CALF SPLEEN PURINE NUCLEOSIDE PHOSPHORYLASE IN COMPLEX WITH AN N(7)-ACYCLOGUANOSINE INHIBITOR. Gertraud Koellner, Marija Luic, Agnieszka Bzowska, David Shugar & Wolfram Saenger, Institut fur Kristallographie, Freie Universitat Berlin, Takustr. 6, D-14159 Berlin, Germany

The complex of calf spleen purine nucleoside phosphorylase with an N(7)- acycloguanosine inhibitor was crystallized in the cubic space group P2₁3 with an unit cell dimension a=94.02Å and one monomer in the asymmetric unit. The biologically active trimer is formed by the crystallographic three-fold axis. The structure was solved by molecular replacement using the model of the human erythrocyte enzyme [Ealick et al., Proc. Nat. Acad. Sci. USA 88, 11540 11544 (1990)]. The complexed calf spleen PNP crystallizes at pH 8.2-8.5 from PEG, which is almost optimal for enzyme activity [Kulikowska et al., Biochim. Biophys. Acta 874, 355- 363 (1986)]. N(7)-acycloguanosine binds in an inverted ('upside-down') orientation with respect to guanosine in the human PNP. The acyclic chain is engaged in several hydrogen bonds. Since the crystals were grown at pH 8.2-8.5. the secondary nitrogen of the acyclic chain (pKa~9.5) should be protonated. It follows that it is the acyclic chain which is predominantly responsible for binding of the inhibitor.

Agnieszka Bzowska, Marija Luic, Werner Schröder, David Shugar, Wolfram Saenger, Gertraud Koellner. (1995) FEBS Letters, 367, 214-218.