Hepsin, SRCR <p>This entry represents the extracellular domain of the serine protease hepsin. The domain is formed primarily by three elements of regular secondary structure: a 12-residue alpha helix, a twisted five-stranded antiparallel beta sheet, and a second, two-stranded, antiparallel sheet. The two beta-sheets lie at roughly right angles to each other, with the helix nestled between the two, adopting an SRCR fold. The exact function of this domain has not been identified, though it probably may serve to orient the protease domain or place it in the vicinity of its substrate [<cite idref="PUB00029725"/>]. </p>