. Author manuscript; available in PMC: 2013 Jan 8.

Published in final edited form as: Nature. 2009 Mar 19;458(7236):305–309. doi: 10.1038/nature07841

Table 1.

Heme iron ligand on and off rates and equilibrium constants in natural and artificial proteins

Heme protein	Ligation	k_{His on} (s^-1)	k_{His off} (s^-1)	k_{CO on} (uM^-1 s^-1)	k_{CO off} (s^-1)	K_dCO (nM)	k_{O2 on} (uM^-1 s^-1)	k_{O2 off} (s^-1)	K_dO2 (nM)	K_dO2/ K_dCO
6 (heme B)	bis-his	310	17		0.034			0.1
Single His (Apparent)^*				0.95 (0.006)		36 (6000)	0.31 (0.017)		32 (600)	(0.1)
Neuroglobin³¹	bis-his	2000	4.5	65 (0.15)	0.014	0.21 (93)	250 (0.56)	0.8	3.2 (1400)	(15)
Myoglobin ⁴⁷	his-aquo (distal his)	n/a	n/a	0.5	0.019	37	14	12	860	23
Human hemoglobin ⁴⁸	his-aquo (distal his)	n/a	n/a	2.2	0.009	4	19	15	770	190
Ascaris hemoglobin ⁴⁹	his-aquo (distal tyr)	n/a	n/a	0.2	0.018	90	1.5	0.004	3	0.03
Microperoxidase⁵⁰	his-aquo	n/a	n/a	20	0.01	0.5
Combinatorial bundles⁴⁴	mixed	?	?	(3-11) (k_app)	0.03-0.11	(6-25)

Apparent binding rates (in parentheses) are slowed by bis-histidine ligation. Corresponding rates not in parentheses are estimated binding rates without histidine impedance facilitating comparison with proteins with different fractions of bis-histidine ligation.