Steve Ealick's Research Group


Toxoplasma gondii adenosine kinase


PDB files:

1LII, T. gondii adenosine kinase bound to adenosine and AMP-PCP

1LIO,T. gondii adenosine kinase apoenzyme

1LIJ, T. gondii adenosine kinase bound to prodrug 7-iodotuberdicin and AMP-PCP

1LIK, T. gondii adenosine kinase bound to adenosine

#2ABS, TgAK + AMP-PCP

*2A9Y, TgAK + N6-dimethyladenosine

*2A9Z, TgAK + N6-dimethyladenosine + AMP-PCP

*2AA0, TgAK + 6-methylmercaptopurine riboside

*2AB8, TgAK + 6-methylmercaptopurine riboside + AMP-PCP

Description:

Adenosine kinase (AK) is a key purine metabolic enzyme from the opportunistic parasitic protozoan Toxoplasma gondii and belongs to the family of carbohydrate kinases that includes ribokinase. To understand the catalytic mechanism of AK, we determined. the structures of the T. gondii apo AK, AK:adenosine complex and the AK:adenosine:AMP-PCP complex to 2.55 Å., 2.50 Å. and 1.71 Å. resolution, respectively. These structures reveal a novel catalytic mechanism that involves an adenosine-induced domain rotation of 30 ° and a newly described anion hole (DTXGAGD), requiring a helix-to-coil conformational change that is induced by ATP binding. Nucleotide binding also evokes a coil-to-helix transition that completes the formation of the ATP binding pocket. A conserved dipeptide, Gly68- Gly69, which is located at the bottom of the adenosine-binding site, functions as the switch for domain rotation. The synergistic structural changes that occur upon substrate binding sequester the adenosine and the ATP g phosphate from solvent and optimally position the substrates for catalysis. The 1.84 Å. resolution structure of an AK:7-iodotubercidin:AMP-PCP complex reveals the basis for the higher affinity binding of this prodrug over adenosine and thus provides a scaffold for the design of new inhibitors and subversive substrates that target the T. gondii AK. The high resolution structure, in complex with an ATP analog enabled us to elucidate more details of the active site.

Adenosine kinase from Toxoplasma gondii is a monomeric protein just as is human adenosine kinase. It has a mixed fold and consists of a large and a small domain. In this diagram, the enzyme is bound to adenosine and AMP-PCP.

Click the image to enlarge.

Click here to see an overlay of the substrate binding sites in apo AK (yellow), Ak:adeosine (green), AK:adenosine:AMP-PCP (blue), and AK:7-iodotubercidin:AMP-PCP (red).

Reference:

Schumacher MA, Scott DM, Mathews II, Ealick SE, Roos DS, Ullman B, and Brennan RG. Crystal structures of Toxoplasma gondii adenosine kinase reveal a novel catalytic mechanism and prodrug binding. J. Mol. Biol. 298:875-893 (2000).

#Zhang Y, el Kouni MH and Ealick SE. Crystal structure of Toxoplasma gondii adenosine kinase in complex with an ATP analog at 1.1 Å resolution. Acta Crystallogr. D. 62:140-145 (2006).

*Zhang Y, el Kouni MH and Ealick SE. Substrate analogs induce an intermediate conformational change in Toxoplasma gondii adenosine kinase. Acta Crystallogr. D. 63:126-134 (2007).



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