Promiscuous binding in a selective protein: the bacterial Na+/H+ antiporter.
The ability to discriminate between highly similar substrates is one of the remarkable properties of enzymes. For example, transporters and channels that selectively distinguish between various solutes enable living organisms to maintain and control their internal environment in the face of a constantly changing surrounding. Herein, we examine in detail the selectivity properties of one of the most important salt transporters: the bacterial Na+/H+ antiporter. Selectivity can be achieved at either the substrate binding step or in subsequent antiporting. Surprisingly, using both computational and experimental analyses synergistically, we show that binding per se is not a sufficient determinant of selectively. All alkali ions from Li+ to Cs+ were able to competitively bind the antiporter's binding site, whether the protein was capable of pumping them or not. Hence, we propose that NhaA's binding site is relatively promiscuous and that the selectivity is determined at a later stage of the transport cycle.
Department of Biological Chemistry, The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel.
Raphael
R
Alhadeff
Assaf
A
Ganoth
Miriam
M
Krugliak
Isaiah T
IT
Arkin
D002412:Cations; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D005453:Fluorescence; D056004:Molecular Dynamics Simulation; D011485:Protein Binding; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D013816:Thermodynamics
D002412:Cations; D029968:Escherichia coli Proteins; C102839:NhaA protein, E coli; D017923:Sodium-Hydrogen Exchangers