Mitochondrial respiration, an important bioenergetic process, relies on operation of four membranous enzymatic complexes linked functionally by mobile, freely diffusible elements: quinone molecules in the membrane and water-soluble cytochromes c in the intermembrane space. One of the mitochondrial complexes, complex III (cytochrome bc1 or ubiquinol:cytochrome c oxidoreductase), provides an electronic connection between these two diffusible redox pools linking in a fully reversible manner two-electron quinone oxidation/reduction with one-electron cytochrome c reduction/oxidation. Several features of this homodimeric enzyme implicate that in addition to its well-defined function of contributing to generation of proton-motive force, cytochrome bc1 may be a physiologically important point of regulation of electron flow acting as a sensor of the redox state of mitochondria that actively responds to changes in bioenergetic conditions. These features include the following: the opposing redox reactions at quinone catalytic sites located on the opposite sides of the membrane, the inter-monomer electronic connection that functionally links four quinone binding sites of a dimer into an H-shaped electron transfer system, as well as the potential to generate superoxide and release it to the intermembrane space where it can be engaged in redox signaling pathways. Here we highlight recent advances in understanding how cytochrome bc1 may accomplish this regulatory physiological function, what is known and remains unknown about catalytic and side reactions within the quinone binding sites and electron transfers through the cofactor chains connecting those sites with the substrate redox pools. We also discuss the developed molecular mechanisms in the context of physiology of mitochondria. Copyright © 2015 the American Physiological Society.

Two-dimensional 1H NMR spectra of Saccharomyces cerevisiae reduced iso-1-cytochrome c have been used to confirm and slightly extend the assignment available in the literature. 1702 NOESY cross-peaks have been assigned, and their intensities have been measured. Through the program DIANA and related protocols (Güntert, 1992), a solution structure has been obtained by using 1442 meaningful NOEs and 13 hydrogen-bond constraints. The RMSD values with respect to the mean structure for the backbone and all heavy atoms for a family of 20 structures are 0.61 +/- 0.09 and 0.98 +/- 0.09 A, the average target function value being as small as 0.57 A2. The larger number of slowly exchanging amide NHs observed in this system compared to that observed in the cyanide derivative of oxidized Ala 80 cytochrome c suggests that the oxidized form is much more flexible and that the backbone protons are more solvent accessible. Comparison of the present structure with the crystal structures of reduced yeast cytochrome c and of the complex between cytochrome c peroxidase and oxidized yeast cytochrome c reveals substantial similarity among the backbone conformations but differences in the residues located in the region of protein-protein interaction. Interestingly, in solution the peripheral residues involved in the interaction with cytochrome c peroxidase are on average closer to the position found in the crystal structure of the complex than to the solid state structure of the isolated reduced from.

The solution structure of oxidized Saccharomycescerevisiae Cys102Ser iso-1-cytochromechas been determined using 1361 meaningful NOEs (of 1676 total) after extending the published proton assignment [Gao, Y., et al. (1990) Biochemistry 29, 6994-7003] to 77% of all proton resonances. The NOE patterns indicate that secondary structure elements are maintained upon oxidation in solution with respect to the solid state and solution structures of the reduced species. Constraints derived from the pseudocontact shifts [diamagnetic reference shift values are those of the reduced protein [Baistrocchi, P., et al. (1996) Biochemistry 35, 13788-13796]] were used in the final stages of structure calculations. After restrained energy minimization with constraints from NOEs and pseudocontact shifts, a family of 20 structures with rmsd values of 0.58 +/- 0.08 and 1.05 +/- 0.10 A (relative to the average structure) for the backbone and all heavy atoms, respectively, was obtained. The solution structure is compared with the crystal structure and the structures of related systems. Twenty-six amide protons were detected in the NMR spectrum 6 days after the oxidized lyophilized protein was dissolved in D2O (pH 7.0 and 303 K); in an analogous experiment, 47 protons were observed in the spectrum of the reduced protein. The decrease in the number of nonexchanging amide protons, which mainly are found in the loop regions 14-26 and 75-82, confirms the greater flexibility of the structure of oxidized cytochrome c in solution. Our finding of increased solvent accessibility in these loop regions is consistent with proposals that an early step in unfolding the oxidized protein is the opening of the 70-85 loop coupled with dissociation of the Met80-iron bond.

Apoptosis is a highly regulated cellular suicide program crucial for metazoan development. However, dysfunction of apoptosis also leads to several diseases. Yeast undergoes apoptosis after application of acetic acid, sugar- or salt-stress, plant antifungal peptides, or hydrogen peroxide. Oxygen radicals seem to be key elements of apoptotic execution, conserved during evolution. Furthermore, several yeast orthologues of central metazoan apoptotic regulators have been identified, such as a caspase and a caspase-regulating serine protease. In addition, physiological occurrence of cell death has been detected during aging and mating in yeast. The finding of apoptosis in yeast, other fungi and parasites is not only of great medical relevance but will also help to understand some of the still unknown molecular mechanisms at the core of apoptotic execution.

Visible spectroscopy was used to measure real-time changes in the oxidation state of cytochrome c (cyt c) and the a-cytochromes (cyt aa(3)) of cytochrome oxidase during mitochondrial outer membrane permeabilization (MOMP) initiated by anisomycin in HL-60 cells. The oxidation state of mitochondrial cyt c was found to be approximately 62% oxidized before MOMP and became approximately 70% oxidized after MOMP. In contrast, the cytosolic pool of cyt c was found to be almost fully reduced. This oxidation change allows cyt c release to be continuously and quantitatively monitored in real time. Anoxia and antimycin were used to fully reduce and fully oxidize, respectively, the mitochondrial pool of cyt c and it was found that the release of cyt c was independent of it oxidation state consistent with a simple model of cyt c passively diffusing down a concentration gradient through a pore or tear in the outer membrane. After MOMP was complete, the flux of cyt c diffusing back into the mitochondria was measured from the residual mitochondrial oxygen consumption after complete inhibition of the bc(1) with antimycin and myxothiazol. The outer membrane was found to be highly permeable after MOMP implying that the reduction of cyt c in the cytosol must be very rapid. The permeability of the outer membrane measured in this study would result in the release of cyt c with a time constant of less than 1 s.

Cytochrome c release from mitochondria induces caspase activation in cytosols; however, it is unclear whether the redox state of cytosolic cytochrome c can regulate caspase activation. By using cytosol isolated from mammalian cells, we find that oxidation of cytochrome c by added cytochrome oxidase stimulates caspase activation, whereas reduction of cytochrome c by added tetramethylphenylenediamine (TMPD) or yeast lactate dehydrogenase/cytochrome c reductase blocks caspase activation. Scrape-loading of cells with this reductase inhibited caspase activation induced by staurosporine. Similarly, incubating intact cells with ascorbate plus TMPD to reduce intracellular cytochrome c strongly inhibited staurosporine-induced cell death, apoptosis, and caspase activation but not cytochrome c release, indicating that cytochrome c redox state can regulate caspase activation. In homogenates from healthy cells cytochrome c was rapidly reduced, whereas in homogenates from apoptotic cells added cytochrome c was rapidly oxidized by some endogenous process. This oxidation was prevented if mitochondria were removed from the homogenate or if cytochrome oxidase was inhibited by azide. This suggests that permeabilization of the outer mitochondrial membrane during apoptosis functions not just to release cytochrome c but also to maintain it oxidized via cytochrome oxidase, thus maximizing caspase activation. However, this activation can be blocked by adding TMPD, which may have some therapeutic potential.

Horseradish peroxidase is an important heme-containing enzyme that has been studied for more than a century. In recent years new information has become available on the three-dimensional structure of the enzyme and its catalytic intermediates, mechanisms of catalysis and the function of specific amino acid residues. Site-directed mutagenesis and directed evolution techniques are now used routinely to investigate the structure and function of horseradish peroxidase and offer the opportunity to develop engineered enzymes for practical applications in natural product and fine chemicals synthesis, medical diagnostics and bioremediation. A combination of horseradish peroxidase and indole-3-acetic acid or its derivatives is currently being evaluated as an agent for use in targeted cancer therapies. Physiological roles traditionally associated with the enzyme that include indole-3-acetic acid metabolism, cross-linking of biological polymers and lignification are becoming better understood at the molecular level, but the involvement of specific horseradish peroxidase isoenzymes in these processes is not yet clearly defined. Progress in this area should result from the identification of the entire peroxidase gene family of Arabidopsis thaliana, which has now been completed.

Cytochrome c (cyt c) is a cationic hemoprotein of ∼100 amino acid residues that exhibits exceptional functional versatility. While its primary function is electron transfer in the respiratory chain, cyt c is also recognized as a key component of the intrinsic apoptotic pathway, the mitochondrial oxidative protein folding machinery, and presumably as a redox sensor in the cytosol, along with other reported functions. Transition to alternative conformations and gain-of-peroxidase activity are thought to further enable the multiple functions of cyt c and its translocation across cellular compartments. In vitro, direct interactions of cyt c with cardiolipin, post-translational modifications such as tyrosine nitration, phosphorylation, methionine sulfoxidation, mutations, and even fine changes in electrical fields lead to a variety of conformational states that may be of biological relevance. The identification of these alternative conformations and the elucidation of their functions in vivo continue to be a major challenge. Here, we unify the knowledge of the structural flexibility of cyt c that supports functional moonlighting and review biochemical and immunochemical evidence confirming that cyt c undergoes conformational changes during normal and altered cellular homeostasis.

Nuclear magnetic resonance (NMR) spectroscopy is a uniquely powerful tool for studying the structure, dynamics and interactions of biomolecules at atomic resolution. In the past 15 years, the development of new isotopic labeling strategies has opened the possibility of exploiting NMR spectroscopy in the study of supra-molecular complexes with molecular weights of up to 1MDa. At the core of these isotopic labeling developments is the specific introduction of [(1)H,(13)C]-labeled methyl probes into perdeuterated proteins. Here, we describe the evolution of these approaches and discuss their impact on structural and biological studies. The relevant protocols are succinctly reviewed for single and combinatorial isotopic-labeling of methyl-containing residues, and examples of applications on challenging biological systems, including high molecular weight and membrane proteins, are presented. Copyright © 2015 Elsevier Ltd. All rights reserved.

The heme and its two axial ligands, His18 and Met80, play a central role in the folding/unfolding mechanism of cytochrome c. Because of the covalent heme attachment, His18 remains bound under typical denaturing conditions, while the more labile Met80 ligand is replaced by an alternate histidine ligand. To distinguish between the two possible non-native histidine ligands in horse cytochrome c, variants with a His26 to Gln or His33 to Asn substitution were prepared using a yeast expression system. Protonation of the non-native histidine ligand in the GuHCl-denatured state results in a pronounced blue shift of the Soret heme absorbance band (low-spin to high-spin transition). While substitution of His26 has no effect on the apparent pKa of this transition (5.7 +/- 0.05), the H33N variant exhibits a substantially higher pKa (6.1 +/- 0.05), indicating that His33 is the dominant sixth heme ligand in denatured cytochrome c and that His26 (or another nitrogenous group) acts as a ligand in the absence of a histidine at position 33. The kinetics of the pH-induced ligand dissociation shows two phases which were assigned to each of the two histidine ligands on the basis of their distinct temperature dependence. Despite their nearly identical equilibrium unfolding transitions, the two histidine mutants show differences in their folding kinetics. While the kinetic behavior of H26Q cyt c is very similar to that of the wild-type, the H33N mutation leads to loss of a kinetic phase with a rate in the 2-10 s-1 range that has previously been attributed to the rate-limiting dissociation of a trapped non-native histidine, which is thus identified as His33.