Human beta-tryptase is a ring-like tetramer with active sites facing a central pore.
Pereira, P.J., Bergner, A., Macedo-Ribeiro, S., Huber, R., Matschiner, G., Fritz, H., Sommerhoff, C.P., Bode, W.(1998) Nature 392: 306-311
- PubMed: 9521329 
- DOI: 10.1038/32703
- Primary Citation of Related Structures:  
1A0L - PubMed Abstract: 
Human tryptase, a mast-cell-specific serine proteinase that may be involved in causing asthma and other allergic and inflammatory disorders, 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 ...