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- W2092309112 abstract "Stopped-flow spectrofluorometry has been used to study the rates of formation and breakdown of the complex between Escherichia coli citrate synthase (EC 4.1.3.7) and its allosteric inhibitor, NADH. For NADH concentrations between 10 and 50 μM, the values of k obs (the pseudo first-order rate constant for the overall reaction of enzyme with NADH) increase linearly with NADH concentration. The rate of complex formation is, therefore, characterized by an apparent second-order rate constant (k F ) which has the value 4.2 × 10 5 M −1 s −1 at pH 6.2 and 21 °C. The size of k F decreases as the pH is raised and can be explained by assuming a process whose rate is proportional to the concentration of an acidic group having pK A = 7.8 ± 0.2. The experimental enthalpy of activation for k F , measured at pH 7.8 between 12.0and 35.5 °C, is 18 ± 8 kJ mol −1 .The breakdown of the NADH–enzyme complex is similarly characterized by an apparent first-order rate constant (k B ). This has the value 0.4 s −1 at pH 6.2 and 21 °C, and increases slightly as the pH is raised. For pH values greater than 8.0, 'pH-jump' experiments indicate a limiting value of at least 3.3 s −1 for k B . The enthalpy of activation of k B as measured by 'pH-jump' experiments is 48 ± 4 kJ mol −1 at pH 8.36, over the temperature range from 4.0 to 23.0 °C. The same enthalpy value is obtained from the rate versus concentration experiments at pH 7.80 and temperatures of 16.0–35.5 °C. Below 16 °C, however, the Eyring plot used to obtain the activation enthalpy becomes increasingly steep at pH 7.80, an observation that suggests that a multistep process is being observed.KCl, an allosteric activator of citrate synthase, does not affect the rate of complex formation but appears to operate by decreasing the stability of the NADH–enzyme complex, so that the complex breaks down more quickly as the KCl concentration is raised." @default.
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- W2092309112 date "1977-07-01" @default.
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- W2092309112 title "Stopped flow fluorescence studies on the binding of reduced nicotinamide adenine dinucleotide to citrate synthase of Escherichia coli: effects of pH" @default.
- W2092309112 doi "https://doi.org/10.1139/o77-103" @default.
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