Modulation of the protein environment in the hydrophilic pore of the ammonia transporter protein AmtB upon GlnK protein binding

The conduction of ammonia/ammonium (NH3/NH4+) through the channel protein AmtB is inhibited by the binding of the signal transduction protein GlnK. In the AmtB-GlnK binding interface, there exists an NH3/NH4+ binding site - Am6. The calculated pKa values at the Am6 sites in both the AmtB-GlnK complex and isolated AmtB implies the dominance of an uncharged NH3 state. The GlnK protein binding causes a significant downshift in the Am6 pKa value of the AmtB. However, this downshift is perfectly compensated by the reorientation of the protein backbone (carbonyl group of Cys312 from the AmtB part) upon AmtB-GlnK complex formation.