 Thrombin

Thrombin is a protease that converts soluble fibrinogen into fibrin, a principal structural component of a blood clot. It cleaves particular peptide bonds and releases short fibrinopeptides, enabling the remaining fibrin units to assemble. Thrombin is generated by activation of the precursor prothrombin.^[ref]

A simple clotting experiment makes the change visible: adding active thrombin to a suitable fibrinogen solution produces an assembling network. The resulting material is not a precipitate of the enzyme itself; a relatively small amount of catalyst can transform a much larger amount of its substrate. This separates the messenger-like activation step, the catalytic reaction and the mechanical structure of the final clot.