Structure of a Zinc Porphyrin-Substituted Bacterioferritin and Photophysical Properties of Iron Reduction
Year: 2020
Type: article
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.
Source: Biochemistry
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
Funders: Welch Foundation, University of Texas at San Antonio, National Institutes of Health, American Chemical Society Petroleum Research Fund, National Cancer Institute, National Institute of General Medical Sciences, National Center for Research Resources, National Center for Advancing Translational Sciences
Awards: Mays Cancer Center at UT Health SA, G2575385162, Pixel Array Detector for X-ray Crystallography, G3434907953, Institute for Integration of Medicine & Science: A Partnership to Improve Health, G3953979934, UTSA RISE Research Training Program, NE-CAT: A Resource for Advanced Macromolecular Crystallography
OpenAlex ID: W3015798351