Crystal structures of Toxoplasma gondii adenosine kinase reveal a novel catalytic mechanism and prodrug binding

Adenosine kinase (AK) is a key purine metabolic enzyme from the opportunist ic parasitic protozoan Toxoplasma gondii and belongs to the family of carbo hydrate kinases that includes ribokinase. To understand the catalytic mecha nism of AK, we determined the structures of the T. gondii apo AK, AK:adenos ine complex and the AK:adenosine:AMP-PCP complex to 2.55 Angstrom, 2.50 Ang strom and 1.71 Angstrom resolution, respectively. These structures reveal a novel catalytic mechanism that involves an adenosine-induced domain rotati on of 30 degrees and a newly described anion hole (DTXGAGD), requiring a he lix-to-coil conformational change that is induced by ATP binding. Nucleotid e binding also evokes a coil-to-helix transition that completes the formati on of the ATP binding pocket. A conserved dipeptide, Gly68-Gly69, which is located at the bottom of the adenosine-binding site, functions as the switc h for domain rotation. The synergistic structural changes that occur upon s ubstrate binding sequester the adenosine and the ATP gamma phosphate from s olvent and optimally position the substrates for catalysis. Finally, the 1. 84 Angstrom resolution structure of an AK:7-iodotubercidin:AMP-PCP complex reveals the basis for the higher affinity binding of this prodrug over aden osine and thus provides a scaffold for the design of new inhibitors and sub versive substrates that target the T. gondii AK. (C) 2000 Academic Press.