Esterase activity of hemoglobin. Differences between HB A and HB S.
The hydrolysis of p-nitrophenyl acetate (p-NPA) is catalyzed by many proteins. We have observed that oxyhemoglobin A also exhibits esterase activity with a rate intermediate between that of bovine albumin and carbonic anhydrase. Kinetic studies of this reaction revealed that the rate of hydrolysis of p-NPA in the presence of oxy Hb S was approximately 2 times slower than that of oxy Hb A. There is general agreement that the catalytic effect of peptides and proteins on the hydrolysis of p-NPA is mediated by histidines. Oxy Hb Deer Lodge (His beta 2 leads to Arg) hydrolyzes p-NPA at a rate approaching that of oxy Hb S. Oxy Hb F, in turn, exhibits a rate indistinguishable from that of oxy Hb A. The effect of 2,3-diphosphoglyceric acid on the reaction is consistent with the participation of His beta 2 in this catalytic effect. The pH dependence of the reaction and studies with free amino acids also lend support to the involvement of a histidine residue. These results point to subtle conformational differences between oxy Hb S and oxy Hb A in solution, probably involving His beta 2 and/or its microenvironment. The catalytic hydrolysis of p-NPA can be considered a useful probe of conformational states of macromolecules.
D004163:Diphosphoglyceric Acids / Q000494:pharmacology; D004950:Esterases / Q000097:blood*; D006441:Hemoglobin A / Q000378:metabolism*; D006451:Hemoglobin, Sickle / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010108:Oxyhemoglobins / Q000378:metabolism
D004163:Diphosphoglyceric Acids; D006451:Hemoglobin, Sickle; D010108:Oxyhemoglobins; D006441:Hemoglobin A; D004950:Esterases