Crystal Structure Analysis of a Serine Proteinase from Streptomyces fradiae at 0.16‐nm Resolution and Molecular Modeling of an Acidic‐amino‐acid‐specific Proteinase

We have determined the three‐dimensional structure of a proteinase from Streptomyces fradiae ATCC 14544 (SFase‐2) at 0.16‐nm resolution. SFase‐2, a typical serine proteinase, has broad substrate specificity. The characterization and crystallographic analysis of this enzyme have been reported previously [Kitadokoro, K., Tsuzuki, H., Nakamura, E., Sato, T. & Teraoka, H. (1994) Eur: J. Biochem. 220, 55–61]. In the present study, data were collected to approximately 0.16‐nm resolution on a Rigaku R‐AXIS IIC imaging plate detector system. Preliminary phases were obtained by molecular replacement methods with a search model derived from the previously determined structure of Streptomyces griseus protease A [Sielecki, A. R., Hendrickson, W. A., Broughton, C. G., Delbaere, L. T., Brayer, G. D. & James, M. N. (1979) J. Mol. Biol. 134, 781–8041. The starting model gave an initial crystallographic R factor of 0.443. Refinement with restrained least‐squares converged at a final R factor of 0.182 for 16128 observed reflections. The final model includes 86 water molecules. The crystal structure showed that the enzyme consists of two domains, each of which is comprised of a β barrel with six‐stranded β sheets and two α helices. The overall tertiary structure of SFase‐2 is similar to the structures of other chyrnotrypsin‐like proteinases from S. griseus, namely proteinase A and proteinase B. The essential residues of the catalytic triad are located on the cleft between the two domains. These two domains have different sequences, but possess similar three‐dimensional structures, indicating that a gene duplication event has occurred to produce these two domains.