| Abstract
| - The binding properties of O2 and CO to recombinant human serum albumin (rHSA) mutantswith 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 coordinationby Tyr-161. The reduced ferrous rHSA(wt)−heme under an Ar atmosphere exists in an unusual mixture offour- and five-coordinate complexes and is immediately autoxidized by O2. To confer O2 binding capabilityon 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 Aratmosphere. These artificial hemoproteins bind O2 at room temperature. Mutation at the other side of theporphyrin, Y161L/L185H, also allowed O2 binding to the heme. In contrast, the single mutant rHSA(I142H)−heme could not bind O2, suggesting that removal of Y161 is necessary to confer reversible O2 binding.Laser flash photolysis experiments showed that the kinetics of CO recombination with the rHSA(mutant)−heme were biphasic, whereas O2 rebinding exhibited monophasic kinetics. This could be due to the twodifferent geometries of the axial imidazole coordination arising from the two orientations of the porphyrinplane in the heme pocket. The O2 binding affinities of the rHSA(mutant)−heme were significantly lowerthan those of hemoglobin and myoglobin, principally due to the high O2 dissociation rates. Changing Leu-161 to Phe-161 at the distal side increased the association rates of both O2 and CO, which resulted inenhanced binding affinity.
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