Abstract
Molybdenum (Mo) is an essential trace element for bacteria that is utilized in myriad metalloenzymes that directly couple to the biogeochemical cycling of nitrogen, sulfur, and carbon. In particular, Mo is found in the most common nitrogenase enzyme, and the scarcity and low bioavailability of Mo in soil may be a critical factor that contributes to the limitation of nitrogen fixation in forests and agroenvironments. To overcome this scarcity, microbes produce exudates that specifically chelate scarce metals, promoting their solubilization and uptake. Here, we have determined the structure and stability constants of Mo bound by protochelin, a siderophore produced by bacteria under Mo-depleted conditions. Spectrophotometric titration spectra indicated a coordination shift from a catecholate to salicylate binding mode for MoVI-protochelin (Mo-Proto) complexes at pH < 5. pKa values obtained from analysis of titrations were 4.8 ± 0.3 for MoVIO2H3Proto− and 3.3 ± 0.1 for MoVIO2H4Proto. The occurrence of negatively charged Mo-Proto complexes at pH 6 was also confirmed by mass spectrometry. K-edge Extended X-ray absorption fine structure spectroscopy confirmed the change in Mo coordination at low pH, and structural fitting provides insights into the physical architecture of complexes at neutral and acidic pH. These findings suggest that Mo can be chelated by protochelin across a wide environmental pH range, with a coordination shift occurring at pH < 5. This chelation and associated coordination shift may impact biological availability and mineral surface retention of Mo under acidic conditions.
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Acknowledgements
We thank United States Department of Agriculture—National Institute of Food and Agriculture (USDA-NIFA) Project 2019-06522 for support. This work was supported by the USDA National Institute of Food and Agriculture, Hatch projects NC02440 and NC02713.Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.
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Doydora, S.A., Baars, O., Harrington, J.M. et al. Salicylate coordination in metal-protochelin complexes. Biometals 35, 87–98 (2022). https://doi.org/10.1007/s10534-021-00352-7
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DOI: https://doi.org/10.1007/s10534-021-00352-7