MYOGLOBIN AND HEMOGLOBIN 357
vitro preference of CO over O 2 binding to Mb and Hb are in direct confl ict
with the data indicating that under physiological conditions, the dioxygen
affi nity of Mb and Hb are orders of magnitude greater than that for CO.^20
For small - molecule, metal – carbon monoxide complexes, the carbon mon-
oxide ligand is almost always in a linear conformation and perpendicular to
the metal. If one assumed bonding of CO to Hb or Mb in its normal linear,
perpendicular mode, steric confl icts as illustrated in Figure 7.11A would occur
and thus one might predict that CO binding would be less favored by physio-
logical systems. Earlier X - ray crystallography of MbCO and Hb(CO) 4 had
been less than clear about the carbon monoxide ligand because resolution in
protein crystallographic structures was not suffi cient to unequivocally distin-
guish between linear and bent structures. Several possible interpretations of
CO binding in a bent conformation may have included, for instance, either
of the binding modes illustrated in Figure 7.11B.^20 However, the simple model
of linear CO perpendicular to the heme metal ion causing steric crowding
versus the bent O 2 molecule relieving steric crowding has proved to be remark-
ably successful, as J. P. Collman and co - workers reported in a 2004 Chemical
Reviews article.^24 This reference is the place to look for updates to all the
material in Section 7.2.
In attempting to understand how the attachment and release of carbon
monoxide, and ultimately dioxygen, happens on a molecular scale, Rodgers
Figure 7.11 (A) Coordination of O 2 and CO to heme iron in Mb and Hb. (B) Possible
Fe – CO bent conformation binding modes.
CONHNN NC
O
O-NHNNNOOHN NC
O
O-steric clashproximal histidine proximal histidineprotein globinplane of porphyrin
ringprotein globinplane of porphyrin
ringdistal histidineBNFe CO
FeCO
or NA