A schematic diagram of fibrinogen E-fragment and alphastatin. (A) The central domain of fibrinogen, fibrinogen E fragment, consists of 2 α chains (black), 2 β chains (dark gray), and 2 γ chains (light gray). Alphastatin is located at the amino terminus of each of the α chains, a region which contains fibrinopeptide A (FpA). (B) Alphastatin, the first 24 amino acids of the α chain, consists of a β bend stabilized by a salt bridge between R19 and D7 (arrow). The thrombin cleavage site is denoted by a vertical line and the sequence of FpA is from A-R.