Preliminary x‐ray analysis of crystals of plasminogen activator inhibitor‐1

Abstract

Crystals of bacterially expressed plasminogen activator inhibitor (PAI-1) suitable for X-ray diffraction analysis have been obtained from 8% (w/v) PEG 1500, pH 8.25. The space group is P1, and the lattice constants are a = 82.17 A, b = 47.82 A, c = 62.89 A, alpha = 90.00 degrees, beta = 106.90 degrees, gamma = 106.84 degrees. The diffraction limit is 2.3 A, and the unit cell contains two molecules of PAI-1. The crystals contain latent PAI-1 which can be partly reactivated by exposure to denaturants.

The Acid Stabilization of Plasminogen Activator Inhibitor-1 Depends on Protonation of a Single Group That Affects Loop Insertion into β-Sheet A

“…It appears that the loop containing helix F must move out of the way in order for the reactive loop to insert completely as strand 4A and that factors that stabilize the helix should slow down the insertion. In the latent inhibitor the presumably neutral His-143 (the crystals were grown at pH 8.2 (36)) is seen at hydrogen bond distance to the ␥-oxygen of Thr-142 and the carbonyl oxygen of Trp-139, both in helix F. In the active inhibitor, a positive charge on the histidine in addition to structural differences may allow the imidazolium ring to rotate into a different position, where it may stabilize helix F by neutralizing its dipole field or by forming a hydrogen bond to sheet A. Although there is no suitable acceptor within distance for the latter bond in latent PAI-1, it can be seen from the differences between latent PAI-1 and nonrelaxed serpins (35) that sheet A in the former has expanded in the region beneath helix F. In the active inhibitor the side chain oxygen of Thr-94 in strand 2A and one of the nitrogens of His-143 may very well be within hydrogen bond distance.…”