BIOINORGANIC CHEMISTRY A Short Course Second Edition

MYOGLOBIN AND HEMOGLOBIN 347

7.2.2 Structure of the Heme Prosthetic Group,

Deoxymyoglobin and deoxyhemoglobin contain pentacoordinate iron(II)
centers in which the metal ion lies out of the plane of the porphyrin ’ s four
pyrrole – nitrogen donor ligands.^10 Perutz has called this state the T or tense
state.^11 The T state, a term describing the quaternary structure of the Hb tet-
ramer, is one of low oxygen affi nity in which the protein is restrained by
binding of the so - called proximal histidine. In the T state the Fe(II) center is
held approximately 0.55 Å outside of the porphyrin plane and none of the four
Hb subunits possess dioxygen ligands. The porphyrin ring also is anchored at
the active site by iron ’ s coordination to the proximal histidine ’ s imidazole
nitrogen (see Figures 7.4 and 7.5 ).^12 In the R state or relaxed quaternary state,

Figure 7.3 Bonding of the O 2 − ion to Fe(III) in Mb or Hb. (Adapted with permission
from Figure 3 of reference 9. Copyright 1994 American Chemical Society.)

O

O

x

z

filled

superoxide ion

sp^2 hybrid orbital

z

empty Fe dz 2 orbital

half filled

superoxide ion

π* orbital

half filled Fe dxz

orbital

O

Fe

Fe

O

Figure 7.4 T and R states for iron hemes. (Adapted with permission from Figure 4 of
reference 12. Copyright 1997, American Chemical Society.)

Fe

Fe

O 2

N

N

HN

O

O

N N N

N

HN protein globin

plane of porphyrin

ring

2.07 Å

plane of porphyrin
ring

T-state protein

2.68 Å 0.55 Å

protein globin

R-state protein

heme distal side

heme proximal side

(histidine attached)

H-bond to

vacant distal distal his

coordinate site