Unfolding of 4-aminobutyrate aminotransferase equilibrium and kinetic studies

The unfolding of pig liver 4-aminobutyrate aminotransferase by urea has been investigated at equilibrium. The overall process was reversible as judged from the recovery of catalytic activity after dilution of urea-treated samples. Unfolding of the enzyme was monitored by circular dichroism and fluorescence spectroscopy. The steepness of the fluorescence and CD changes between 2 and 8 M urea, and the lack of any discernible plateau suggests that unfolding of the protein is a cooperative process. The unfolding of 4-aminobutyrate aminotransferase as a function of urea concentration was monitored by fluorescence measurements of the tryptophanyl residues. The kinetic results indicate that the aminotransferase unfolds in a single kinetic phase. Unfolded 4-aminobutyrate aminotransferase recovers immediately its catalytic activity upon dilution with buffers of neutral pH. Based on equilibrium and kinetic results, a two-state model for the unfolding of the aminotransferase is proposed.