Steve Ealick's Research Group


Trichomonas vaginalis Purine Nucleoside Phosphorylase


PDB files:

1Z33 unliganded TvPNP

1Z34 TvPNP complexed with 2-fluoro-2'- deoxyadenosine (F-dAdo)

1Z35 TvPNP complexed with 2-fluoroadenosine (F-Ado)

1Z36 TvPNP complexed with Formycin A (FMA)

1Z37 TvPNP complexed with adenosine (Ado)

1Z38 TvPNP complexed with inosine (Ino)

1Z39 TvPNP complexed with 2'-deoxyinosine (dIno)

Description:

Trichomonas vaginalis is an anaerobic protozoan parasite that causes one of the most common sexually transmitted infections in the world. Parasitic protozoa lack de novo synthesis of purine nucleotides and depend on purine salvage to replenish their purine nucleotide pools. This metabolic deficiency opens up the possibility of using a “rational approach” to new drug discovery against protozoan parasites via purine salvage enzyme inhibitor searching and design. Purine nucleoside phosphorylase (PNP) catalyzes the reversible phosphorolysis of (2′-deoxy)purine ribonucleosides to free base and (2′-deoxy)ribose-1-phosphate. The monomer unit in mammals has a molecular weight of about 31 kDa, while the monomer in prokaryotes is about 26 kDa T. vaginalis PNP (TvPNP) is very similar to E. coli PNP (EcPNP) and quite different from human PNP in that the active sites have different amino acids, giving them different specificity and different inhibition profiles. The differences in enzyme specificity offer possibilities for new "designer drug" approaches to control the growth of the parasite.

The monomer unit of TvPNP is shown with deoxyinosine bound at the active site, which is predominantly hydrophobic. Compare this monomer unit to those in E. coli PNP, human PNP and bovine PNP.

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The active enzyme in T. vaginalis is hexameric as it is in E. coli, while in mammals it is trimeric. TvPNP hexamer can be considered to be a “trimer of dimers”, with contacts between subunits within dimers that are more extensive than those between the three dimers. Two adjacent subunits are involved in the formation of each active site. The active sites in hexameric PNP's are more accessible than those in trimeric PNP. Compare this structure to the oligomers of E. coli PNP, human PNP and bovine PNP.

 

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This schematic diagram of the active site shows the F-dAdo complex . The principal active site difference between the active site of TvPNP and that of EcPNP is at Thr156, which is alanine in EcPNP. In the complex of TvPNP with F-dAdo, Thr156 causes the purine base to tilt and shift by 0.5 Å compared to the binding scheme of F-dAdo in E. coli PNP.

 

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Reference:

Zhang Y, Wang W-H, Wu S-W, Ealick SE, and Wang CC. Identification of the Subversive Substrate of Trichomonas vaginalis Purine Nucleoside Phosphorylase and the Crystal Structure of the Enzyme-Substrate Complex. J. Biol. Chem. 280:22318-22325 (2005).



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