Structural and Functional Divergence of the Aldolase Fold in Toxoplasma gondii.

Parasites of the phylum Apicomplexa are highly successful pathogens of humans and animals worldwide. As obligate intracellular parasites, they have significant energy requirements for invasion and gliding motility that are supplied by various metabolic pathways. Aldolases have emerged as key enzymes involved in these pathways, and all apicomplexans express one or both of fructose 1,6-bisphosphate (F16BP) aldolase and 2-deoxyribose 5-phosphate (dR5P) aldolase (DERA). Intriguingly, Toxoplasma gondii , a highly successful apicomplexan parasite, expresses F16BP aldolase ( Tg ALD1), d5RP aldolase ( Tg DERA), and a divergent dR5P aldolase-like protein ( Tg DPA) exclusively in the latent bradyzoite stage. While the importance of Tg ALD1 in glycolysis is well established and Tg DERA is also likely to be involved in parasite metabolism, the detailed function of Tg DPA remains elusive. To gain mechanistic insight into the function of different T. gondii aldolases, we first determined the crystal structures of Tg ALD1 and Tg DPA. Structural analysis revealed that both aldolases adopt a TIM barrel fold accessorized with divergent secondary structure elements. Structural comparison of Tg ALD1 and Tg DPA with members of their respective enzyme families revealed that, while the active-site residues are conserved in Tg ALD1, key catalytic residues are absent in Tg DPA. Consistent with this observation, biochemical assays showed that, while Tg ALD1 was active on F16BP, Tg DPA was inactive on dR5P. Intriguingly, both aldolases are competent to bind polymerized actin in vitro . Altogether, structural and biochemical analyses of T. gondii aldolase and aldolase-like proteins reveal diverse functionalization of the classic TIM barrel aldolase fold. [ABSTRACT FROM AUTHOR]