Antithrombin, molecular model. Antithrombin is a glycoprotein produced in the liver that inhibits enzymes involved in blood clotting. Some people have a genetic disorder that means that they are deficient in antithrombin, which causes them to suffer frequent abnormal blood clots that can block blood vessels (thrombosis). In this molecular model, the secondary structure is shown. This is made up of alpha helices (red spirals) and beta sheets (green arrows), joined by grey linking regions. The arrangement of these domains makes up the molecule's tertiary structure; its overall shape.