Role of the intersubunit disulfide bond in the unfolding pathway of dimeric red kidney bean purple acid phosphatase

Quantitative equilibrium denaturation studies on oligomeric proteins have the potential to provide information on the role of subunit interactions in protein function and structure. We studied the equilibrium denaturation of red kidney bean purple acid phosphatase (KBPAP), a homodimer with a single disulfide bond between the two subunits, with an objective to understand the role of the intersubunit disulfide bond in KBPAP structure. Binding of 8-anilino-1-naphthalenesulfonic acid, enzymatic activity, size-exclusion chromatography, tryptophan fluorescence and circular dichroism studies revealed that the protein undergoes unfolding through at least three intermediates. Susceptibility of KBPAP for denaturation increases on reduction of the disulfide and aggregation was the predominant product of denaturation. In terms of stability, an intersubunit disulfide bond contributes to 25% of the overall stability of the dimer.