Protein stability function relations: native ?-lactoglobulin sulphhydryl-disulphide exchange with PDS
✍ Scribed by Apenten, Richard K Owusu; Galani, Despina
- Publisher
- John Wiley and Sons
- Year
- 2000
- Tongue
- English
- Weight
- 107 KB
- Volume
- 80
- Category
- Article
- ISSN
- 0022-5142
No coin nor oath required. For personal study only.
✦ Synopsis
Intermolecular sulphhydryl±disulphide exchange with b-lactoglobulin dimer occurs when this dissociates to form monomers exposing two SH groups. This notion is re-evaluated in the light of recent structural data suggesting that the degree of SH group exposure in b-lactoglobulin is unaffected by dissociation. b-Lactoglobulin was treated with 2,2'-dipyridyl disulphide (PDS). The rate of sulphhydryl±disulphide exchange was measured at sub-denaturation temperatures of 25±60 °C. Parallel studies were conducted by reacting PDS with reduced glutathione (GSH). The SH group of GSH was up to 31 000 times more reactive than b-lactoglobulin. At pH 7 the reaction activation enthalpy (DH # ) and entropy (DS # ) was 26 kJ mol À1 and À100 J mol À1 K À1 respectively for GSH. For blactoglobulin, DH # was 157.2 kJ mol À1 and DS # was 254 J mol À1 K À1 . At pH 2.6, DH # was 14.4 kJ mol À1 and DS # was À213 J mol À1 K À1 for GSH. The corresponding results for b-lactoglobulin were 20.3 kJ mol À1 and À147 J mol À1 K À1 . These and other thermodynamic results are discussed in terms of the effects of blactoglobulin conformational structure and stability on SH group reactivity. For native b-lactoglobulin at neutral pH, intermolecular sulphhydryl±disulphide exchange appears to involve the dissociated monomer. SH group activation probably arises from the lower structural stability of the monomer relative to the dimer. At pH 2.6 the mechanism of SH±disulphide exchange does not require protein dissociation and probably involves breathing motions or localised changes in protein structure.