Scientists report the X-ray crystal structure of full-length human plasminogen, and describe binding of blood clots to trigger conversion of the protein into the functional fibrinolytic enzyme plasmin. The Monash University-led team in Australia hopes their findings will help in the development of new clot-busting plasminogen activators, or plasmin inhibitors as potential anticancer therapeutics. James C. Whisstock, Ph.D., and colleagues report their findings in Cell Reports, in a paper titled “The X-ray Crystal Structure of Full-Length Human Plasminogen.”
The closed, activation-resistant conformation of circulating plasminogen is composed of a Pan-apple (PAp) domain, five kringle domains (KR1-5), and a serine protease (SP) domain. The kringle domains mediate interactions with fibrin clots and cell-surface receptors, and these interactions allow plasminogen to take on an open form that can be cleaved and converted to plasmin by tissue-type and urokinase-type plasminogen activator.
Working with researchers at the Australian Syncortron, the new X-ray data reported by the Monash University team suggests that the PAp and SP domains, together with chloride ions, are responsible for anchoring the molecule into its closed conformation through interactions with the kringle array. Effectively, the N-terminal PAp domain makes extensive contacts with KR4 and KR5. In addition, KR4 forms another interface with the activation loop and the SP domain. The interactions overall act to trap the KR3/KR4 linker region in a critical position that protects the proenzyme from unwanted activation.
While this at first sight appears to completely block the binding site, the researchers also surprisingly found that there was evident instability in one region of the molecule. This instability allows the KR5 domain to move away from the core of the plasminogen structure, resulting in transient exposure of the lysine binding site.
“The PAp domain makes an imperfect interaction with KR5 that is insufficient to stably tether this domain to the closed plasminogen core,” the authors explain. “The resulting mobility in KR5 thus most likely represents the Achilles heel of closed plasminogen to activating ligands. Once the KR5 LBS is exposed, interactions with lysine residues present in binding partners may irreversibly set the molecule on the pathway of conformational change.”
The team maintains their findings will provide valuable new insights into how plasminogen-activating drugs work, and aid in new drug design. “Now with the structure of plasminogen and an enhanced understanding of how it is converted to plasmin, we finally have a platform to develop new and more effective clot-busting therapeutics,” states co-senior author Paul Couglin, M.D., at the Australian Centre for Blood diseases.