ISSN:
1573-4943
Keywords:
Sulphydryl groups
;
multiple
;
hemoglobin
;
kinetics
;
ionizable groups
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract Pigeon hemoglobin has eight reactive sulphydryl groups per (tetramer) molecule, as determined by Boyer titration with p-chloromercuribenzoate. However, only four of these are titra-table with 5,5′-dithiobis(2-nitrobenzoate) under the same experimental conditions. The time course of the reaction of pigeon hemoglobin with 5,5′-dithiobis(2-nitrobenzoate) is biphasic. In thepH range 6–9, the fast phase is between one and two orders of magnitude faster than the slow phase. For the fast phase,k app, the apparent second-order rate constant, increases monotonously withpH. Quantitative analysis reveals that the reactionof the sulphydryl group responsible for this phase is coupled to the ionization of two groups with pK a values of 6.15±0.1 and 8.5±0.1. These pK a values are assigned to HisHC3(146)β and to the CysF9(93)β sulphydryl group, respectively. For the slow phase thek app vs.pH profiles are bowl-shaped. Analysis reveals that the reaction of the sulphydryl group to which this phase may be attributed is coupled to the ionization of two groups with mean pK a values of 6.53±0.1 and 8.25±0.1. Examination of the structure of hemoglobin allows us to assign these values to HisG19(117)β and CysB5(23)β, respectively. The CysB5(23)β sulphydryl is in the region of the molecule where amino acid substitutions have been found to give rise to significant changes in the oxygen affinity of hemoglobin [Huanget al. (1990),Biochemistry 29, 7020–7023.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01025129
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