Acid-induced denaturation and refolding of prothrombin.

Structural transitions of the blood coagulation factor prothrombin (extracted from goat blood) in response to reduction of pH were investigated by fluorescence, circular dichroism and light scattering measurements. The study revealed the presence of a partially unfolded state at around pH 3.5, characterized by marked enhancement of fluorescence from ANS bound to the protein, increase of bimolecular rate constant for tryptophan fluorescence quenching and a sharp peak in the light scattering intensity. Further lowering of the pH caused reversal of the trend of variation of these parameters, suggesting that prothrombin folds back to a compact state containing native-like secondary structural elements. The refolded state at low pH (<pH 3) fits the description of the A-state, the end-point of acid-induced denaturation process of several other monomeric proteins, and is a possible candidate for the class of folding intermediates known as molten globules.