The kinetics of the recombination reaction between apomyoglobin and alkaline haematin.
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ABSTRACT:
SUBMITTER: Adams PA
PROVIDER: S-EPMC1164671 | biostudies-other | 1977 Apr
REPOSITORIES: biostudies-other
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Project description:A simple rapid-mixing technique is described which allows the recombination reaction between apomyoglobin and haemin to be studied within 0.3s of the splitting of myoglobin by dilute HCl. Evidence is presented that indicates that the recombination process occurs between folded 'native' apomyoglobin and monomeric haemin. Postulation of a one (or more)-intermediate recombination process, as suggested by other studies, is not necessary to explain the results. The effect, on the kinetics and mechanism of recombination, of the time of exposure to acid pH of the split myoglobin solution was investigated. The effect of temperature on the recombination kinetics was also studied.
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Project description:1. Pyridine reacts with alkaline haematin to form a dipyridine dihydroxy dimeric haematin in which there is no competition between pyridine and OH(-) for coordination positions on the iron of haematin. 2. Imidazole competes directly with OH(-) to form the monomeric imidazole parahaematin in alkaline media. 3. The monomeric imidazole parahaematin aggregates readily to form a species that is precipitated on standing. 4. A structure is proposed for the haematin-pyridine compound in alkaline media.
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Project description:1. The steady-state rate of hydrolysis of 2,4-dinitrophenyl phosphate catalysed by Escherichia coli phosphatase is identical with that of 4-nitrophenyl phosphate over the pH range 5.5-8.5. 2. The increase in the rate of the enzyme-catalysed decomposition of nitrophenyl phosphates in the presence of tris at pH8.1 and 5.9 is consistent with the hypothesis that tris increases the rate of decomposition of a phosphoryl-enzyme intermediate. At pH8.1 the rate of decomposition of the phosphoryl-enzyme is approximately twice as fast as the rate of its formation, whereas at pH5.9 the rate of formation of the phosphoryl-enzyme is considerably faster than its decomposition. 3. Pre-steady-state measurements of the initial transient of the liberation of 2,4-dinitrophenol during the reaction of the enzyme with 2,4-dinitrophenyl phosphate confirmed the above pH-dependence of the ratio of the rates of phosphorylation and dephosphorylation of the enzyme. At optimum pH (above pH8), when the phosphorylation of the enzyme by the substrate is rate-determining, this step must be controlled by a rearrangement of the enzyme or enzyme-substrate complex.
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Project description:The kinetics of the reaction between O(2) and haemoglobin bound to haptoglobin 1-1 were investigated by the stopped-flow and temperature-jump techniques. The reaction, which follows second-order kinetics in the lower concentration range, becomes independent of O(2) concentration above about 150mum-O(2).
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Project description:The potential energy surface (PES) of the C6H5 + NH2 reaction has been investigated by using ab initio CCSD(T)//B3LYP/6-311++G(3df,2p) calculations. The conventional transition-state theory (TST) and the variable reaction coordinate-TST (VRC-TST) have been used to predict the rate constants for the channels possessing tight and barrierless transition states, respectively. The Rice-Ramsperger-Kassel-Marcus/Master equation (RRKM/ME) theory has been utilized to determine the pressure-dependent rate constants for these reactions. The PES shows that the reaction begins with an exothermic barrierless addition of NH2 to C6H5 producing the vital intermediate state, namely, aniline (C6H5NH2, IS1). Once IS1 is generated, it can further isomerize to various intermediate states, which can give rise to different products, including PR4 (4,5,6-trihydro-1-amino phenyl + H2), PR5 (3,4,5,6-tetrahydro phenyl + NH3), PR6 (2,3,5,6-tetrahydro-1-imidogen phenyl + H2), PR9 (3,4,5,6-tetrahydro-1-imidogen phenyl + H2), and PR10 (2,5,6-trihydro-1-amino phenyl + H2), of which the most stable product, PR5, was formed by the most favorable channel going through the two advantageous transition states T1/11 (-28.9 kcal/mol) and T11P5 (-21.5 kcal/mol). The calculated rate constants for the low-energy channel, 1.37 × 10-9 and 2.16 × 10-11 cm3 molecule-1 s-1 at T = 300, P = 1 Torr and T = 2000 K, P = 760 Torr, respectively, show that the title reaction is almost pressure- and temperature-dependent. The negative temperature-dependent rate coefficients can be expressed in the modified Arrhenius form of k 1 = 8.54 × 1013 T -7.20 exp (-7.07 kcal·mol-1/RT) and k 2 = 2.42 × 1015 T -7.61 exp (-7.75 kcal·mol-1/RT) at 1 and 10 Torr, respectively, and in the temperature range of 300-2000 K. The forward and reverse rate coefficients as well as the high-pressure equilibrium constants of the C6H5 + NH2 ↔ IS1 process were also predicted; their values revealed that its kinetics do not depend on pressure at low temperature but strongly depend on pressure at high temperature. Moreover, the predicted formation enthalpies of reactants and the enthalpy changes of some channels are in good agreement with the experimental results.
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Project description:Understanding arsenic reactions with ferric hydroxides is important in understanding arsenic transport in the environment and in designing systems for removing arsenic from potable water. Many experimental studies have shown that the kinetics of arsenic adsorption on ferric hydroxides is biphasic, where a fraction of the arsenic adsorption occurs on a time scale of seconds while full equilibrium may require weeks to attain. This research employed density functional theory modeling in order to understand the mechanisms contributing to biphasic arsenic adsorption kinetics. The reaction energies and activation barriers for three modes of arsenate adsorption to ferric hydroxides were calculated. Gibbs free energies of reaction depended on the net charge of the complexes, which is a function of the system pH value. Physical adsorption of arsenate to ferric hydroxide proceeded with no activation barrier, with Gibbs free energies of reaction ranging from -21 to -58 kJ/mol. The highest Gibbs free energies of reaction for physical adsorption resulted from negative charge assisted hydrogen bonding between H atoms on the ferric hydroxide and O atoms in arsenate. The conversion of physically adsorbed arsenate into monodentate surface complexes had Gibbs free energies of activation ranging from 62 to 73 kJ/mol, and Gibbs free energies of reaction ranging from -23 to -38 kJ/mol. The conversion of monodentate surface complexes to bidentate, binuclear complexes had Gibbs free energies of activation ranging from 79 to 112 kJ/mol and Gibbs free energies of reaction ranging from -11 to -55 kJ/mol. For release of arsenate from uncharged bidentate complexes, energies of activation as high as 167 kJ/mol were encountered. Increasingly negative charges on the complexes lowered the activation barriers for desorption of arsenate, and in complexes with -2 charges, the highest activation barrier was 65 kJ/mol. This study shows that the slow kinetics associated with arsenic adsorption and desorption can be attributed to the high Gibbs free energies of activation for forming and breaking bonds with the ferric hydroxide.
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