356 IRON-CONTAINING PROTEINS AND ENZYMES
porphyrin illustrated in Figure 7.9B although many others have been
synthesized.9,19- Attaching the fi ve - coordinate system to a rigid support — that is, silica
gel, reducing its mobility and ability to add a sixth ligand.
7.2.7 Binding of CO to Myoglobin, Hemoglobin, and Model Compounds
Selective binding of dioxygen rather than carbon monoxide in wild - type bio-
logical systems is complicated by the fact that naturally occurring metallopro-
teins including Mb and Hb produce CO during their degradation processes.^12
Therefore, hemes must be able to carry out their oxygen transport and storage
functions in the presence of signifi cant concentrations of CO. In addition, CO
binds to myoglobin and hemoglobin with affi nities of, respectively, 25 and 200
times those of O 2. Collman proposes that the discrimination occurs through
steric constraints imposed by amino acid residues on the distal side of the
porphyrin and by selective hydrogen bonding favoring O 2 over CO coordina-
tion. Dioxygen is capable of a bent geometry when bound to the heme facili-
tating the hydrogen bond to the distal histidine, whereas CO ’ s preferred linear
binding mode not only prevents hydrogen bond formation but also results in
steric clashes with neighboring amino acids as shown in Figure 7.11.
Not only is carbon monoxide produced during catabolism of hemes, but it
is also necessary for maintenance of neuronal function and cell signaling in
the vasculature and therefore is present normally in the vicinity of Mb and
Hb. For synthetic systems of picket - fence, pocket, or capped porphyrins, CO
affi nity exceeds that for O 2 sometimes by many orders of magnitude. The in
Figure 7.10 Picket - fence porphyrin models. (A) Model for T - state and (B) model for
R - state. (Adapted with permission from Figure 5 of reference 12. Copyright 1997,
American Chemical Society.)
NNCH 3CH 3MeNN
CH 3MeSteric HindranceModel for R-StateB Me(Picket Fence)(N-MeIm)
Model for T-StateAMe(Picket Fence)(1,2-Me 2 Im)