Transition of hemoglobin between two tertiary conformations: determination of equilibrium and thermodynamic parameters from the reaction of 5,5'-dithiobis(2-nitrobenzoate) with the CysF9[93]beta sulfhydryl group.
The equilibrium constant of the reaction of 5,5'-dithiobis(2-nitrobenzoate) with the CysF9[93]beta sulfhydryl group of hemoglobin decreases by 2 to 3 orders of magnitude between pH 5.6 and 9. The reaction is coupled to the ionizations of two groups on the protein. At 25 degrees C one group has a pK(a) of 5.31+/-0.2 when hemoglobin is in its (tertiary) r conformation, typified by the thiolate anion form of CysF9[93]beta; this changes to 7.73+/-0.4 in the (tertiary) t conformation, typified by the mixed disulfide form of the sulfhydryl. The second group ionizes with a pK(a) of 7.11+/-0.4 in the r conformation; this changes to 8.38+/-0.2 in the t conformation. K(rt), the equilibrium constant for the r<-->t isomerization process, is 0.22+/-0.06. The standard enthalpy and entropy changes for the isomerization are DeltaH(o)(rt)=24.2 kJ mol(-1) and DeltaS(o)(rt)=68.8 JK(-1)mol(-1), respectively.
Okonjo
Kehinde Onwochei
KO
Department of Chemistry, University of Ibadan, Ibadan, Nigeria. kehindeokonjo@yahoo.com
Adediji
A Temilade
AT
Fodeke
Adedayo A
AA
Adeboye
Omolara
O
Ezeh
Chibuzo V
CV
D000818:Animals; D001665:Binding Sites; D002263:Carboxyhemoglobin / Q000737:chemistry; D002417:Cattle; D003545:Cysteine / Q000737:chemistry; D004228:Dithionitrobenzoic Acid; D006454:Hemoglobins / Q000737:chemistry*; D006801:Humans; D066298:In Vitro Techniques; D008706:Methemoglobin / Q000737:chemistry; D010108:Oxyhemoglobins / Q000737:chemistry; D011487:Protein Conformation; D011817:Rabbits; D013439:Sulfhydryl Reagents; D013816:Thermodynamics
D006454:Hemoglobins; D010108:Oxyhemoglobins; D013439:Sulfhydryl Reagents; C024963:aquomethemoglobin; D008706:Methemoglobin; D002263:Carboxyhemoglobin; D004228:Dithionitrobenzoic Acid; D003545:Cysteine