Enthalpies and entropies have been determined for the reversible binding of 02 Structural and dynamic studies of hemoproteins have led to three distinct but not necessarily exclusive proposals for the effects of the protein on the reactivity of heme toward 02 and CO. Two of these postulates, the proximal base tension effect (1, 2) and the proximal imidazole deprotonation effect (3-5), have recently been documented by studies of model compounds (6-11). The third postulate, of a distal side steric effect, has been the subject of some controversy in model compound studies (7,9).In hemoproteins studied by x-ray crystallography, the Fell-CO has the ligand "pushed" off the axis by a nearby histidine or other distal side residue (12)(13)(14), in contrast to the observations in simple complexes (15). This effect was postulated to decrease the affinity of CO (16). A corollary suggestion is that Fel-OO, being bent already, should not suffer this steric effect, and CO binding in the hemoproteins should be impaired relative to that in simple model complexes (7,16).In contrast to these predictions, the model compounds chelated protoheme and chelated mesoheme were reported to bind both 02 and CO in aqueous suspension at 20°C with affinities and kinetics similar to those of the high-affinity proteins R-state hemoglobin and hemoglobin chains (8,17,18). A number of other simple model systems have CO affinities similar to those of the chelated hemes (6,9,19,20) although the corresponding studies with 02 have not been made.Based upon these observations we suggested that in R-state hemoglobin or isolated hemoglobin chains the protein served only to keep the heme five-coordinated and to prevent oxidation, imparting no special effect on the heme affinity for CO or 02-Other hemoproteins having lower CO or 02 affinities were presumed to accomplish this alteration from the standard hemoglobin chain behavior by one of the effects mentioned above (18).The more elaborate iron complexes such as cyclophane heme (21,22), capped heme (23), or picket-fence heme (7,24,25), all of which have bulky groups around one face of the heme, either do not mimic the hemoproteins or do so only in part. Picket-fence heme was reported to bind 02 in toluene or in solid state (7) with an affinity similar to that of chelated heme or R-state hemoglobin but to bind CO "irreversibly" in the solid state (24) and with greatly increased affinity in solution (25). These observations led to the conclusion that model compounds bind CO more strongly than do hemoproteins (24), in contrast to the proposal discussed above.We now report the thermodynamics of the binding of 02 and CO (Eqs. 1 and 2) and the replacement reaction (Eq. 3) which are very nearly the same as those of hemoglobin (R-state) Kinetics were determined with a microprocessor-controlled laser flash photolysis apparatus as described (9,26
