Abstract
The effect of protein stability on the reactivity of the free sulphydryl (SH) group in β-lactoglobulin-A ( β-LgA) provides a model for the study of protein stability-function relations (PSFR). The free energy change for protein unfolding (Δ G O) and SH group exposure (Δ G SH) were determined from (i) the urea unfolding curve for β-LgA and (ii) the kinetics of β-LgA SH/disulphide exchange with 2-pyridine disulphide (2-PDS) in 0–8 M urea (pH 3). Protein unfolding profiles determined from extrinsic fluorescence and SH-group reactivity measurements were not coincident. β-LgA formed a stable intermediate ( X) state in the presence of 4 M urea with Δ G O=20 (±0.03) kJ/mol. From the low rate of SH/disulphide exchange in 4 M urea, the SH-group within β-LgA was efficiently masked within the X-state. SH reactivity increased after β-LgA was unfolded in 6–8 M urea with, Δ G SH=43(±6.4) kJ/mol. Such results are discussed in terms of possible interrelationships between protein unfolding stability and SH reactivity in β-LgA.
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