Pepsin inhibitor-3 consisting of two domains, each comprising an antiparallel beta-sheet flanked by an alpha-helix. In the enzyme-inhibitor complex, the N-terminal beta-strand of PI-3 pairs with one strand of the active site flap region of pepsin [1]. The two domains are tandem repeats of sequence, and has therefore been termed repeated domain.

1: Structural basis for the inhibition of porcine pepsin by Ascaris pepsin inhibitor-3. Ng KK, Petersen JF, Cherney MM, Garen C, Zalatoris JJ, Rao-Naik C, Dunn BM, Martzen MR, Peanasky RJ, James MN; Nat Struct Biol 2000;7:653-657. PMID:10932249