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Human tryptase, a mast-cell-specific serine proteinase that may be involved in causing asthma and other allergic and inflammatory disorders [1-3], is unique in two respects: it is enzymatically active only as a heparin-stabilized tetramer, and it is resistant to all known endogenous proteinase inhibitors. The 3-Angstrom crystal structure of human beta-tryptase in a complex with 4-amidinophenyl pyruvic acid shows four quasi-equivalent monomers arranged in a square flat ring of pseudo 222 symmetry. Each monomer contacts its neighbours at two different interfaces through six loop segments. These loops are located around the active site of beta-tryptase and differ considerably in length and conformation from loops of other trypsin-like proteinases. The four active centres of the tetramer are directed towards an oval central pore, restricting access for macromolecular substrates and enzyme inhibitors. Heparin chains might stabilize the complex by binding to an elongated patch of positively charged residues spanning two adjacent monomers. The nature of this unique tetrameric architecture explains many of tryptase's biochemical properties and provides a basis for the rational design of monofunctional and bifunctional tryptase inhibitors.