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 .beta.-structure content is about the same in both of these states. A temperature-induced noncooperative melting process takes place from 10 to 67.degree. C with a decrease of the antiparallel .beta.-form content by about one third. The remaining part of the .beta.-form melts with a more intensive heart absorption, with a maximum at 87.degree. 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".