Prothrombin domains: circular dichroic evidence for a lack of cooperativity.

The far-ultraviolet circular dichroism spectra of bovine and human prothrombin, prothrombin fragment 1, prethrombin 1, prothrombin fragment 2, and prethrombin 2 (prethrombin 2des(1--13)) were determined and the method of Chen et al. [Chen, Y. H., Yang, J. T., & Martinez, H. M. (1972) Biochemistry 11, 4120--4131; Chen, Y. H., Yang, J. T., & Chau, K. H. (1974) Biochemistry 13, 3350--3359] was used to calculate the apparent alpha-helix, beta-sheet, and random-coil contents of each protein. Prothrombin and its activation components were found to contain a large amount of aperiodic secondary structure and there was little species difference between the spectra and, thus, secondary structures. The hypothesis that the prothrombin activation components exist as relative ly noncooperative "domains" within the prothrombin molecule was tested by comparing the circular dichroism spectrum of prothrombin with the sum of the spectra of the components. It support of the hypothesis, no gross alterations in the spectra and, hence, secondary structures of the components were found to have occurred upon activation.