Noncooperative temperature melting of a globular protein without specific tertiary structure: Acid form of bovine carbonic anhydrase B

Heat denaturation of native globular proteins is a cooperative process usually connected with the melting of the main part of their regular secondary structure. In this paper, a noncooperative temperature‐induced melting of the regular secondary structure in the carbonic anhydrase B at pH 2.6 in heavy water is observed by ir spectroscopy. The molecules of carbonic anhydrase B in an acid medium, unlike the native ones, do not have a specific tertiary structure. Nevertheless, the β‐structure content is about the same in both of these states. A temperature‐induced noncooperative melting process takes place from 10 to 67°C with a decrease of the antiparallel β‐form content by about one third. The remaining part of the β‐form melts with a more intensive heat absorption, with a maximum at 87°C. The whole melting process is practically reversible. We assume that the observed noncooperative process displays a general property of a new type of structural state of the globular protein—the “molten globule state.”