Kinetics of carboxymethylation of histidine hydantoin.

The reaction of the imidazole group of histidine hydantoin with bromoacetate was studied as a model for carboxymethylation of histidine residues in proteins. pK values of 6.4 and 9.1 (25 degrees C) and apparent heats of ionization of 7.8 and 8.7 kcal/mol were determined for the imidazole and hydantoin rings, respectively. At pH values corresponding to the isoelectric points for histidine hydantoin, the rates of carboxymethylation at 12, 25, 37, and 50 degrees C were determined; the modified hydantoins were hydrolyzed to the corresponding histidine derivatives for quantitative amino acid analysis. At pH 7.72 and 25 degrees C, the imidazole tele-N was alkylated (k = 3.9 X 10(-5) M-1 s-1) twice as fast as the pros-N. The monocarboxymethyl derivatives were carboxymethylated at the same rate at the pros-N (k = 2.1 X 10(-5) M-1 s-1) but 3 times faster at the tele-N (k = 11 X 10(-5) M-1 s-1). The enthalpies of activation determined for carboxymethylation of the imidazole ring and its monocarboxymethyl derivatives were similar (15.9 +/- 0.7 kcal/mol). delta S for the four carboxymethylations was -25 +/- 2 eu. The electrostatic component of delta S (delta S es) was calculated from the influence of the dielectric constant on the reaction rate at 25 degrees C. delta S es was slightly negative (-4 +/- 1 eu) for mono- or dicarboxymethylations, indicating some charge separation in the transition state. The nonelectrostatic entropy of activation was -21 +/- 2 eu for all four carboxymethylations.     

Thermodynamic studies of the reversible association of Escherichia coli ribosomal subunits.

The kinetics of association of Escherichia coli 30S and 50S ribosomal subunits have been carried out as a function of temperature after a magnesium jump from 1.5 to 3 mM. Turbidimetric recordings combined with a stopped-flow apparatus were used to follow the kinetics. The data show that the rates of formation and dissociation of the 70S particles at 3 mM Mg2+ and +25 degrees C were, respectively: k2 = 10(5) M-1 s-1, k1 = 4,5 X 10(-3) s-1; lowering the temperature decreases the rate constants with activation energies equal to E2 = 7.5 kcal/mol, E1 = 26.5 kcal/mol and enhances the association equilibrium towards the 70S species with an enthalpy change (delta H degrees assoc = -19.9 kcal/mol) dominant over the entropy change (delta S degrees assoc = -33 cal/(deg mol)). These thermodynamic parameters were compared to those obtained from studies on the interactions of codon-anticodon in yeast phenylalanine transfer RNA as well as of ribooligonucleotides. The kinetic and thermodynamic data are shown to be consistent with 16S-23S RNA interaction.     

Nitrogenase of Azotobacter chroococcum. Kinetics of the reduction of oxidized iron-protein by sodium dithionite.

The kinetics of the reduction of oxidized Fe-protein of nitrogenase from Azotobacter chroococcum by sodium dithionite were studied by stopped-flow and rapid-freezing e.p.r. (electron-paramagnetic-resonance) spectroscopy. The appearance of the gav. = 1.94 e.p.r. signal (0.24 electron integrated intensity/mol) was associated with a one-electron reduction by SO2--with k greater than 10(8)M-1-S-1 at 23 degrees C. A value of k = 1.75s-1 was obtained for the rate of dissociation of S2O42- into 2SO2-- at 23 degrees C. Further reductions by SO2-- occurred in three slower phases with rate constants in the range 10(4) -10(6)M-1-S-1. These latter phases have no corresponding e.p.r. signal changes and are probably associated with enzymically inactive protein. The high rate of reduction by SO2-- of the Fe-protein alone (k greater than 10(8)M-1-S-1) relative to the rate of oxidation of the Fe-protein in the catalytically active Fe:Mo-Fe protein complex (k = 2.2 X 1O(2)s-1) and the observation that in the steady state the Fe-protein is substantially oxidized means that at normal assay concentrations another reaction must limit the rate of reduction of Fe-protein during turnover.     

Reactivity of cuprous stellacyanin as a quinone and semiquinone reductase

The reactivity of cuprous stellacyanin as a quinone and semiquinone reductase has been examined. Rate constants (25.0 degrees C) measured for the oxidation of stellacyanin by 1,4-benzoquinone and benzosemiquinone are 2.3 X 10(4) M-1 s-1 (delta H not equal to = 4.4 kcal/mol, delta S not equal to = -24 eu) and 5.1 X 10(6) M-1 s-1, respectively [pH 7.0, I = 0.1 M (phosphate)]. The agreement of these rate constants with those calculated on the basis of relative Marcus theory is discussed. Stellacyanin is more effective than laccase in quenching benzosemiquinone, suggesting that the physiological role of this metalloprotein is to regulate the concentration of free radicals generated through the laccase-catalyzed oxidation of phenols.     

Rapid spectral scan and stopped-flow studies of carbon monoxide binding to bovine adrenocortical cytochrome P-450scc

Carbon monoxide binding with both cholesterol-free (low-spin) and cholesterol-bound (high-spin) reduced forms of purified cytochrome P-450scc has been investigated by rapid-scan and stopped-flow spectrometry. CO binding occurs within 150 ms at 25 degrees C for both forms of P-450scc, with a typical absorption maximum at 450 nm. Isosbestic points occur at the following wavelengths: between reduced-CO and reduced cholesterol-free P-450scc at 434 and 471 nm; between reduced-CO and reduced cholesterol-bound P-450scc at 433 and 469 nm. Both the 'on' (k1) and 'off' rate constants (k-1) are found to be independent of pH between pH 5 and 9. The mean values of k1 for cholesterol-free (1.8 +/- 0.2) X 10(5) M-1 X s-1) and cholesterol-bound [1.9 +/- 0.1) X 10(5) M-1 X s-1) P-450scc are almost identical, while the mean value of k-1 for the former [2.3 +/- 0.3) X 10 s-1) is about double that of the latter [1.2 +/- 0.1) X 10 s-1). This suggests the instability of the reduced-CO complex in the absence of cholesterol.     

Reactions of prostaglandin H synthase in the presence of the stabilizing agents diethyldithiocarbamate and glycerol

Both cyclooxygenase and peroxidase reactions of prostaglandin H synthase were studied in the presence and absence of diethyldithiocarbamate and glycerol at 4 degrees C in phosphate buffer (pH 8.0). Diethyldithiocarbamate reacts with the high oxidation state intermediates of prostaglandin H synthase; it protects the enzyme from bleaching and loss of activity by its ability to act as a reducing agent. For the reaction of diethyldithiocarbamate with compound I, the second-order rate constant k2,app, was found to fall within the range of 5.8 x 10(6) +/- 0.4 x 10(6) M-1.s-1 less than k2,app less than 1.8 x 10(7) +/- 0.1 x 10(7) M-1.s-1. The reaction of diethyldithiocarbamate with compound II showed saturation behavior suggesting enzyme-substrate complex formation, with kcat = 22 +/- 3 s-1, Km = 67 +/- 10 microM, and the second-order rate constant k3,app = 2.0 x 10(5) +/- 0.2 x 10(5) M-1.s-1. In the presence of both diethyldithiocarbamate and 30% glycerol, the parameters for compound II are kcat = 8.8 +/- 0.5 s-1, Km = 49 +/- 7 microM, and k3,app = 1.03 x 10(5) +/- 0.07 x 10(5) M-1.s-1. The spontaneous decay rate constants of compounds I and II (in the absence of diethyldithiocarbamate) are 83 +/- 5 and 0.52 +/- 0.05 s-1, respectively, in the absence of glycerol; in the presence of 30% glycerol they are 78 +/- 5 and 0.33 +/- 0.02 s-1, respectively. Neither cyclooxygenase activity nor the rate constant for compound I formation using 5-phenyl-4-pentenyl-1-hydroperoxide is altered by the presence of diethyldithiocarbamate.(ABSTRACT TRUNCATED AT 250 WORDS)     

The reaction of superoxide radical with iron complexes of EDTA studied by pulse radiolysis.

The reactions of Fe3+-EDTA and Fe2+-EDTA with O2- and CO2- were investigated in the pH range 3.8--11.8. Around neutral pH O2- reduces Fe3+-EDTA with a rate constant which is pH dependent kpH 5.8--8.1 = 2 - 10(6)--5 - 10(5) M-1 - s-1. At higher pH values this reaction becomes much slower. The CO2- radical reduces Fe3+-EDTA with kpH 3.8--1- = 5 +/- 1 - 10(7) M-1 - s-1 independent of pH. At pH 9--11.8, Fe2+-EDTA forms a complex with O2- with kFe2+-EDTA + O2 = 2 - 10(6)--4 - 10(6) M-1 - s-1 which is pH dependent. We measured the spectrum of Fe2+-EDTA-O2- and calculated epsilon 290 over max = 6400 +/- 800 M-1 - cm-1 in air-saturated solutions. In O2-saturated solutions another species is formed with a rate constant of 7 +/- 2 s-1. This intermediate absorbs around 300 nm but we were not able to identify it.     

Electrostatic effects in electron transfer reactions of [2Fe-2S] ferredoxins with inorganic reagents.

The kinetics of electron transfer from the reduced [2Fe-2S] ferredoxins from the cyanobacterium Anabaena 7120 and the protozoan Trichomonas vaginalis to select cobalt coordination compounds have been studied in order to gain insight into the mechanism of electron transfer and intrinsic reactivity of [2Fe-2S] active sites. With tripositive cobalt complexes, reactions of both proteins displayed saturation kinetics; values of association constants of 12,900 and 1,400 M-1 and limiting rate constants of 7.6 and 3.5 s-1 were found for oxidation of T. vaginalis and Anabaena ferredoxins, respectively, by Co(NH3)6(3+) at room temperature and I = 0.1 M. An activation enthalpy of 12.1 kcal/mol and activation entropy of -14.3 cal/mol K for oxidation of T. vaginalis ferredoxin by Co(NH3)6(3+) contrasted with corresponding values of 13.4 kcal/mol and -10.5 cal/mol K for the Spirulina platensis protein, which is homologous to Anabaena ferredoxin. The dependence of the reaction rates on ionic strength were measured to probe the importance of electrostatics on the reactivity of the proteins. Analysis of the ionic strength dependence of the oxidation of the proteins by Co(NH3)6(3+) by the "parallel plate" model of Watkins et al. (1994, Protein Sci 3:2104-2114) afforded values for active site charges of -0.7 and -1.1 and limiting rate constants at infinite ionic strength of 25,800 and 76 M-1 S-1 for T. vaginalis and Anabaena ferredoxins, respectively. These results suggest that the [2Fe-2S] center of the protozoal ferredoxin is more accessible and adjacent to a less highly charged, more compact patch of negative charges than the photosynthetic protein.     

Effect of micelle diameter on tryptophan dynamics in an amphipathic helical peptide in phosphatidylcholine

The effect of dimyristoylphosphatidylcholine (DMPC) on the conformation and environment of the single tryptophan residue of a model amphipathic helical polypeptide has been investigated by fluorescence quenching with a water-soluble, neutral quencher (acrylamide) and multiple-frequency phase fluorometry. The peptide H-Ser-Ser-Ala-Asp-Trp-Leu-Lys-Ala-Phe-Tyr-Asp-Lys-Val-Ala-Glu-Lys-Leu-Ly s-Glu- Ala-Phe-Ser-Ser-Ser-OH [18As; Kanellis, P., Romans, A.Y., Johnson, B.J., Kercret, H., Chiovetti, R., Jr., Allen, T.M., & Segrest, S.P. (1980) J. Biol. Chem. 255, 11464] was synthesized by solid-phase techniques. Peptide was incubated at 26 degrees C with DMPC at various peptide:lipid weight ratios. The diameter of the resulting disk-shaped micelles increases with increasing lipid concentration from 12.0 +/- 0.4 nm at a 1:1 weight ratio of peptide to lipid to a maximum of 48.7 +/- 1.0 nm at a 1:13 ratio. At a weight ratio of 1:5, the average diameter is 22.7 +/- 0.6 nm. Decreasing the peptide:lipid ratio of the micelle resulted in a blue-shift in the fluorescence emission maximum (from 337 nm at 1:1 to 334 nm at 1:5), an increase in the fluorescence lifetime of the tryptophan measured by the phase shift method at 18 MHz (from 3.12 ns at 1:1 to 3.61 ns at 1:5), a decrease in the rate of fluorescence quenching by acrylamide (from 0.87 x 10(9) M-1 s-1 at 1:1 to 0.42 x 10(9) M-1 s-1 at 1:5), and an increase in the activation energy for quenching (from 6.7 kcal/mol at 1:1 to 12.7 kcal/mol at 1:5).(ABSTRACT TRUNCATED AT 250 WORDS)     

Reduction of laccase type 1 copper by 3,4-dihydroxyphenylalanine and other catechol derivatives

3,4-Dihydroxyphenylalanine (DOPA) is not a preferred substrate of Rhus vernicifera laccase, as rate constants for the anaerobic reduction of the type 1 cupric atom by L-DOPA (6.3 X 10(1) M-1 s-1), D-DOPA (2.6 X 10(1) M-1 s-1), and L-DOPA methyl ester (2.6 X 10(1) M-1 s-1) are considerably smaller than k1 (catechol) (7 X 10(2) M-1 s-1) and rate constants characteristic of numerous other nonphysiological organic substrates (25 degrees C, pH 7.0, I = 0.5 M). The reactions of DOPA derivatives with laccase are unique, however, in that a two-term rate law pertains: kobsd = k0 + k1[phenol]; k0(L-DOPA) = 7 X 10(-2) s-1. The reactivities of other catechol derivatives (pyrogallol, gallic acid, and methyl gallate) with laccase type 1 copper were also examined.     

A stepwise mechanism for the permeation of phloretin through a lipid bilayer

The thermodynamics of interactions between phloretin and a phosphatidylcholine (PC) vesicle membrane are characterized using equilibrium spectrophotometric titration, stopped-flow, and temperature- jump techniques. Binding of phloretin to a PC vesicle membrane is diffusion limited, with an association rate constant greater than 10(8) M-1s-1, and an interfacial activation free energy of less than 2 kcal/mol. Equilibrium binding of phloretin to a vesicle membrane is characterized by a single class of high-affinity (8 micro M), noninteracting sites. Binding is enthalpy driven (delta H = -4.9 kcal/mol) at 23 degrees C. Analysis of amplitudes of kinetic processes shows that 66 +/- 3% of total phloretin binding sites are exposed at the external vesicle surface. The rate of phloretin movement between binding sites located near the external and internal interfaces is proportional to the concentration of un-ionized phloretin, with a rate constant of 5.7 X 10(4) M-1s-1 at 23 degrees C. The rate of this process is limited by a large enthalpic (9 kcal/mol) and entropic (-31 entropy units) barrier. An analysis of the concentration dependence of the rate of transmembrane movement suggests the presence of multiple intramembrane potential barriers. Permeation of phloretin through a lipid bilayer is modeled quantitatively in terms of discrete steps: binding to a membrane surface, translocation across a series of intramembrane barriers, and dissociation from the opposite membrane surface. The permeability coefficient for phloretin is calculated as 1.9 X 10(-3) cm/s on the basis of the model presented. Structure- function relationships are examined for a number of phloretin analogues.     

The reduction of ferrate(VI) to ferrate(V) by ascorbate

The reduction of ferrate(VI) by ascorbate has been studied under anaerobic conditions in the pH range between 6.8 and 11.5 at 24 degrees C. A mechanism is proposed that is consistent with the observed rate constants k11 (HFeO4- + AH-) = (5.6 +/- 0.6) x 10(6) M-1 s-1, k12(FeO4(2-) + AH-) = (1.3 +/- 0.1) x 10(6) M-1 s-1 and the pK(HFeO4- in equilibrium with H(+) + FeO4(2-) = 7.9. Stoichiometric studies show that at high ratios of [AH-]/[FeO4(2-)], one ferrate(VI) oxidizes three molecules of ascorbate to the corresponding ascorbyl (A-) radicals.     

The DNA guanyl radical: kinetics and mechanisms of generation and repair

The one-electron oxidation of DNA bases and single-stranded DNA was studied by pulse radiolysis of aqueous solutions from pH 7-7.4 at 20 degrees C. Thallic ions, Tl(II), were found to rapidly oxidize the purine nucleotides, deoxyguanosine 5'-monophosphate, k[Tl(II) + dGMP2-] = 3.4.10(9) M-1.s-1, and deoxyadenosine 5'-monophosphate, k[Tl(II) + dAMP2-] = 1.3.10(8) M-1.s-1. The reactivities of Tl(II) ions with model pyrimidine DNA bases, 1-methylcytosine and 1-methylthymine, were too low to be measured by pulse radiolysis, k less than 10(7) M-1.s-1. The Tl(II)-mediated oxidation of ssDNA, k = 2.8.10(8) M-1.s-1, produces DNA-guanyl radical, DNA-G.(-H), exclusively. The DNA-guanyl radical is found to be a potent oxidant in neutral media, E7 = 1.04 +/- 0.05 V. It rapidly oxidizes the aromatic amino acids in glycyl-tryptophan and tyrosine methyl ester, k = 3.6.10(7) M-1.s-1 and k = 1.7.10(8) M-1.s-1, respectively. These electron transfer processes indicate that a positive 'hole' may be transferred from DNA to a DNA-associated protein. The positive 'hole' in DNA can also be repaired by antioxidants, which are electron donors. The chemical repair of the DNA-guanyl radical by negatively charged antioxidants is slower than that by positively charged and neutral antioxidants.     

Reduction of methemerythrin by deoxymyoglobin: a protein-protein redox reaction not involving electron-transfer proteins.

The stoichiometry and kinetics of reaction of methemerythrin with the deoxy forms of myoglobin and hemoglobin have been examined at I = 0.2 M and 25 degrees C. One mole of methemerythrin (on the basis of the monomer unit containing two irons) reacts with 2 mol of deoxymyoglobin and with 0.5 mol of deoxyhemoglobin. All reactions are second order. Rate constants for reaction with deoxymyoglobin are 0.25 M-1s-1 (Phascolopsis gouldii) and 5.6 M-1s-1 (Themiste pyroides) at pH 6.3. There is little effect of raising the ionic strength to 1.35 M and only a small decrease in rate when the pH is adjusted to 8.2. The rate constant for reaction of deoxyhemoglobin with P. gouldii methemerythrin is approximately 0.1 M-1s-1 at pH 6.3. Metmyohemerythrin from T. pyroides reacts slightly slower than the octamer form (k = 2.0 M-1s-1 at pH 6.3 and 7.0). Oxymyoglobin is converted to metmyoglobin by methemerythrin. The electron-transfer path is discussed and a self-exchange rate constant for hemerythrin assessed as 10(-3) M-1s-1 on the basis of Marcus's theory.     

Microcalorimetric method to determine competitive binding. Action of a psychotropic drug (dipotassium chlorazepate) on L-tryptophan . human serum albumin complex.

A mathematical treatment and an original microcalorimetric method are developed to verify an eventual competitive binding between any two substances for the same macromolecule. To apply this method, a competitive binding of L-tryptophan and one benzodiazepin (dipotassium chlorazepate) for human serum albumin is perfectly demonstrated. The association constants and the enthalpy variations are equal to 14 000 +/- 2000 M-1 and --6.6 +/- 0.2 kcal/mol for human serum albumin . tryptophan complex and 13 000 +/- 1000 M-1 and --10.0 +/- 0.2 kcal/mol for human serum albumin . chlorazepate complex. In all cases the stoichiometry is equal to one. The binding of tryptophan to human serum albumin is partially stereospecific; the association constant and the enthalpy variation for D-tryptophan complex are equal, respectively, to 1000 +/- 200 M-1 and --2.6 +/- 0.3 kcal/mol.     

Transferrins, the mechanism of iron release by ovotransferrin.

Iron release from ovotransferrin in acidic media (3 < pH < 6) occurs in at least six kinetic steps. The first is a very fast (相似文献   

Electron paramagnetic resonance, kinetics of formation and decomposition studies of (bis(hydroxyethyl)amino-tris(hydroxymethyl)-methane)oxochromate(V): a model chromium(V) complex for DNA damage studies

A new chromium complex, (bis(hydroxyethyl)amino-tris(hydroxymethyl)methane)oxochromate(V), has been characterized by epr spectroscopy. The chromium(V) complex was formed by the ligand displacement reaction of bis(2-ethyl-2-hydroxybutanato) oxochromate(V) with bis(hydroxyethyl)amino-tris(hydroxy-methyl)methane (BT). Both epr and kinetic data indicate that the reaction proceeds through a chromium(V) intermediate. Kinetics of formation of the intermediate exhibit a rate saturation at higher [BT] (> 30 mM) indicating a rate law constituting an equilibrium between the parent Cr(V) complex and the bis-tris ligand followed by a pure first order process. The g-value of the intermediate is consistent with a Cr(V) complex in which the BT is coordinated in a bidentate fashion replacing a coordinated hydroxy butanoic acid ligand, affording a mixed ligand complex. The equilibrium step (K = 36 M-1) consists of monodentate coordination by the BT ligand and the limiting first order rate constant (1.9 x 10(-2) s-1) manifests the rate of chelation by the polydentate ligand. The intermediate is converted to the product upon further chelation through the complete displacement of the remaining 2-ethyl-2-hydroxy butanoic acid by a first order process (k = 0.023 s-1). The epr data support a pair of products that are in rapid equilibrium. In these products, BT functions either as a tetra or a penta-dentate ligand coordinating through four or five alkoxy sites. The enthalpy and entropy of activations related to the two chelation steps were found to be 32 +/- 2 kJ/mol and -(1.7 +/- 0.2) x 10(2) J/mol K for the intermediate, and 36 +/- 1 kJ/mol and -(1.5 +/- 0.2) x 10(2) J/mol K for the product. Our data support an associative mechanism for the chelation steps. The Cr(V)-BT product is more stable than the parent complex. The second order disproportionation rate constant for the Cr(V)-BT complex was evaluated to be 0.1 M-1 s-1 compared to 8.0 M-1 s-1 for the parent complex. This is the first example of a chromium(V) complex with a non-macrocyclic ligand coordinating through oxygen donor atoms which is stable in aqueous solution at neutral pH over a long period of time.     

Ligand binding equilibrium and kinetic measurements on the dimeric myoglobin of Busycon canaliculatum and the comparative ligand binding of diverse non-cooperative heme proteins

The rate constants and delta H degrees for the non-cooperative dimeric Busycon myoglobin are: oxygen, k' = 4.75 X 10(7) M-1 sec-1, k = 71 sec-1, and CO, l'= 3.46 X 10(5) M-1 sec-1, l = 0.0052 sec-1 at 20 degrees C, pH 7, delta H degrees = -3 kcal/mol for O2 and CO.2. Log-log plots of k vs K for oxygen and of l' vs L for CO binding for numerous non-cooperative hemoglobins and myoglobins point to a large steric influence of the protein on heme ligation reactions. Many of the proteins behave as "R" state for one ligand, but "T" for the other.     

Kinetics of formation of 8-oxy-2'-deoxyguanosine-5'-monophosphate under the effect of heat: determination of rate constants and activation energy]

Rate constants of 8-oxy-dGMP (8-hydroxy-dGMP) formation upon incubating dGMP in H2O solutions at different temperatures were determined with differential UV-spectroscopy. Extrapolation of rate constant values obtained at elevated temperatures to 37 degrees C gives k = 5.8 x 10(-10) s-1.M-1. The activation energy for the process was estimated to be 24 kcal/mole. In D2O solutions essential lowering of the activation energy (13 kcal/mole) and rising of rate constant (k = 3.7 x 10(-9) s-1.M-1 at 37 degrees C) were observed. The strong influence of D2O on the process points to the possible participation of singlet oxygen in a heat-induced formation of 8-oxy-dGMP. The obtained values of rate constants and activation energy induced by heat show that of all types of DNA damages currently known such as single strand scission, depurination, cytosine deamination and oxidation of guanyl residues to the 8-oxo-derivatives- the last process seems to be the strongest damage of DNA resulting in such biological consequences as mutagenesis, carcinogenesis and aging.     

Some redox properties of myohemerythrin from retractor muscle of Themiste zostericola

Distinct semimetmyohemerythrin species are produced by one-electron oxidation of deoxymyohemerythrin and one-electron reduction of metmyohemerythrin. The former, (semimetmyo)o, changes (greater than or equal to 90%) to the latter, (semimetmyo)R, with k = 1.0 x 10(-2) s-1, delta H = 15.1 kcal mol-1 and delta S = -17 eu. Oxidation of (semimetmyo)o by Fe(CN)6(3)- rapidly produces an unstable metmyohemerythrin form which converts to the final metmyohemerythrin with k = 4.6 x 10(-3) s-1, delta H = 16.8 kcal mol-1, and delta S = -13 eu. The two met forms react at the same rate with N3-, but the unstable form reacts very rapidly with S2O4(2-) in contrast to stable metmyohemerythrin. (Semimetmyo)R or a mixture of metmyohemerythrin and deoxymyohemerythrin equilibrate very slowly to a mixture containing all three species. The rate constants for disproportionation and comproportionation are 0.89 M-1 s-1 and 9.4 M-1 s-1, respectively. EPR spectra near liquid He temperatures and optical absorption spectra have been used to characterize and measure the rates at 25 degrees C, pH 8.2, and I = 0.15 M. The comparative behavior of octameric and monomeric protein is discussed.