Ko. Okonjo et al., EFFECT OF A3[6]BETA(GLU-]VAL) MUTATION ON REACTIVITY OF THE CYSF9[93]BETA SULFHYDRYL-GROUP OF HUMAN HEMOGLOBIN-S, Journal of the Chemical Society. Faraday transactions, 92(10), 1996, pp. 1739-1746
Citations number
31
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
The pH dependence profiles of the apparent second-order rate constant,
k(app), for the reaction of the oxy, carbon monoxy and aquomet deriva
tives of human haemoglobin S with 5,5'-dithiobis(2-nitrobenzoate) (DTN
B) in buffers of ionic strength 50 mmol dm(-3) are complex. The pK(a)
s of the ionizable organic phosphate binding groups which influence th
e reactivity of the CysF9[93]beta sulfhydryl group have been determine
d from quantitative analyses of the complex profiles. These pK(a) valu
es were not significantly different from those of haemoglobin A. In th
e presence of inositol hexakisphosphate (inositol-P-6) each profile as
sumes a simple form resembling the titration curve of a diprotic acid.
The pK(a) s of HisHC3[146]beta and CysF9[93]beta were determined from
quantitative analyses of the simple profiles. The mean values obtaine
d, 6.6 +/- 0.2 and 8.84 +/- 0.04, respectively, were the same as those
of haemoglobin A. Comparison of the k(app) data for haemoglobin S wit
h those of haemoglobin A shows that, irrespective of the presence or a
bsence of inositol-P-6 presence, the carbon monoxy and aquomet derivat
ives of haemoglobin A react more rapidly than the corresponding haemog
lobin S derivatives. In contrast, in the absence of inositol-P-6, oxyh
aemoglobin A reacts faster than oxyhaemoglobin S; in the presence of t
he organic phosphate both haemoglobins react at about the same rate. A
t an ionic strength of 200 mmol dm(-3) in the absence of inositol-P-6,
the pH dependence profiles of k(app) for the oxy, carbon monoxy and a
quomet derivatives of haemoglobins A and S are simple. Quantitative an
alyses of these profiles give mean pK(a) values of 5.4 +/- 0.1 and 8.9
+/- 0.2 for HisHC3[146]beta and CysF9[93]beta, respectively. The oxy
and carbon monoxy derivatives of both haemoglobins react at about the
same rate, but aquomethaemoglobin A reacts significantly more rapidly
than aquomethaemoglobin S.