Carbon monoxide

As documented in Table 4.2, carbon monoxide (CO) generally binds more strongly to hemoglobin than does dioxygen, hence causing carbon-monoxide poisoning. In addition to being readily available from car exhausts and tobacco smoke to convert oxyhemoglobin to carbonmonoxyhemoglobin, CO is produced in the catabolism of heme molecules. 1I7 Thus under even the most favorable of conditions, about 3 percent of human hemoglobin is in the carbonmonoxy form. When CO binds to a single metal atom in nonbiological systems, without exception it does so through the carbon atom and in a linear manner:

Model systems for carbonmonoxy (also called carbonyl) hemoglobin show a geometry similar to that of the Fe-C == 0 group, linear or nearly so and essentially perpendicular to the porphyrin plane. I 10,118-121 The biochemical literature is littered with reports that this is not the geometry adopted by CO in binding to hemoglobins. 122-128 We will return to this topic later in this chapter, since the physiological consequences are potentially important. Carbon monoxide binds weakly as a a-donor ligand to four-coordinate cobalt( II) systems. 129 Despite a bout of artifactual excitement,130 CO has never been observed to bind significantly to five-coordinate COIl systems with a nitrogenous axial base to yield octahedral six-coordinate species. 131 The sulfur analogue thiocarbonyl (CS), although not stable as a free entity, binds very strongly to iron-porphyrin species in a linear manner.