Table 2. Axial Ligand Types and Reduction Potentials of Various Cytochromes^a.

cytochrome	axial ligand	heme type	E (mV)b	mutant	E (mV)	ref
Nitrosomonas europaea diheme cyt c peroxidase	His/Met	class I	450			(153, 154)
Rhodocyclus tenuis THRC cyt c		class IV	420			(155)
HP1	His/Met		420
HP2	His/Met		110
LP1	bis-His		60
LP2	His/Met
Rhodopseudomonas viridis THRC cyt c		class IV	380			(156,157)
H1 (c559)	His/Met		330
H3 (c556)	His/Met		20
H2 (c552)	bis-His		–60
H4 (c554)	His/Met
Rhodobacter capsulatas cyt c2	His/Met	class I	373	Gly29Ser	330	(158−160)
				Pro30Ala	258
				Tyr67Cys	348
				Tyr67Phe	308
Chlamydomonas reinhardtii cyt f	His/Ntr-Tyr	cyt f	370	Tyr1Phe	369	(161)
				Tyr1Ser	313
				Val3Phe	373
				Phe4Leu	348
				Phe4Trp	336
				Tyr1Phe/Phe4Tyr	370
				Tyr1Ser/Phe4Leu	289
				Val3Phe/Phe4Trp	342
Rhodospirillum rubrum cyt c2	His/Met	class I	324			(156)
Pseudomonas aeruginosa cyt c nitric oxide reductase	His/Met	class I	310			(162)
	bis-His	cyt b	345
Pseudomonas aeruginosa cyt c peroxidase	His/Met	class I	320			(163)
Arthrospira maxima cyt c6	His/Met	class I	314			(164)
Saccharomyces cerevisiae iso-2-cyt c	His/Met	class I	288	Asn52Ile	243	(130)
Saccharomyces cerevisiae iso-1-cyt c	His/Met	class I	272	Arg38Lys	249	(131, 165−173)
			285	Arg38His	245
			290	Arg38Gln	242
				Arg38Asn	238
				Arg38Leu	231
				Arg38Ala	225
				Asn52Ala	257
				Asn52Ile	231
				Tyr67Phe	234
				Phe82Leu	286
				Phe82Tyr	280
				Phe82Ile	273
				Phe82Trp	266
				Phe82Ala	260
				Phe82Ser	255
				Phe82Gly	247
Pseudomonas aeruginosa cyt c551	His/Met	class I	276			(156)
horse cyt c	His/Met	class I	262	Met80Ala	82	(158, 174)
				Met80His	41
				Met80Leu	–42
				Met80Cys	–390
rat cyt c	His/Met	class I	260	Pro30Ala	258
				Pro30Val	261
				Tyr67Phe	224
Rhodopseudomonas palustris cyt c556	His/Met	class II	230			(80)
Escherichia coli cyt b562	His/Met	cyt b (class II)	168	Phe61Gly	90	(175, 176)
				Phe65Val	173
				Phe61Ile/Phe65Tyr	68
				His102Met	240
				Arg98Cys/His102Met	440
Alicycliphilus denitrificans cyt c′	His/Met	class II	132			(80)
Rhodopseudomonas palustris cyt c′	His/Met	class II	102			(80)
cytochrome b5	bis-His	cyt b		form A	80	(177)
				form B	–26
Desulfovibrio vulgaris cyt c553	His/Met	class I	37	Met23Cys	29	(156, 178)
			20 ± 5	Gly51Cys	28
				Met23Cys/Met23Cys	88
				Gly51Cys/Gly51Cys	105
bovine liver microsomal cyt b5	bis-His	cyt b	3	protoheme IX dimethyl ester	70	(179)
Saccharomyces cerevisiae cyt b2	bis-His	cyt b	–3			(156)
Chromatium vinosum cyt c′	His	class II	–5			(80)
rat liver microsomal cyt b5	bis-His	cyt b	–7 ± 1			(129, 180)
Rhodospirillum rubrum cyt c′	His/Met	class I	–8			(80)
tryptic bovine hepatic cyt b5	His/Met	class I	–10 ± 3	Val61Lys	17	(181)
				Val61His	11
				Val61Glu	–25
				Val61Tyr	–33
Allochromatium vinosum triheme cyt c	bis-His	class III	–20			(182)
	His/Met		–200
	His-Cys/Met		–220
Rhodobacter sphaeroides cyt c′	His/Asn	cyt c	–22			(183)
cyt b6f complex	bis-His	cyt b	–45			(184)
			–150
Thermosynechococcus elongates PS cyt c550	His/Met	class I	–80	in the absence of mediators	200	(185)
MamP magnetochrome	His/Met	class I	–76			(186)
rat liver OM cyt b5	bis-His	cyt b	–102	His63Met	110	(187, 188)
				Val45Leu/Val61Leu	–148
				protoheme IX dimethyl ester	–36
Desulfovibrio desulfuricans Norway cyt c3	bis-His	class III	–132			(78)
	bis-His		–255
	bis-His		–320
	bis-His		–360
Chlorella nitrate reductase cyt b557	bis-His	cyt b	–164			(189, 190)
Ectothiorhodospira shaposhnikovii cyt b558	bis-His	cyt b	–210			(191)
Azotobacter vinelandii bacterioferritin	bis-His	cyt b	–225			(192)
(in the presence of a nonheme iron core)			–475
Desulfovibrio vulgaris Hildenborough cyt c3	bis-His	class III	–280			(192, 193)
	bis-His		–320
	bis-His		–350
	bis-His		–380
Synechocystis sp. cyt c549	bis-His		–250			(78)
Arthrospira maxima cyt c549	His/Met		–260			(164)

^aAdapted with permission from ref (78). Copyright 2004 Elsevier.

^bAll reduction potentials listed in this review are versus standard hydrogen electrode (SHE) or normal hydrogen electrode (NHE).