Table I.

Ligand Type Distribution in b- and c-type Hemes

		b-Type heme		c-Type heme		Sum
		Occurrence	[%]	Occurrence	[%]	Occurrence	[%]
Redundant	His-His	149	6.5	603	61.2	752	23.0
	His-Met	10	0.4	247	25.1	257	7.9
	His-Xaa	14	0.6	31	3.1	45	1.4
	His-Ex	1514	66.4	68	6.9	1582	48.4
	Xaa-Xaa	4	0.2	0	0.0	4	0.1
	Xaa-Ex	529	23.2	26	2.6	555	17.0
	Other	61	2.7	10	1.0	71	2.2
	Total	2281		985		3266
Nonredundant	His-His	17	23.9	42	68.9	59	44.7
	His-Met	1	1.4	10	16.4	11	8.3
	His-Xaa	2a	2.8	2a,b	3.3	4	3.0
	His-Ex	31	43.7	4	6.6	35	26.5
	Xaa-Xaa	1c	1.4	0	0.0	1	0.8
	Xaa-Ex	17	23.9	2d	3.3	19	14.4
	Other	2	2.8	1	1.6	3	2.3
	Total	71		61		132

Distribution of ligands to b- and c-type hemes in the redundant and nonredundant heme protein data sets used in this survey. Xaa, amino acids other than His and Met. These include Asn, Asp, Cys, Tyr, Glu, and Lys in the redundant data set. Nonredundant data set:

^aHis-Tyr,

^bLys-Met,

^cHis-Cys,

^dLys-Ex. Ex: denotes hemes without a 6th ligand or an exchangeable, nonamino acid ligand (e.g., O₂).