11-3 |
Sulfhydryl group reactivity expressed by native beta-lactoglobulin and bovine serum albumin towards pyridine disulfide |
R. K. OWUSU APENTEN, Department of Food Science, The Pennsylvania State Univeristy, College of Agricultural Sciences, 111 Borland Laboratory, University Park, PA 16802, C. Chee, and P. H. Ong, Department of Food Science, University of Leeds, Woodhouse Lane, West Yorkshire, Leeds, United Kingdom. Protein sulfhydryl (SH) groups are important for ligand binding, redox reactions and for SH-disulfide exchange. This last process is involved in the development of protein functionality in foods. Less well understood is the role of SH-groups on the storage characteristics of protein ingredients. We hypothesize that the SH-group reactivity expressed by different proteins within a mixture will impact on protein storage characteristics. The objectives in this portion of our ongoing studies were to determine kinetic and thermodynamic parameters for SH-group reactions of beta-lactoglobulin (BLG) and bovine serum albumin (BSA). SH-group reactivity was measured with pyridine disulfide (PDS) dissolved in phosphate buffer (50 mM, pH 7.0). Protein reactions with PDS were monitored continuously by spectrophotometry at 304 nm. The rate constant (k) for the BSA reaction with PDS was 40 (±0.54) M -1 s -1. The corresponding k-value for BLG was 0.038 (±0.002) M -1 s -1. Both proteins react slowly as compared to glutathion (GSH). This low molecular weight compound has a solvent-exposed SH-group. By reacting GSH with PDS we determined the reaction-rate limit for a solvent-accessible SH-group (k*) as 532 M -1 s -1. The Gibbs free energy change for protein SH-group exposure, DG ex (=- RT ln k / k*) was 6,467 J mol -1 for BSA and 23,622 J mol -1 for BLG. These results suggest that the SH-group of BSA is more solvent-accessible and reactive compared to the SH-group of BLG. Differences in SH-group reactivity and DG ex may be related to the free energy required for a limited conformational change in BSA or BLG. The concentration ratio BLG: BSA is 8:1 compared to a sulfhydryl reactivity ratio of 1:1052. Therefore, BSA may contribute more than BLG to the net SH-group reactivity associated with whey proteins.
Session 11, Food Chemistry: Proteins
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