Electron Exchange between Fe(II)-Horse Spleen Ferritin and Co(III)/Mn(III) Reconstituted Horse Spleen andAzotobacter vinelandiiFerritins

Abstract

Azotobacter vinelandii bacterioferritin (AvBF) containing 800-1500 Co or Mn atoms as Co(III) and Mn(III) oxyhydroxide cores (Co-AvBF, Mn-AvBF) was synthesized by the same procedure used previously for horse spleen ferritin (HoSF). The kinetics of reduction of Co-AvBF and Mn-AvBF by ascorbic acid are first-order in each reactant. The rate constant for the reduction of Mn-AvBF (8.52 M(-1) min(-1)) is approximately 12 times larger than that for Co-AvBF (0.72 M(-1) min(-1)), which is consistent with a previous observation that Mn-HoSF is reduced approximately 10-fold faster than Co-HoSF [Zhang, B. et al. (2005) Inorg. Chem. 44, 3738-3745]. The rates of reduction of M-AvBF (M = Co and Mn) are more than twice that for the reduction of the corresponding M-HoSF. HoSF containing reduced Fe(II) cores (Fe(II)-HoSF), prepared by methyl viologen and CO, also reduces M-HoSF and M-AvBF species, with both cores remaining within ferritin, suggesting that electrons transfer through the ferritin shell. Electron transfer from Fe(II)-HoSF to Co-AvBF occurs at a rate approximately 3 times faster than that to Co-HoSF, indicating that the Co cores in AvBF are more accessible to reduction than the Co cores in HoSF. The presence of nonconductive (SiO2) or conductive (gold) surfaces known to bind ferritins enhances the rate of electron transfer. A more than approximately 4-fold increase in the apparent reaction rate is observed in the presence of gold. Although both surfaces (SiO2 and gold) enhance reaction by providing binding sites for molecular interaction, results show that ferritins with different mineral cores bound to a gold surface transfer electrons through the gold substrate so that direct contact of the reacting molecules is not required.