Table S2.

100 K X-ray crystallographic data collection and refinement statistics

Data and refinement	WT pH 4.0	N44D pH 4.6	N44E-Mes pH6.0
	PDB ID 4XQD	PDB ID 4XQ4	PDB ID 4XQW
Data collection
Space group	C2	C2	P212121
Cell dimensions
a, b, c, Å	80.86, 69.63, 78.19	80.94, 69.75, 78.28	47.99, 59.63, 69.55
α, β, γ, °	90, 117.5, 90	90, 117.5, 90	90, 90, 90
Resolution, Å	20.00–1.50 (1.53–1.50)*	27.20–1.22 (1.29–1.22)	50.00–1.50 (1.53–1.50)
No. reflections unique	56,145 (2,752)	114,048 (16,604)	31,754 (1,494)
Rmerge	0.098 (0.380)	0.113 (0.421)	0.066 (0.427)
I/σ(I)	7.5 (2.1)	4.7 (1.8)	17.0 (3.1)
Completeness, %	91.2 (88.9)	99.6 (99.7)	96.5 (92.7)
Redundancy	2.3 (2.0)	3.3 (3.1)	3.8 (3.8)
Refinement
Resolution, Å	20–1.50	15–1.25	33–1.50
No. reflections	56,100	105,797	31,711
Rwork/Rfree	0.192/0.223	0.141/0.168	0.174/0.179
No. atoms
Protein	2,943	3,042	1,473
Iodide	6	5	5
Ligand	8 (Tris)	N/A	12 (Mes)
Water	459	597	234
B-factors
Protein	17.4	12.7	18.1
Iodide	23.9	16.5	25.2
Ligand	26.8 (Tris)	N/A	18.8 (Mes)
Water	27.4	26.2	28.0
rms deviations
Bond lengths, Å	0.006	0.008	0.006
Bond angles, °	1.122	1.318	1.191

^*Values in parentheses are for the highest-resolution shell. Data were collected from 1 crystal for each structure.