Acute toxicity of cyanide in aerobic respiration: Theoretical and experimental support for murburn explanation (original) (raw)
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A re-examination of the reactions of cyanide with cytochrome c oxidase
Biochemical Journal, 1984
Experiments were performed to examine the cyanide-binding properties of resting and pulsed cytochrome c oxidase in both their stable and transient turnover states. Inhibition of the oxidation of ferrocytochrome c was monitored as a function of cyanide concentration. Cyanide binding to partially reduced forms produced by mixing cytochrome c oxidase with sodium dithionite was also examined. A model is presented that accounts fully for cyanide inhibition of the enzyme, the essential feature of which is the rapid, tight, binding of cyanide to transient, partially reduced, forms of the enzyme populated during turnover. Computer fitting of the experimentally obtained data to the kinetic predictions given by this model indicate that the cyanidesensitive form of the enzyme binds the ligand with combination constants in excess of 106 M-1-s-1 and with KD values of 50 nM or less. Kinetic difference spectra indicate
A re-examination of the reactions of cyanide with cytochrome c oxidase
Biochemical Journal, 1984
Experiments were performed to examine the cyanide-binding properties of resting and pulsed cytochrome c oxidase in both their stable and transient turnover states. Inhibition of the oxidation of ferrocytochrome c was monitored as a function of cyanide concentration. Cyanide binding to partially reduced forms produced by mixing cytochrome c oxidase with sodium dithionite was also examined. A model is presented that accounts fully for cyanide inhibition of the enzyme, the essential feature of which is the rapid, tight, binding of cyanide to transient, partially reduced, forms of the enzyme populated during turnover. Computer fitting of the experimentally obtained data to the kinetic predictions given by this model indicate that the cyanide-sensitive form of the enzyme binds the ligand with combination constants in excess of 10(6) M-1 X s-1 and with KD values of 50 nM or less. Kinetic difference spectra indicate that cyanide binds to oxidized cytochrome a33+ and that this occurs rapidl...
Cyanide inhibition and pyruvate-induced recovery of cytochrome c oxidase
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2010
The mechanism of cyanide's inhibitory effect on the mitochondrial cytochrome c oxidase (COX) as well as the conditions for its recovery have not yet been fully explained. We investigated three parameters of COX function, namely electron transport (oxygen consumption), proton transport (mitochondrial membrane potential Δψ(m)) and the enzyme affinity to oxygen (p₅₀ value) with regard to the inhibition by KCN and its reversal by pyruvate. 250 μM KCN completely inhibited both the electron and proton transport function of COX. The inhibition was reversible as demonstrated by washing of mitochondria. The addition of 60 mM pyruvate induced the maximal recovery of both parameters to 60-80% of the original values. When using low KCN concentrations of up to 5 μM, we observed a profound, 30-fold decrease of COX affinity for oxygen. Again, this decrease was completely reversed by washing mitochondria while pyruvate induced only a partial, yet significant recovery of oxygen affinity. Our results demonstrate that the inhibition of COX by cyanide is reversible and that the potential of pyruvate as a cyanide poisoning antidote is limited. Importantly, we also showed that the COX affinity for oxygen is the most sensitive indicator of cyanide toxic effects.
Cyanide does more to inhibit heme enzymes, than merely serving as an active-site ligand
The toxicity of cyanide is hitherto attributed to its ability to bind to heme proteins' active site and thereby inhibit their activity. It is shown herein that the long-held interpretation is inadequate to explain several observations in heme-enzyme reaction systems. Generation of cyanide-based diffusible radicals in heme-enzyme reaction milieu could shunt electron transfers (by non-active site processes), and thus be detrimental to the efficiency of oxidative outcomes.
Kinetics and mechanism for the binding of HCN to cytochrome c oxidase
Biochemistry, 1995
The kinetics of cyanide binding to cytochrome c oxidase were systematically studied as a function of [HCN], [oxidase], pH, ionic strength, temperature, type and concentration of solubilizing detergent, and monomer-dimer content of oxidase. On the basis of these results a minimum reaction mechanism is proposed in which the spectrally visible rapid and slow cyanide binding reactions are two consecutive first-order reactions, not parallel reactions with different conformers of cytochrome c oxidase. The fast reaction (k'obs) follows saturation type kinetics to form an HCN complex that subsequently undergoes a slow reaction (k"obs). The fast k'obs reaction is independent of ionic strength but is strongly dependent upon pH. Two pK values were evaluated from the bell-shaped rate versus pH profile; one is due to an ionizable group on the protein (pK, = 7. 4 3 , while the other is that of HCN (~K H C N = 9.15). Therefore, oxidase is reactive toward HCN only when the group on the protein is unprotonated. The slow k"&s reaction is not a reaction of oxidase with either CN-or HCN; in fact, the product formed by the fast k'obs reaction, the oxidase-HCN complex, still undergoes the slow k" process even if all of the excess KCN is removed. The apparent rate constant of the slower phase (k"obs) is independent of all the variations done in this study, and it probably corresponds to either a slow conformational change in the protein or a change in ligand coordination at one of the metal centers after HCN binds to the bimetallic center of oxidase. Based upon the bell-shaped pH dependence of the fast phase and the pH independence of the slow phase, the mechanism also predicts that a single conformer of cytochrome c oxidase can exhibit either monophasic or biphasic cyanide binding kinetics depending upon the pH. At either very low or very high pH, the two rates become comparable in magnitude, which makes the reaction appear to be monophasic even though both reactions still occur. The amount of monomeric or dimeric oxidase only slightly affects the magnitude of krobs and k"&s values, and both processes are clearly present in both types of oxidase. Cytochrome c oxidase (EC 1.9.3.1) is a multisubunit enzyme of the mitochondrial respiratory chain that transfers electrons from cytochrome c to dioxygen. Its function is to couple these electron transfers to the translocation of protons across the inner membrane (
The Journal of biological chemistry, 1990
Cyanide binding to bovine heart cytochrome c oxidase at five redox levels has been investigated by use of infrared and visible-Soret spectra. A C-N stretch band permits identification of the metal ion to which the CN- is bound and the oxidation state of the metal. Non-intrinsic Cu, if present, is detected as a cyanide complex. Bands can be assigned to Cu+CN at 2093 cm-1, Cu2+CN at 2151 or 2165 cm-1, Fe3+CN at 2131 cm-1, and Fe2+CN at 2058 cm-1. Fe2+CN is found only when the enzyme is fully reduced whereas the reduced Cu+CN occurs in 2-, 3-, and 4-electron reduced species. A band for Fe3+CN is not found for the complex of fully oxidized enzyme but is for all partially reduced species. Cu2+CN occurs in both fully oxidized and 1-electron-reduced oxidase. CO displaces the CN- at Fe2+ to give a C-O band at 1963.5 cm-1 but does not displace the CN- at Cu+. Another metal site, noted by a band at 2042 cm-1, is accessible only in fully reduced enzyme and may represent Zn2+ or another Cu+. Bi...
The active sites of the enzymes superoxide reductase (SOR) and cytochrome P450 feature square pyramidal FeN4S centers, with a thiolate in axial position trans to the substrate binding site but with differing equatorial nitrogenous ligands. The respective catalytic cycles also share a common intermediate – a ferric-(hydro)peroxo species. The detailed catalytic mechanisms are still a matter of debate for both enzymes, as some of their key catalytic intermediates have very short lifetimes. Inhibitors such as cyanide were therefore often employed to probe active sites of these enzymes and identify important structural features controlling reactivity; among these studies, ENDOR spectral data on ferric-cyanide complexes were previously reported. Here, density functional calculations are employed in order to more accurately correlate the experimental data with electronic structure elements. The data are shown to be in good agreement with experiment and also provide new insight.