Plasminogen is the inactive precursor of the protease plasmin, which has a primary role in the removal of blood clots (fibrinolysis). Plasminogen can exist in two conformations, such that it circulates in the blood in the closed conformation and opens up upon recruitment to fibrin clots or cell surfaces, via binding to lysine residues on the surface of the target molecules. Such conformational change results in the exposure of the activation loop, allowing activation by plasminogen activators. Two natural variants of plasminogen with different glycosylation profiles- glycoforms 1 and 2 (G1 and G2) are found in the blood. These two glycoforms play distinctive biological roles in vivo. Recently, the structure of plasminogen of both G1 and G2 plasminogen had been published by our laboratory (1). However, the detailed molecular bases involved in the conformational change is not well understood.

In this study, we address the issue of conformational change in real time using Small-Angle X-ray Scattering (SAXS). We studied the conformational changes of plasminogen using a group of lysine analogues, such as transexamic acid (TXA) and 6-aminocaproic acid (EACA). We have also carried out a comparative study to determine the EACA-dependent conformational change of plasminogen with and without chloride ions which plays an important role in stabilizing the closed plasminogen conformation. As well as that we investigated the effects of a group of plasminogen activator called SMTPs in their ability to induce conformational change. In these studies, we have shown that SAXS is a useful technique in the studies conformational changes of plasminogen in the presence of small molecule plasminogen modulators.