Extended Data Table 1 ∣. Equilibrium dissociation constants (K_d) of M^II-(AB)₂ complexes.

Metal-competition assays were performed using Fura-2 in Chelex100-treated buffer solution (pH 7.4) containing 20 mM MOPS and 150 mM NaCl. K_d values in 20 mM NH₄HCO₃ were determined using Mag-Fura-2 (Co^II and Ni^II) and Newport Green DCF (Cu^II) as metal-binding competitor. Errors are derived from non-linear least square fitting analyzed in Dynafit.

		(AB)2	(AB)2 (20 mM NH4HCO3)
Mn II	K d1	2.7 (±0.4) × 10−6
	K d2	4.3 (±1.1) × 10−6
Co II	K d1	1.1 (±0.2) × 10−8	5.2 (±1.8) × 10−8
	K d2	4.1 (±0.7) × 10−8	1.1 (±0.3) × 10−7
Ni II	K d1	4.5 (±1.1) × 10−9	3.0 (±0.7) × 10−8
	K d2	7.6 (±1.0) × 10−9	6.1 (±0.7) × 10−8
Cu II	K d1	2.9 (±0.5) × 10−13	4.3 (±1.4) × 10−9
	K d2	-	-
Zn II	K d1	1.4 (±0.3) × 10−8
	K d2	1.3 (±0.4) × 10−8