Folding of the reduced form of the thioredoxin from bacteriophage T4

The folding pattern for bacteriophage T4 thioredoxin is similar to that of the oxidized form [Borden, K. L. B., & Richards, F. M. (1990) Biochemistry 29, 3071-3077]. Equilibrium and kinetic studies were carried out by fluorescence and circular dichroism techniques. The same box model proposed for the oxidized form, with four identifiable states, can accommodate most of the data: N----Uc----Ut----It----N, where N is the native state, Uc is the unfolded species with Pro 66 in the cis form, Ut is the unfolded species with Pro 66 in the trans form, and It is a trans-Pro 66 intermediate with a volume comparable to that of N. However, the relative importance of the different components is shifted between the oxidized and reduced proteins. In spite of the small size of the disulfide loop, the Cys 14-Cys 17 bond appears to be important in stabilizing It. The tertiary structure as monitored by near-UV CD and fluorescence indicates that the reduced form is significantly less stable than its oxidized counterpart; however, the two secondary structures, as seen by far-UV CD, are very similar. The intermediate It behaves as though it is cold denaturated at 4 degrees C.