The binding properties of O-2 and CO to recombinant human serum albumin (rHSA) mutants with a prosthetic heme group have been physicochemically and kinetically characterized. Iron(III) protoporphyrin IX (hemin) is bound in subdomain IB of wild-type rHSA [rHSA(wt)] with weak axial coordination by Tyr-161. The reduced ferrous rHSA(wt)-heme under an Ar atmosphere exists in an unusual mixture of four- and five-coordinate complexes and is immediately autoxidized by O-2. To confer O-2 binding capability on this naturally occurring hemoprotein, a proximal histidine was introduced into position Ile-142 or Leu-185 by site-directed mutagenesis. A single mutant (I142H) and three double mutants (I142H/Y161L, I142H/ Y161F, and Y161L/L185H) were prepared. Both rHSA(I142H/Y161L)-heme and rHSA(I142H/Y161F)heme formed ferrous five-N-coordinate high-spin complexes with axial ligation of His-142 under an Ar atmosphere. These artificial hemoproteins bind O-2 at room temperature. Mutation at the other side of the porphyrin, Y161L/L185H, also allowed O-2 binding to the heme. In contrast, the single mutant rHSA(I142H)heme could not bind O-2, suggesting that removal of Y161 is necessary to confer reversible O-2 binding. Laser flash photolysis experiments showed that the kinetics of CO recombination with the rHSA(mutant)heme were biphasic, whereas O-2 rebinding exhibited monophasic kinetics. This could be due to the two different geometries of the axial imidazole coordination arising from the two orientations of the porphyrin plane in the heme pocket. The O-2 binding affinities of the rHSA(mutant)-heme were significantly lower than those of hemoglobin and myoglobin, principally due to the high O-2 dissociation rates. Changing Leu161 to Phe-161 at the distal side increased the association rates of both O-2 and CO, which resulted in enhanced binding affinity.