relatively slowly but are completely utilized in reducing it to ferrocytochrome c; and hydrated electrons and hydrogen atoms, which react with it very rapidly but yield ferrocytochrome c only in part, showing intramolecular consecutive reactions and further attack on the ferrocytochrome c protein. From a detailed comparison between malate and hydrogen atoms it is argued that malate reacts directly and selectively with a specific part of the ferricytochrome c surface while hydrogen atoms react with other parts of the protein too, yielding radicals which in part transfer intramolecularly to yield ferrocytochrome c.

A comparison of the physical and chemical properties of four cytochromes c from Azotobacter vinelandii

1. A modified method for the separation and purification of four cytochromes c from Azotobacter vinelandii is described. Two new cytochromes c have been purified and are designated cytochromes c(551) and c(555). 2. Additional evidence is presented to establish the dihaem nature of cytochrome c(4). Ultracentrifugation data indicated similar molecular weights for the native and the denatured protein. Cleavage with CNBr yielded seven peptides; the amino acid compositions of the purified peptides were determined. Only one haem peptide was recovered. 3. Cytochromes c(551) and c(555) were characterized as acidic proteins of molecular weights about 12000. The spectral properties, isoelectric points, ;maps' of peptides from CNBr cleavage and amino acid compositions were determined for these two proteins. 4. The spectral properties, isoelectric points, molecular weights, CNBr peptide ;maps', amino acid compositions, relative oxidation-reduction potentials and e.p.r. (electron-paramagnetic-resonance) spectra of the four cytochromes c were compared. Cytochrome c(4) and cytochrome c(551) appear to be distinct proteins. The distinction between cytochromes c(5) and c(555) was not as clear, and our data are inadequate to establish firmly that they are distinct proteins. 5. The dihaem nature of cytochrome c(4) is evident in its e.p.r. spectrum. The e.p.r. spectra are similar to the spectra of mammalian cytochromes c.

Photoacoustic spectroscopy of biological materials

A new technique for performing optical spectroscopy on solids has been developed. Photoacoustic spectra of cytochrome c and hemoglobin show how this technique can be used to obtain information about optical absorptions and subsequent de-excitations in solid biological materials, particularly those which cannot readily be studied by conventional means.

Electron transfer to ferricytochrome c: reaction with hydrated electrons and conformational transitions involved

The reaction of horse-heart cytochrome c with hydrated electrons has been studied by the pulseradiolysis technique. In neutral solution, the ferriheme group was reduced in a bimolecular reaction that takes place at a rate equal to that of the decay of the e(-) (aq), and approaches the diffusion-controlled limit. This reduction is assigned mainly to a direct reaction, proceeding via the exposed edge of the porphyrin projecting into the cytochrome c crevice. The reaction absorption spectrum observed 20 musec after an electron pulse was very similar, yet blue-shifted relative to, the difference spectrum between the reduced and oxidized forms of cytochrome c. However, this shift vanishes in a slow monomolecular reaction, which seems to reflect the conformational relaxation of the protein to the final equilibrium state of its reduced form. In alkaline solutions, the transition of cytochrome c molecules into an irreducible conformation causes a proportionate decrease in the amount of ferricytochrome c reduced in the direct reaction. The rate of conformational transition of the protein into the reactive form is now the limiting step for a substantial part of the reduction that takes place via this slow monomolecular reaction. Pseudomonas cytochrome c 551 which, in contrast to horse-heart cytochrome c, is a negatively charged protein at neutral pH reacts with e(-) (aq) at a rate lower than does the horse-heart protein. The reduction of the heme group follows that of the e(-) (aq) decay with a small, yet significant, delay.

Reconstitution of horse heart cytochrome c: interaction of the components obtained upon cleavage of the peptide bond following methionine residue 65

Horse heart cytochrome c can be divided into a heme peptide of 65 residues (H65CNBr) and a nonheme peptide of 39 residues (N39CNBr) by treatment of the molecule with cyanogen bromide. Upon mixture of the two peptides in aqueous solution, a 1: 1 complex with properties closely resembling those of the parent heme protein can be formed. The reaction can conveniently be effected in sodium acetate buffer of pH 4.7, with the H65CNBr in the reduced form. The heme peptide is predominantly in the high-spin state under these conditions, and, upon the addition of N39CNBr, is converted to a complex with absorption and circular dichroism spectra which correspond closely to those of ferrocytochrome c. If N39CNBr is added to the oxidized form of H65CNBr, the spectral properties of the product differ appreciably from those of the parent protein. A complex with absorption and circular dichroism spectra comparable to those of ferricytochrome c can readily be obtained, however, through oxidation of the product of the reaction of N39CNBr with H65CNBr in the reduced state. The complex formed in this manner has an absorption band at 695 nm, exhibits no tryptophan or tyrosine fluorescence, and is active in the succinate oxidase system.