College of Biological Sciences – BioTechnology Institute

University of Minnesota

"Exquisite Molecular Specificity: the Importance of a Short Hydrogen Bond in Bacterial Phosphate Transport" 

Arsenate and phosphate are abundant on our planet and exhibit tremendous similarities: nearly identical pK_a values, similarly charged oxygen atoms, and thermochemical radii that differ by only 4%. However, phosphate is indispensable whereas arsenate is toxic. Can proteins discriminate between these two anions, and how would they do so?

We focused on the cellular phosphate uptake mechanism that faces this challenge, especially in arsenate-rich environments. More precisely, we examined the periplasmic Phosphate-Binding Proteins (PBPs) of the ABC-type transport system that mediates phosphate uptake into bacterial cells. We were able to show that all tested PBPs can efficiently discriminate phosphate over arsenate by at least 500-fold. We have solved the structure of one of these PBPs, bound with phosphate and arsenate, at sub-Ångstrom resolutions (0.88-0.98Å). Both anions are bound and immobilized by a constellation of dipole-anion interactions and unfavorable interactions. This binding mode is responsible for the 4% larger arsenate distorting a short, unique hydrogen bond.

Investigations of the chemical nature of the short hydrogen bond using sub-Ångstrom X-ray crystallography, ¹H NMR spectroscopy, H/D isotopic effect analysis on the wild-type protein and mutants reveal that this interaction is (i) an asymmetrical, Low Barrier Hydrogen Bond (LBHB), (ii) sensitive to the pK_a values of the involved heavy atoms, and that (iii) perturbations of chemical environment affects bond length, bond strength, as well as key properties of the phosphate transporter. In fact, we find that this interaction is fined-tuned to balance the affinity and the discrimination properties of the transporter, allowing bacteria to thrive in a variety of environments, including in arsenate-rich niches.

To learn more about Dr. Elias’ research interests, please visit his webpage https://www.eliaslab.org/

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