Nanoscale images of protein complex reveal secret to blood clotting chain reaction

If you have already accidentally cut yourself off on broken glass or a piece of paper, you may have noticed that bleeding can be difficult to stop. Scientists have long wondered how the stunt of events leading to blood clotting is triggered, especially since the process has consequences of life and death. Too little coagulation and you bleed, while too much can cause a heart attack or a stroke.

New detailed 3D structures of blood coagulation proteins, made possible by cryogenic electron microscopy (cryo-EM), have resolved a mystery of thorny biological chemists for over 30 years. Cryo-EM is a structural approach where organic samples are trapped in a layer of non-crystalline ice cream and imagined using powerful electron microscopes.

James Morrissey, Ph.D., professor of biochemistry at UM Medical School, has been studying the different proteins involved in coagulation since the 1980s. To discover their structure, he has teamed up with the expert in cryo-EM Melanie Ohi, Ph.D., professor of cellular biology and development at the UM Medical School and teacher of research at the UM Life Sciences Institute. Their study is published in the journal Blood.

“Most blood coagulation proteins are soluble proteins that circulate in your blood, and there is a missing protein that is hosted on the surface of cells outside the vascular system,” he said.

The normal coagulation process implies an enzyme with two subunits, the tissue factor and the VIIA factor. When this combo connects on a cell surface, he launches the coagulation cascade, he said. The cell membrane plays an important role in the question of whether coagulation will occur.

“A happy and healthy rest cell will not link these blood coagulation proteins,” he said. But an injury causes particular phospholipids of the lipid bicouche that composes the cell membrane to switch to the outside of the cell, where blood coagulation proteins bind to themselves, he explained.

In the case of a cut of abundant bleeding paper, “it could be that if you do not damage enough cells and do not expose enough of these phospholipids from the surface of damaged cells to recruit blood coagulation proteins, then coagulation is slowed down,” noted Morrissey.

This process was extremely difficult to study, so Morrissey and his colleagues used Cryo-EM to determine the 3D structures which enabled them to build an atomic model of protein interactions when associated with a lipid nanodisc.

They found that the tissue factor / factor VIIa factor is linked to the second protein in the coagulation cascade, called factor X, by moving a small tissue factor site out of the way, modifying the structure so that the X factor can agree on this site, like two pieces of puzzle adapting.

“It was exciting to determine a structure that helps to solve the long -standing mystery to explain why the tissue factor cannot activate the blood coagulation cascade only when specific lipids are outside a cell. For me, this work highlights two research forces at the University of Michigan to establish successful interdisciplinary collaborations and access to one of the best microscopy facilities The world.

The results help to explain “something that people have known for years: that this part of the tissue factor was important to recognize the substrate and allow the reaction to take place. But no one has found the way proteins really amortize,” said Morrissey.

Anticoagulant drugs like warfarin, prescribed to treat dangerous blood coagulation, work by weakening the ability of these proteins to interact with the cell membrane, but they worked without scientists fully understanding what was going on. This fundamental scientific work finally provides new perspectives on the mechanism behind.