Conformational and functional studies of chemically modified cytochrome c: nitrated and iodinated cytochromes c

Tissue-specific regulation of cytochrome c by post-translational modifications: respiration, the mitochondrial membrane potential, ROS, and apoptosis

Cytochrome c (Cyt c) plays a vital role in the mitochondrial electron transport chain (ETC). In addition, it is a key regulator of apoptosis. Cyt c has multiple other functions including ROS production and scavenging, cardiolipin peroxidation, and mitochondrial protein import. Cyt c is tightly regulated by allosteric mechanisms, tissue-specific isoforms, and post-translational modifications (PTMs). Distinct residues of Cyt c are modified by PTMs, primarily phosphorylations, in a highly tissue-specific manner. These modifications downregulate mitochondrial ETC flux and adjust the mitochondrial membrane potential (ΔΨm), to minimize reactive oxygen species (ROS) production under normal conditions. In pathologic and acute stress conditions, such as ischemia-reperfusion, phosphorylations are lost, leading to maximum ETC flux, ΔΨm hyperpolarization, excessive ROS generation, and the release of Cyt c. It is also the dephosphorylated form of the protein that leads to maximum caspase activation. We discuss the complex regulation of Cyt c and propose that it is a central regulatory step of the mammalian ETC that can be rate limiting in normal conditions. This regulation is important because it maintains optimal intermediate ΔΨm, limiting ROS generation. We examine the role of Cyt c PTMs, including phosphorylation, acetylation, methylation, nitration, nitrosylation, and sulfoxidation and consider their potential biological significance by evaluating their stoichiometry.-Kalpage, H. A., Bazylianska, V., Recanati, M. A., Fite, A., Liu, J., Wan, J., Mantena, N., Malek, M. H., Podgorski, I., Heath, E. I., Vaishnav, A., Edwards, B. F., Grossman, L. I., Sanderson, T. H., Lee, I., Hüttemann, M. Tissue-specific regulation of cytochrome c by post-translational modifications: respiration, the mitochondrial membrane potential, ROS, and apoptosis.

Characterization of zinc-substituted cytochrome c by circular dichroism and resonance Raman spectroscopic methods

Iron(III) in cytochrome c is replaced with zinc(II) by a modification of a method published by others, and the procedure is described in full detail. Three forms of cytochrome c-those containing iron(III), iron(II), and zinc(II)-are examined by circular dichroism spectroscopy and resonance Raman spectroscopy. Spectra of both kinds show that introduction of zinc(II) ions does not appreciably alter the overall structure and conformation of cytochrome c. Resonance Raman spectra indicate the size of the porphyrin "core" that is inconsistent with six-coordination and consistent with five-coordination. Unlike the iron(III) and iron(II) ions, which are bound to two axial ligands (His 18 and Met 80), the zinc(II) ion in cytochrome c seems to be bound to only one, most probably His 18. Evidence pertaining to the question of axial coordination is discussed.

Differential effect of iodination of ovotransferrin and its two half-molecule domains on binding to transferrin receptors on chick embryo red blood cells

Iodination of the C-terminal half-molecule domain of ovotransferrin (OTF) causes a significant reduction in binding to transferrin receptors on chick reticulocytes when compared to the binding observed with holo-OTF or the N-terminal half-molecule domain. (In such studies binding of iodinated half-molecule is measured in the presence of equimolar unlabelled complementary half-molecule). In particular iodination of the C-terminal half-molecule domain by the solid-phase reagent Iodogen resulted in half the binding found when ICl was used. The iodinated N-terminal half-molecule domain labelled by either Iodogen or ICl showed consistently higher binding than was observed with the C-terminal half-molecule or Fe2OTF. Although the molecular basis for the reduced binding of these proteins relative to the N-terminal half-molecule has not been definitively established, the implication is that there is a Tyr in the C-terminal domain which is involved in receptor recognition and binding. Addition of one or more bulky iodine atoms to the Tyr interferes with the interaction. Tryptic peptide maps of unlabelled holo-OTF and half-molecule domains and of the half-molecule domains labelled by both ICl and Iodogen are presented. The maps indicate limited access of the tyrosine residues to iodination especially in the C-terminal half-molecule domain. Equilibrium binding experiments have been carried out to compare the Kd (the apparent dissociation constant for the interaction between OTF and the transferrin receptors on chick-embryo red blood cells) with the Bmax, (binding at infinite free-ligand concentration) for Fe2OTF labelled using ICl, Iodogen, Enzymobeads and Chloramine-T. The effect of labelling Fe2OTF by Bolton-Hunter reagent has also been assessed. These studies show that ICl appears to be the reagent of choice for labelling Fe2OTF and its half-molecule domains.

Ionization of tyrosine and lysine residues in native and modified horse cytochrome c

1H-n.m.r. and 13C-n.m.r. spectroscopy of horse cytochrome c and 1H-n.m.r. spectroscopy of the lysine-modified proteins N epsilon-acetimidyl-, N epsilon-amidino-, N epsilon-trifluoroacetyl- and N epsilon-maleyl-cytochrome c have shown that, although the lysine modifications do not greatly perturb the protein structure at pH7 and 27 degrees C, at higher temperature or at alkaline pH some parts of the structure are markedly perturbed. At pH7 and 27 degrees C the region of the protein about Ile-57 is affected in all the modified proteins, though not all to the same degree. N epsilon-Maleylation most seriously affects the protein structure, and the fully maleylated protein is readily unfolded. At 27 degrees C all four of the tyrosine residues of native horse cytochrome c have pKa values above 11, but in N epsilon-acetimidyl-cytochrome c the pKa of one tyrosine residue is 10.2.

Structural role of the tyrosine residues of cytochrome c

The tertiary structures of horse, tuna, Neurospora crassa, horse [Hse65,Leu67]- and horse [Hse65,Leu74]-cytochromes c were studied with high-resolution 1H n.m.r. spectroscopy. The amino acid sequences of these proteins differ at position 46, which is occupied by phenylalanine in the horse proteins but by tyrosine in the remaining two, and at positions 67, 74 and 97, which are all occupied by tyrosine residues in horse and tuna cytochrome c but in the other proteins are substituted by phenylalanine or leucine, though there is only one such substitution per protein. The various aromatic-amino-acid substitutions do not seriously affect the protein structure.

13C-NMR spectroscopy of acetyltyrosyl-guanidinated horse heart cytochrome c

The tyrosine residues of guanidinated horse heart cytochrome c have been specifically acetylated by reaction with N-[1-13C]acetylimidazole (90 atom%). Acetylation was monitored by 13C-NMR spectroscopy. The tyrosine residues were found to show widely varying reactivities ranging from one that is completely and exclusively acetylated at low reagent concentration (residue 67) to one that is acetylated only when the protein is unfolded (residue 97). Homogeneous derivatives were prepared containing one (either residue 67 or 97), three 48, 67 and 74), or four (residues 48, 67, 74 and 97) O-[1-13C]acetyl groups. 13C-NMR spectra of selected derivatives were obtained at pH 5.8, in the presence of cyanide ion, in the ferrous and ferric oxidation states, and after denaturation with 6M guanidine hydrochloride. The O-[1-13C]acetyltyrosyl resonances gave chemical shift values ranging from 171.8 to 176.0 ppm. These resonances were assigned to specific groups based on the known order of reactivity of the tyrosyl side chains toward N-acetylimidazole. The chemical shift of O-[1-13C]acetyltyrosyl 67 was found to be particularly sensitive to changes in protein structure. The proximity of this group to the heme makes it subject to distance-dependent paramagnetic and ring current effects. Acetylation of tyrosyl 74 gives rise to a pH-dependent equilibrium between conformers in the ferric state and a conformation change in the ferrous state. Acetylation of this residue also leads to an absorbance decrease at 695 nm that can be related to the 13C-NMR-detected conformational equilibrium. Addition of cyanide ion abolished this equilibrium.

Perchlorate binding to cytochrome c: a magnetic and optical study

1. The effects of perchlorate on cytochrome c have been investigated by 1H and 35Cl NMR, electron paramagnetic resonance and optical spectroscopy. 2. The pK values for the formation and disappearance of the major alkaline conformation were found to be displaced from 9.3 to 8.3 and from 10.4 to 10.9, respectively. The displacement was dependent on the ClO4(-) concentration below 0.1 M. 3. Competition experiments between perchlorate and chloride show that ClO4(-) binds both to the neutral and alkaline forms but with a higher affinity for the latter. The appearance of a new binding site in the alkaline form accounts for the markedly enhanced relaxation rate of 35ClO4(-) in this pH range. Complex formation between cyanide and the alkaline species results in the loss of this binding site, which probably is located close to or within the heme crevice. 4. The neutral form of ferricytochrome c also binds perchlorate strongly as evidence by the unique appearance of a high-spin signal dependent on pH and perchlorate concentration. This signal disappears with the same pK value as the neutral form. The effects of perchlorate on cytochrome c are due to specific binding of this ion.

Cyanide reactivity of cytochrome c derivatives

The kinetic rates and equilibrium association constants for cyanide binding have been measured for a series of cytochrome c derivatives as a probe of heme accessibility. The series included horse and yeast cytochromes iodinated at Tyr 67 and 74, horse cytochrome formylated at Trp 59 in both a low and high redox potential form, the Met 80 sulfoxide derivative of horse cytochrome and the N-acylisourea heme propionate derivative of tuna cytochrome. Native cytochromes c are well known to bind cyanide slowly in a reaction simply first order both in cytochrome and cyanide up to at least 100 mM in cyanide. The derivative demonstrate markedly different kinetics which indicate the following conclusions. (1) In spite of chemical modification at different loci, all the derivatives have highly similar reactivity, suggesting common ligation structures and mechanisms for reaction. (2) Compared to native cytochromes, reaction rates are 10-20 fold greater. This is in accord with a more accessible heme crevice, but not a completely opened crevice. For the completely opened case, rate increases are expected to be between three and five orders of magnitude. (3) Reaction rates are either independent of cyanide concentration (zero order) or show only slight variation. A mechanism which accounts for the data over four orders of magnitude in concentration postulates a protein conformation step, opening of the heme crevice, as the rate determining step. This conformation change has a limiting rate of 6 . 10(-2) s-1.

Semisynthetic analogs of cytochrome c reconstructed from natural and synthetic peptides

A biologically active semisynthetic hybrid of horse heart cytochrome c has been prepared by combining the heme peptide 1 through 65 (HP 1-65), prepared by CNBr cleavage of natural cytochrome c, with a semisynthetic peptide corresponding to positions 66 through 104. A fully protected synthetic peptide 66--79 was prepared by a modified solid phase peptide synthesis procedure and was converted to its N-hydroxysuccinimide ester. A peptide corresponding to residues 81--104 of cytochrome c was also isolated from the CNBr cleavage mixture and its epsilon-amino groups and tyrosyl hydroxyl group were protected selectively with the t-butyloxycarbonyl group. This partially protected peptide was reacted with t-butyloxycarbonyl methionine N-hydroxysuccinimide ester to give a derivative having methionine at position 80. This product was deprotected, purified and then t-butyloxycarbonyl groups were again introduced specifically on the epsilon-amino groups to give the peptide, Boc(Lys,Tyr)80--104. A semisynthetic peptide corresponding to residues 66 through 104 of cytochrome c was prepared by condensing the synthetic peptide 66--79 N-hydroxysuccinimide ester with t-butyloxycarbonyl (Lys,Tyr)80--104. The semisynthetic product was deprotected, purified and combined under anaerobic conditions with a heme peptide, HP 1-65, that was isolated from the products of CNBr cleavage of native cytochrome c. The reconstituted semisynthetic cytochrome c was purified by ion exchange chromatography and was shown to have the same oxygen uptake as native cytochrome c when assayed in the succinate oxidase system.