Okonjo, K. O. and Nwozo, Sarah
Ligand-dependent reactivity of the CysB5 b sulfhydryl group of the major haemoglobin of chicken.
Journal of the Chemical Society (Faraday Transactions), 93 (7).
Chicken haemoglobin contains eight reactive sulfhydryl groups per (tetramer) molecule, as determined by Boyer titration with p-chloromercury(II)benzoic acid. However, only four of these sulfhydryls are reactive towards 5,5@-dithiobis(2-nitrobenzoic acid) (DTNB). They are at the F9 and B5 positions of each of the two b subunits in the molecule. The time course of the DTNB reaction is biphasic. With oxyhaemoglobin, k the apparent second-order rate constant of the fast phase, increases app,
monotonically with pH, the simple proÐle resembling the titration curve of a diprotic acid; the pH-dependence of k for the app slow phase is bowl-shaped. With carbonmonoxyhaemoglobin and aquomethaemoglobin, k for the fast phase is bowl-shaped app whilst k for the slow phase increases monotonically with pH. Quantitative analyses of the simple proÐles show that the app reactivity of the sulfhydryl group to which they may be attributed is subject to the inÑuence of two ionizable groups on the
molecule, with mean pK values of 6.4^0.1 and ca. 8.4^0.3. These values are assigned to HisHC3b and CysF9b, a pKa respectively. Quantitative analyses of the bowl-shaped proÐles show that the reactivity of the sulfhydryl group to which they may be attributed is subject to the inÑuence of two ionizable groups on the protein, with mean pK of 6.85^0.05 and 8.3^0.2. as These values are assigned to HisG19b and CysB5b, respectively. It is highly signiÐcant that the CysB5b sulfhydryl
groups of carbonmonoxy- and aquomet-haemoglobin react ca. 100 times faster than that of oxyhaemoglobin. By contrast, the di†erence in the reactivities of the CysF9b sulfhydryls of the three haemoglobin derivatives is no more than four-fold. This indicates that, in chicken haemoglobin, changes in the haem ligand give rise to structural changes in the neighbourhood of the
CysB5b sulfhydryl which are far more signiÐcant than those in the neighbourhood of the CysF9b sulfhydryl.
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