Organometallic interaction of group 12 metals by Methanobactin chalkophore from Methylocystis sp. SB2
Excitation emission spectrum and a schematic illustration of the proposed molecular mechanism for a notable fluorescence enhancement with transition metal complexation. [Reprinted with permission by Elsevier from Eckert, P. A., et al. 2021.]
Methanotrophic bacteria catalyze the aerobic oxidation of methane to methanol using enzymes with copper (Cu)- based active sites. To facilitate the acquisition of Cu ions some methanotrophic bacteria secrete small postranslationally modified peptides known as methanobactins. Analogous to siderophores and iron, methanobactins strongly bind Cu ions and function as an extracellular Cu recruitment relay. In addition to binding Cu, methanobactins will bind most transition metals and near-transition metals and protect the host methanotroph as well as other bacteria from metal toxicity. Investigating the mechanisms of methanobactin-metal interactions is essential for understanding chemical speciation, competitive interactions, and biological processes involved in metal transformations.
This work offers insights into mechanistic aspects of transition metal complexation by mb-SB2 and demonstrates its influence on the speciation and biogeochemical cycling of Hg and other transition metals. Results suggest that small metal-active peptides need to be considered in evaluating biological processes participating in metal transformations in natural environments and lay a groundwork for spectroscopic analysis of the peptide and its complexes.
Methanotrophic bacteria are typically found at oxicanoxic interfaces in wetlands, soils, and aquatic systems and thus may have significant influence on the biogeochemical cycling of Hg and other metals. We characterized the interactions of methanobactin from Methylocystis sp. SB2 (mb-SB2) with transition metals using UV-Vis absorbance, fluorescence, and extended X-ray absorption fine structure spectroscopy (EXAFS) complemented by time-dependent density functional theory (TD-DFT) calculations. The metal binding site in mb-SB2 is comprised of two enethiolate groups, each conjugated with nitrogen-containing heterocycles, which facilitate interactions with a wide range of transition metal ions. The complexation of metal ions is reflected in the electronic structure of the conjugated system. Our spectro- scopic data shows that mb–SB2–metal complexes may assume a range of intra- and intermolecular configurations that are distinct for each metal and depend on the metal to methanobactin ratio. We further report time-dependent changes in sample absorbance and fluorescence spectra, which occur on a wide range of experimental timescales. EXAFS data and TD-DFT calculations are consistent with tetrahedral coordination for Zn2+, Cd2+ and linear coordination for Hg2+. Furthermore, we propose a mechanism of complexation-hindered isomerization for a fluorescence enhancement observed upon the interaction of methanobactins with transition metals. This work represents the first combined computational and experimental spectroscopy study of methanobactins complexes with transition metals. Our results suggest that the methanobactins may influence the speciation and biogeochemical cycling of several group 11 and 12 transition metals.
Eckert, P., A. Johs, J. D. Semrau, A. A. DiSpirito, J. Richardson, R. Sarangi, E. Herndon, B. Gu, and E. M. Pierce. 2021. “Spectroscopic and computational investigations of organometallic complexation of group 12 transition metals by methanobactins from Methylocystis sp. SB2.” Journal of Inorganic Biochemistry. 222:(111496). DOI:10.1016/j.jinorgbio.2021.111496.
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