TABLE 1

All GPCR structures resolved at 3.1 Å or better, with small ions bound to 7TM domain

Receptor	PDB code	Resolution Range, Å	Ion	Site
A_2AAR	4EIY, 5IU4, 5IU7, 5IU8, 5IUA, 5IUB, 5K2A, 5K2B, 5K2C, 5K2D, 5MZJ, 5MZP, 5N2R, 5NLX, 5NM2, 5NM4, 5OLG, 5OLH, 5OLV, 5OLZ, 5OM1, 5OM4, 5VRA	1.7–2.8	Na⁺	D2.50, conserved, tight binding^{^a}
DRD4	5WIV	2.1	Na⁺	D2.50, conserved, tight binding
δ-OR	4N6H, 4RWD	1.8, 2.7	Na⁺	D2.50, conserved, tight binding
PAR1	3VW7	2.2	Na⁺	D2.50, conserved, tight binding
PAR2	5NDD	2.8	Na⁺	D2.50, conserved, tight binding
CLTR1	6RZ4, 6RZ5	2.5–2.7	Na⁺	D2.50, conserved, tight binding
β1AR	3ZPR, 4BVN, 5A8E	2.1–2.7	Na⁺	D2.50, conserved, tight binding
β1AR	2VT4, 2Y00, 2Y02, 2Y03, 2Y04, 2YCW, 3ZPQ, 3ZPR, 4AMJ, 4BVN, 5A8E, 6H7J, 6H7K, 6H7L, 6H7M, 6H7N, 6H7O	2.1–3.1	Na⁺	ECL2, backbone, tight binding
β2AR	4LDE, 4LDL	2.8, 3.1	Na⁺	ECL2, backbone, tight binding
SMO, class F	5L7D	3.2	Na⁺	Loose binding
A_2AAR	5OM1	2.1	Cl⁻	Helices VI and VIII (H230, R293), loose binding
PAR1	3VW7	2.2	Cl^-	Helices II, III, VI, (K135, R200, K307), loose binding
Rhodopsin	1GZM, 1F88, 1U19, 1L9H, 1HZX, 2HPY, 2G87, 2PED, 3C9L, 3OAX	2.2–3.0	Zn²⁺	Additive in crystallization protocols. Loose binding in multiple sites.
PAF	5ZKQ	2.9	Zn²⁺	Helices I, VI, VII (H4, H8, E259, H268), tight binding
SMO, class F	4QIM, 4N4W	2.6, 2.8	Zn²⁺	Loose binding
M2R	5YC8	2.5	Hg²⁺	Used as anomalous scatterers for phasing. Loose binding in multiple sites.
Rhodopsin	1U19, 1L9H, 1HZX, 1F88, 2PED, 2G87, 2HPY, 3OAX	2.2–3.0	Hg²⁺
5HT_2B	6DRY, 6DRZ	2.9, 3.1	PO₄³⁻	Loose binding
DRD4	5WIU, 5WIV	2, 2.1	PO₄³⁻	ICL2 and Helix III, tight binding
H1R	3RZE	3.1	PO₄³⁻	7TM bundle, ECL, (K179, K191, Y431, H450), tight binding
μ-OR	5C1M	2.1	PO₄³⁻	Loose binding
A_2AAR	3EML	2.6	SO₄²⁻	Loose binding
CB1	5U09	2.6	SO₄²⁻	Loose binding
CCR2	5T1A	2.8	SO₄²⁻	Helix VIII, ICL1, tight binding
ETB1	5X93, 5GLI	2.2, 2.5	SO₄²⁻	Inside 7TM, intracellular (R199, R208, K210), tight binding, common^b
PD2	6D27, 6D26	2.7, 2.8	SO₄²⁻	Inside 7TM, intracellular (R143, R2243), tight binding, common^b
PD2	6D27, 6D26	2.7, 2.8	SO₄²⁻	+ Other loose binding sites
EP3	6M9T	2.5	SO₄²⁻	Loose binding
Rhodopsin	3PQR, 4J4Q, 4PXF, 5DYS, 5EN0, 5TE3	2.3–2.9	SO₄²⁻	Loose binding
β2AR	2RH1, 5D5A	2.4, 2.5	SO₄²⁻	Inside 7TM, intracellular (T68, R131), tight binding, common^b
β2AR	2RH1, 5D5A	2.4, 2.5	SO₄²⁻	VI, VII (K270, K273, R328), tight binding
μ-OR	4DKL	2.8	SO₄²⁻	Inside 7TM, intracellular (T103, R179), tight binding, common^b
μ-OR	4DKL	2.8	SO₄²⁻	Other six loose binding sites

↵^a Directly coordinated by two or more ionic interactions with charged residues.
↵^b SO₄²⁻ bound at a common intracellular site in several receptors.