Abstract

FODEKE, Adedayo A.. Organic phosphate binding inhibits high pH t-isomerization of the ß-chain in straw-coloured fruit bat (Eidolon helvum) haemoglobin. S.Afr.j.chem. (Online) [online]. 2017, vol.70, pp.181-188. ISSN 1996-840X.  http://dx.doi.org/10.17159/0379-4350/2017/v70a25.

Understanding the systematic structural changes accompanying allosteric effector binding to haemoglobin should provide some clues to the understanding of structure-function relationship in other multimeric enzymes. The affinities of the CysF9[93]ß sulfhydryl group of oxy-, carbomonoxy- and aquomet-derivatives of straw-coloured fruit bat (Eidolon helvum) haemoglobin (SCFB-Hb) for 5,5'-dithiobis(2-nitrobenzoate) (DTNB) were measured in the range 5.6 £ pH £ 9.0 using stripped and inositol hexakisphosphate (inosito-P₆) bound haemoglobin. The data were analyzed on the basis of findings that the tertiary structure of the product of the reaction of DTNB with haemoglobin CysF9[93]ß sulfhydryl group exists in two conformations; r and t. The result shows that the affinity of DTNB for SCFB-Hb in both r and t conformations are coupled to the ionizations of two ionizable groups, HisH21[143]ß and HisFG4[97]ß. In the r conformation, the presence of inositol-P₆ reduces the pK_a of HisH21[143]ß by 1.24 units and that of HisFG4[97]ß by 2.74. In the t conformation, inositol-P₆ raises the pK_a of HisH21[143]ß by 1.10 pK_a units whereas that of HisFG4[97]ß was increased by 0.78 pK_a units. Change in pK_a of ionization of the ionizable groups and isomerization of the tertiary conformations are important modulators of organic phosphate binding.

Keywords : Sulfhydryl group; inositol hexakisphosphate; ionizable group; stripped haemoglobin; r↔t isomerization; 5,5'-dithiobis(2-nitrobenzoate).

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