THE PH-DEPENDENCE OF THE EQUILIBRIUM BETWEEN THE FAST-FOLDING AND SLOW-FOLDING FORMS OF BOVINE PROTHROMBIN FRAGMENT-1 - A ROLE FOR PH-INDUCED SELF-ASSOCIATION

Examination of the pH- and ionic strength (mu)-dependence of the equil ibrium between fast- and slow-folding forms of bovine prothrombin frag ment 1 reveals a sharp dependence of K-eq ([% fast-folding form]/[% sl ow-folding form]) and % fluorescence quenching (%Q) on pH at low mu, a nd the absence of a pH-dependence of K-eq at high mu (0.1 M NaCl) and much reduced pH-dependence of %Q, suggesting that the ionization proce ss is coupled to other processes, such as self-association. The observ ed low mu pH effect on K-eq amounts to a 10% increase in the % of the fast-folding form of prothrombin at low pH. We hypothesize the existen ce of a pH-dependent self-association of bovine prothrombin fragment 1 . This process is associated with a conformation change involving an i onizing group with pK(a) in the neighborhood of 6.5 and a change in K- eq from 0.27 to 0.45. (C) 1995 Academic Press, Inc.