Steve Ealick's Research Group

Calf Spleen Purine Nucleoside Phosphorylase

PDB files:

1PBN (native)

1A9O (with phosphate)

1A9P (with 9-deazainosine and phosphate)

1A9Q (with inosine)

1A9S (with inosine and sulfate)

1A9T (with 9-deazahypoxanthine and ribose 1-phosphate)

1A9R (with hypoxanthine)

3PNP (with phosphate and Mg2+; trimer)

4PNP (with 9-deazainosine and phosphate;trimer)

*2AI1Yb2 (guanosine-2′,3′-O-ethylphosphonate)

*2AI2 Yb3 (9-deazainosine-2′,3′-O-ethylphosphonate)

*2AI3 Yb1 (guanosine-2′,3′-O-methylphosphonate)


Purine nucleoside phosphorylase (PNP) is a key enzyme in the purine salvage pathway, which provides an alternative to the de novo pathway for the biosynthesis of purine nucleotides. PNP catalyzes the reversible phosphorolysis of 2′-deoxypurine ribonucleosides to the free bases and 2-deoxyribose 1-phosphate. The bovine PNP structure reveals several new details of substrate and inhibitor binding, including two phosphate-induced conformational changes involving residues 33-36 and 56-69 and a previously undetected role for His64 in phosphate binding. In addition, a well-ordered water molecule is found in the PNP active site when purine base or nucleoside is also present. In contrast to human PNP, only one phosphate binding site was observed. Although binary complexes were observed for nucleoside, purine base, or phosphate, ribose 1-phosphate binding occurs only in the presence of purine base.

Each subunit has an α/β structure with a central eight-strand mixed sheet and a smaller five-strand sheet. This model of the bovine PNP monomer has inosine and sulfate modeled at the active site.


Click the image to enlarge.


The active form of bovine PNP is a trimer of identical subunits, as it is in human PNP. There is one active site per subunit.


Click the image to enlarge.

This model shows the active site in bovine PNP, complexed with both sulfate and inosine, which are shown in red. Residues from one subunit are shown in blue and that (159) from an adjacent subunit is magenta.


Mao C, Cook,WJ, Zhou M, Fedorov AA, Almo SC and Ealick SE. Calf Spleen Purine Nucleoside Phosphorylase Complexed with Substrates and Substrate Analogs. Biochemistry 37:7135-7146 (1998).

*Toms AV, Wang W, Li Y, Ganem B and Ealick SE. Novel Multisubstrate Inhibitors of Mammalian Purine Nucleoside Phosphorylase. Acta Crystallogr. D. 61:1449-58 (2005).


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