Table II.

Summary of Residues of Cyt b₆ and SU IV in Sequence Compilations of Tables IA, B and Figs. 2A, B That Are Most Highly Conserved (Degli Esposti et al., 1993)

Residue	Invariance	Location (Figs. 2A, B)	Function
	Cytochrome b6
Gly 36	Completeb	n-side, A helix	Heme pocketa
Gly 50	Essentially completeb	A helix	—
Gly 78	Essentially completeb	“ab” region	Turn between “ab” and B helices
Arg 82	(+) Residue essentially completeb	p-side interface, B helix	Stop transfer, B helix
His 85	Completeb	p-side, B helix	Ligand of p-side hemec,d
Ser 90	Incompleteb	B helix	—
His 99	Completeb	n-side, B helix	Ligand of n-side hemec,d
Arg 102	(+) Residue completeb	n-side interface, B helix	Stop transfer, B helix
Trp 117	Completeb	n-side, C helix	—
Gly 120	Essentially completeb	n-side, C helix	—
Gly 134	Essentially completeb	p-side, C helix	—
Tyr 135	Incompleteb	p-side, C helix	—
Trp 145	Incompleteb	p-side, “cd” helix	—
Thr 178	Essentially completeb	p-side interface—“cd” helix	—
His 186	Completeb	p-side, D helix	Ligand of p-side hemec,d
His 201	Completeb	n-side, D helix	Ligand of n-side hemec,d One extra residue between two His ligands on D helixc
	SU IVe
Asp 249	Essentially completeb	n-side interface, E helix	—
Ile 289	Incompleteb	p-side, “ef” loop	—
Pro 291	Completeb	p-side, “ef” loop	—
Glu 292	Essentially completeb	p-side, “ef” loop	—
Trp 293	Essentially completeb	p-side, “ef” loop	—
Leu 302	Essentially completeb	p-side, “ef” loop	—
Lys 308	Incompleteb	p-side interface, F helix	Stop transfer, F helix
Gly 311	Essentially completeb	p-side interface, F helix	Turn between “ef” and F helix

^aTron et al., 1991; Yun et al., 1991;

^bDegli Esposti et al., 1993;

^cWidger et al., 1984;

^dSaraste, 1984;

^enote that the numbering of SU IV is different from that used by Degli Esposti et al. (1993) who omitted the first 30 residues of the SU IV sequence. Use of the numbering system of the latter authors would cause overlap of numbering with the cyt b₆ sequence from residue 195–214. The basis of the numbering system used in the present work is based on the concept that SU IV is a functional extension of cyt b₆.