Table 1.

Reported selectivity of various proton conducting molecules

Channel	Ratio	Reference Ion	Criterion	Key Residue(s)	Reference Nos.
Voltage-gated H+ HV1	>2 × 108	PD/PTMA	Vrev	Asp112 (hHV1)	107, 362
	≥107	PH/PTMA	Vrev	Asp51 (kHV1)	74, 114, 462
	≥106	PH/PCs,K,Na	Vrev		124, 187, 250
	≥1.8 × 106	PH/PCl	Vrev		187
Elodea H+	108 to 109	PH/PK	Vm	—	328
Channelrhodopsin-2	3–6 × 106	PH/PNa	Vrev	Glu90	32, 142
Channelrhodopsin-1	High	PH/PNa	Vrev	Glu162	364
M2 (full length)	107	PH/PK	Flux	His37	331
	104 to 106	PH/PK	Flux		288
M2 (residues 22–62)	3.6 × 105	PH/PK	Flux		402
BM2	High	PH/PK	Vrev	His19	310, 345
TRP-ML1	104 to 106	PH/PCs	Vrev	—	467
TRPM7	High	PH/PNMDG	Vrev	Asp1054	242, 386
Sour taste cell proton current	High	PH/PNa	Vrev		67
Sugar sensor SGLT3	High	PH/PNa	Vrev	Glu457	36
Arachidonic acid-induced, EAAT4 glutamate transporter	High	PH/PNa	Vrev	—	152
Colicin A	1.2 × 104	PH/PK	Vrev	—	459
Mutant aquaporin	8–34 × 103	PH/PK	I	R195V, R195S	295, 524
Taste receptor	4 × 103	PH/PK	Vrev	—	261
TRPV1	87–1,127	PH/PCs	Vrev		205, 509
Na+ (V-gated)	252–274	PH/PNa	Vrev		346, 347
Gramicidin A	43–344	PH/PNa	Vrev		363
Synthetic proton channel (LSLLLSL)3	>40	gH/gLi	iH detection	none	283
Na+ (amiloride sensitive)	7–25	PH/PNa	Vrev		181, 394
ASIC1a	≥5	PH/PNa	Vrev	—	69

Permeability ratios were determined from measured V_rev values using the Goldman-Hodgkin-Katz (GHK) equation (216), from the assumption that membrane potential V_m measurements reflect a diffusion potential generated by the cations present, from detection of single-channel currents i_H, from I using the GHK current equation, or from flux estimates. D, deuterium; NMDG, N-methyl-d-glucamine. It should be noted that despite the lower limits given in this table, there is no evidence that H_V1 is less than perfectly selective.