Transition of haemoglobin between two tertiary conformations: Inositol hexakisphosphate increases the transition constant and the affinity of sheep haemoglobin for 5,5'-dithiobis(2-nitrobenzoate).
The equilibrium constant (K(equ)) for the reaction of 5,5'-dithiobis(2-nitrobenzoate) - DTNB - with the CysF9[93]beta sulphydryl group of the haemoglobins of the sheep decreases by about two orders of magnitude between pH approximately 5.6 and 9.2: from a mean of 7.2+/-1 to a mean of 0.044+/-0.01. Calculations from the pH dependence of K(equ) show that in the r-t tertiary conformational transition of haemoglobin the t isomer population is 50.7 and 61.8% for the major and minor haemoglobins, respectively. In the presence of inositol hexakisphosphate (inositol-P(6)), K(equ) increases for both haemoglobins by about an order of magnitude through most of the pH range. The t isomer population also increases to 82.1 and 79.6% for the major and minor haemoglobins, respectively. These results indicate that inositol-P(6) increases the affinity of the sulphydryl for DTNB by increasing the population of the t isomer. It is highly probable that a minimum four-state model that includes the r-t transition is required for a full understanding of haemoglobin function.
Okonjo
Kehinde Onwochei
KO
Department of Chemistry, Covenant University, Ota, Ogun State, Nigeria; Department of Chemistry, University of Ibadan, Ibadan, Nigeria. kehindeokonjo@yahoo.com
Adeogun
Idowu
I
Babalola
J Oyebamiji
JO
D000595:Amino Acid Sequence; D000818:Animals; D002263:Carboxyhemoglobin / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry; D004228:Dithionitrobenzoic Acid / Q000737:chemistry*; D006454:Hemoglobins / Q000737:chemistry* / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010833:Phytic Acid / Q000494:pharmacology*; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012756:Sheep
D006454:Hemoglobins; D010833:Phytic Acid; D002263:Carboxyhemoglobin; D004228:Dithionitrobenzoic Acid; D003545:Cysteine