Abstract:
:The iron storage protein bacterioferritin (Bfr) binds up to 12 hemes b at specific sites in its protein shell. The heme b can be substituted with the photosensitizer Zn(II)-protoporphyrin IX (ZnPP), and photosensitized reductive iron release from the ferric oxyhydroxide {[FeO(OH)] n } core inside the ZnPP-Bfr protein shell was demonstrated [Cioloboc, D., et al. (2018) Biomacromolecules19, 178-187]. This report describes the X-ray crystal structure of ZnPP-Bfr and the effects of loaded iron on the photophysical properties of the ZnPP. The crystal structure of ZnPP-Bfr shows a unique six-coordinate zinc in the ZnPP with two axial methionine sulfur ligands. Steady state and transient ultraviolet-visible absorption and luminescence spectroscopies show that irradiation with light overlapping the Soret absorption causes oxidation of ZnPP to the cation radical ZnPP•+ only when the ZnPP-Bfr is loaded with [FeO(OH)] n . Femtosecond transient absorption spectroscopy shows that this photooxidation occurs from the singlet excited state (1ZnPP*) on the picosecond time scale and is consistent with two oxidizing populations of Fe3+, which do not appear to involve the ferroxidase center iron. We propose that [FeO(OH)] n clusters at or near the inner surface of the protein shell are responsible for ZnPP photooxidation. Hopping of the photoinjected electrons through the [FeO(OH)] n would effectively cause migration of Fe2+ through the inner cavity to pores where it exits the protein. Reductive iron mobilization is presumed to be a physiological function of Bfrs. The phototriggered Fe3+ reduction could be used to identify the sites of iron mobilization within the Bfr protein shell.
journal_name
Biochemistryjournal_title
Biochemistryauthors
Benavides BS,Valandro S,Cioloboc D,Taylor AB,Schanze KS,Kurtz DM Jrdoi
10.1021/acs.biochem.9b01103subject
Has Abstractpub_date
2020-04-28 00:00:00pages
1618-1629issue
16eissn
0006-2960issn
1520-4995journal_volume
59pub_type
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