The pH-Dependence of the Equilibrium between the Fast- and Slow-Folding Forms of Bovine Prothrombin Fragment 1: A Role for pH-Induced Self-Association?

Abstract

Examination of the pH- and ionic strength (μ)-dependence of the equilibrium between fast- and slow-folding forms of bovine prothrombin fragment 1 reveals a sharp dependence of Keq ([% fast-folding form]/[% slow-folding form]) and % fluorescence quenching (%Q) on pH at low μ, and the absence of a pH-dependence of Keq at high μ (0.1 M NaCl) and much reduced pH-dependence of %Q, suggesting that the ionization process is coupled to other processes, such as self-association. The observed low μ pH effect on Keq amounts to a 10% increase in the % of the fast-folding form of prothrombin at low pH. We hypothesize the existence of a pH-dependent self-association of bovine prothrombin fragment 1. This process is associated with a conformation change involving an ionizing group with pKa in the neighborhood of 6.5 and a change in Keq from 0.27 to 0.45.