Circular dichroism of mammalian cytochrome c 1

Structural characterization of isolated mitochondrial cytochrome c1

Resonance Raman spectroscopy (RRS) has been employed to characterize cytochromes c1 isolated from bc1 complexes of beef heart mitochondria and Rhodopseudomonas sphaeroides. The data obtained in this study extend the physical characterization of cytochromes c1 and focus on the effects of the local protein environment on the heme active site. While the general characteristics of the cytochromes c1 are similar to those of smaller soluble cytochromes c, the behavior of several core-size and ligation-sensitive heme modes reveal that significant systematic differences exist between those species. These, most likely, result from changes in the heme axial-ligand interactions.

Resolution of the circular dichroism spectra of the mitochondrial cytochrome bc1 complex

The circular dichroic spectrum of the mitochondrial cytochrome bc1 complex isolated from bovine heart has been resolved into the contributions from the prosthetic groups: cytochrome c1, the 'Rieske' iron-sulphur centre and the two b cytochromes. It is apparent that firstly, the circular dichroism (CD) properties of cytochrome c1 within the bc1 complex differ from those found in the isolated cytochrome c1 and secondly, both the oxidized and reduced b cytochromes exhibit an intense spectrum of bilobic shape, with the wavelengths of the cross-over points closely corresponding to those of the maxima in the optical absorbance spectra. These latter CD features are discussed in relation to the proposed structure of cytochrome b.

The circular-dichroic properties of the 'Rieske' iron-sulphur protein in the mitochondrial ubiquinol: cytochrome c reductase

We have studied the c.d. spectra of the 'Rieske' iron-sulphur protein isolated from the ubiquinol: cytochrome c reductase (bc1 complex) of bovine heart mitochondria. Both the oxidized and the reduced form of the 'Rieske' protein display a series of well-resolved c.d. features resembling those reported for the 'Rieske'-type iron-sulphur protein purified from the bacterium Thermus thermophilus [Fee, Findling, Yoshida, Hille, Tarr, Hearshen, Dunham, Day, Kent & Münck (1984) J. Biol, Chem. 259, 124-133]. In particular, the difference spectra, reduced minus oxidized, of both proteins have a distinctive negative band at 497 nm. The c.d. features characteristic of the isolated 'Rieske' protein were found in the dichroic spectra of the whole bc1 complex in the region between 450 and 520 nm. The reduction of the enzyme by ascorbate or ubiquinol is accompanied by the formation of a negative band at about 500 nm that corresponds, in all its c.d. properties, to the specific dichroic absorption of the reduced 'Rieske' iron-sulphur protein.

Circular-dichroism studies of the cytochrome b-c1 complex of Saccharomyces cerevisiae

1. Circular dichroism studies on the Soret region of the cytochrome b-c1 complex of yeast reveal a change in the dichroism of cytochrome c1 depending on the redox state of cytochrome b, indicating a conformational interaction between both cytochromes. 2. This interaction is not influenced by binding of the inhibitor antimycin A to the complex, so that the interaction does not appear to be involved in the mechanism of electron transport through the complex. 3. Antimycin A binding causes a complex set of changes in the CD spectrum of the complex, which can be attributed to a severe and specific distortion of the environment of the chromophore of cytochrome b.

Controlled digestion with trypsin as a structural probe for the N-terminal peptide of soluble and membranous cytochrome c

When purified bovine cytochrome c1 is digested with trypsin under controlled conditions, the heme polypeptide is preferentially converted from a species of molecular weight 30,600 to a heme polypeptide of molecular weight 29,000. The trypsin sensitive peptide bond is located in the N-terminal region of the cytochrome. Both the reduced and oxidized cytochrome are susceptible to hydrolysis by trypsin at the same locus, but the reduced cytochrome is cleaved at an initial rate approximately twofold greater than the oxidized cytochrome. Membranous cytochrome c1, as occurring in cytochrome b-c1 complex or succinate-cytochrome c reductase complex, is not susceptible to trypsin proteolysis under similar conditions, nor after more extensive treatment of the membranes with trypsin, in spite of the fact that cytochrome c1 presumably comes into contact with cytochrome c at the membrane surface during electron transport. These findings are consistent with a model for the structure of cytochrome c1 in situ in which the cytochrome is an integral membrane protein, located primarily in the membrane continuum, while still having the heme-containing portion of the protein available at the membrane surface for electron transfer to cytochrome c.

A trypsin-resistant heme peptide from cardiac cytochrome c1

A tryptic resistant heme peptide has been prepared and purified from cardiac cytochrome c1. This purified peptide is not further hydrolyzed by reactions of other proteolytic enzymes, such as pronase. The peptide contains 2 residues each of serine, cysteine and valine, and 1 residue each of alanine, methionine, tyrosine, histidine, arginine, proline, glutamic acid (glutamine) and aspartic acid. The intensity of the absorption spectrum of the peptide has been found to be dependent upon, but the positions of the absorption maxima do not vary with, concentration. The heme peptide does not show multiple splitting of absorption peaks at liquid N2 temperatures as does the intact cytochrome C1. However, cyanide rapidly reacts with the peptide and causes significant spectral changes. CD spectra of the peptide exhibit a typical profile of a non-structured heme peptide with positive CD bands in the Soret region and around 250 nm, and a broad negative extreme of 320-360 nm. The similarities and differences between the tryptic resistant heme peptides from cytochromes c1 and c have been compared.