Redox potential changes during ATP-dependent corrinoid reduction determined by redox titrations with europium(II)-DTPA

Corrinoids are essential cofactors of enzymes involved in the C₁ metabolism of anaerobes. The active, super-reduced [Co^I ] state of the corrinoid cofactor is highly sensitive to autoxidation. In O-demethylases, the oxidation to inactive [Co^II ] is reversed by an ATP-dependent electron transfer catalyzed by the activating enzyme (AE). The redox potential changes of the corrinoid cofactor, which occur during this reaction, were studied by potentiometric titration coupled to UV/visible spectroscopy. By applying europium(II)-diethylenetriaminepentaacetic acid (DTPA) as a reductant, we were able to determine the midpoint potential of the [Co^II ]/[Co^I ] couple of the protein-bound corrinoid cofactor in the absence and presence of AE and/or ATP. The data revealed that the transfer of electrons from a physiological donor to the corrinoid as the electron-accepting site is achieved by increasing the potential of the corrinoid cofactor from -530 ± 15 mV to -250 ± 10 mV (E_SHE , pH 7.5). The first 50 to 100 mV of the shift of the redox potential seem to be caused by the interaction of nucleotide-bound AE with the corrinoid protein or its cofactor. The remaining 150-200 mV had to be overcome by the chemical energy of ATP hydrolysis. The experiments revealed that Eu(II)-DTPA, which was already known as a powerful reducing agent, is a suitable electron donor for titration experiments of low-potential redox centers. Furthermore, the results of this study will contribute to the understanding of thermodynamically unfavorable electron transfer processes driven by the power of ATP hydrolysis.