TRANS-CIS ISOMERIZATION OF PROLINE-22 IN BOVINE PROTHROMBIN FRAGMENT-1 - A SURPRISING RESULT OF STRUCTURAL CHARACTERIZATION

The calcium ion-mediated interaction of bovine prothrombin (BF1) with negatively charged phospholipid membranes is assumed to be largely via the Gla domain of BF1 with the fold of the cia domain essential for b inding. It has been reported that Pro22 undergoes classical trans to c is isomerization in the presence of calcium ions with the cis conforma tion of Pro22 of BF1 responsible for membrane binding [Evans, T.C., Jr ., and Nelsestuen, G. L. (1996) Biochemistry 35, 8210-8215]. However, Pro22 was found to be in the trans conformation in the crystal structu re of BF1. In the present work, we have used molecular dynamics simula tions to investigate the relative importance of the two conformations of Pro22 to the structural and dynamical properties of BF1. The initia l trans conformation of Pro22 in BF1 was slowly converted to cis-Pro22 using constrained dynamics. The second-generation AMBER force field i n conjunction with the particle mesh Ewald method to accommodate long- range interaction was employed in the trajectory calculations. Compari son of the BF1(trans-Pro22) and BF1(cis-Pro22) equilibrated structures reveals surprisingly that the overall structural changes associated w ith the trans-cis isomerization is minimal and only minor modification s to the hydrogen bond network and the network of N-terminus Alal take place. The calculated electrostatic potential energy surfaces of the two protein structures also appear to be very similar, indicating the near equality of the local interaction site environments in the protei n prior to lipid binding.