. 2021 Apr 28;296:100724. doi: 10.1016/j.jbc.2021.100724

Table 1.

Cavity residence times of Na⁺ and anion substrate (in ns) in the tested hAE1 and hNBCe1 systems during the 250 ns long MD trajectories

System	Bound ions	Mutation/protonation	Anion binding, ns	Na⁺ binding, ns
hAE1	+HCO₃⁻ (Trial 1)	-	250	-
	+HCO₃⁻ (Trial 2)	-	250	-
	+Cl⁻ (Trial 1)	-	250	-
	+Cl⁻ (Trial 2)	-	30^a	-
	+Na⁺+CO₃²⁻	-	250	250
	+CO₃²⁻	-	250	-
	+Na⁺	-	-	0.5
	+Na⁺+Cl⁻	-	78.5^a	14
	+Na⁺+HCO₃⁻	-	238	3.5
	+HCO₃⁻	protE681	43	-
	+Cl⁻	protE681	54.5^a	-
hNBCe1	+Na⁺+CO₃²⁻	-	250	250
	+HCO₃⁻	-	72.5	-
	+Cl⁻	-	7^a	-
	+CO₃²⁻	-	3	-
	+Na⁺	-	-	40
	+Na⁺+Cl⁻	-	9^a	21.5
	+Na⁺+HCO₃⁻	-	40	35
	+Na⁺+CO₃²⁻	protD754	250	46.5

hAE1, human anion exchanger 1; hNBCe1, human electrogenic sodium bicarbonate cotransporter 1; MD, molecular dynamics; OF, outward-facing.

Indicates the cavity residence time of the Cl⁻ ion, bound to the protein at the very beginning of the MD simulation. This Cl⁻ ion is eventually replaced by another Cl⁻ from the surrounding solution. The Cl⁻ ions from the solution permeate freely the large hydrated OF cavities of hAE1 and hNBCe1, and there are multiple binding/unbinding events involving different Cl⁻ ions (see Fig. S4 for more details). Despite their mobility, a Cl⁻ ion is present at site S1 or S2 for the majority of the MD steps in the hAE1 systems leading to a pronounced anion density at these sites.