Blood clotting occurs in response to blood vessel damage. This requires the specific recruitment of platelets (specialised blood cells) to the site of injury as one of the first (of many) events that prevents bleeding. This process is highly dependent upon a protein known as von Willebrand factor (VWF) that circulates in blood. The ability of VWF to perform this task is regulated by an enzyme that is also present in the blood, ADAMTS13, and that, under very single substrate specific circumstances, cleaves VWF into smaller forms that are less capable of recruiting platelets. Clinically, deficiency in VWF is the most common inherited bleeding disorder, whereas people with ADAMTS13 deficiency suffer from a life-threatening thrombotic disorder with a ~90% mortality rate. More subtle differences in the blood levels/function of VWF and ADAMTS13 are also important determinants of an individual's risk of bleeding and thrombosis, and also influence the likelihood of both heart attack and stroke. ADAMTS13 is a very highly specific proteolytic enzyme that cleaves only one protein (VWF) and does so at just a single site, and even then, only under very specific conditions of blood flow. ADAMTS13 is made up of multiple domains. The metalloprotease domain of this enzyme contains the active site that cleaves VWF, whereas the other variably contribute to the binding of ADAMTS13 to VWF. Despite this knowledge, how ADAMTS13 recognises and cleaves VWF so specifically remains unclear. To understand this at a molecular level, we will ascertain the structure of different domains fragments of ADAMTS13, both in free forms and in stabilising complexes with specific antibody fragments that can aid in determining structures. In addition, we will also elucidate the structure of ADAMTS13 fragments whilst bound to the corresponding fragments of VWF. We will characterise the binding and cleavage of these VWF fragments by ADAMTS13 and also explore the influence of calcium binding to this process. The information from this project will provide important insights into how ADAMTS13 functions at a molecular level its unique single substrate specific cleavage of VWF. This data will provide the opportunity to rationally engineer ADAMTS13 to improve its efficacy as a therapeutic agent, for which it is currently under development as a more specific clotbuster for the treatment of thrombotic disease.