Biochemical and structural studies on trypanosomatid pyruvate kinases
Glycolytic enzymes have been indicated as potential drug targets in trypanosomatid parasites such as Trypanosoma brucei (T. brucei), Trypanosoma cruzi (T. cruzi) and Leishmania spp. Pyruvate kinase (PYK) catalyses the final reaction in the glycolytic pathway to produce ATP and pyruvate from ADP and phosphoenolpyruvate (PEP), and has been validated by RNAi experiments as a suitable drug target in T. brucei. This thesis describes biochemical and structural studies of PYKs from T. cruzi (TcPYK) and T. brucei (TbPYK), providing not only a foundation but also new clues for PYK-specific inhibitor screening and structure-based drug design. Soluble TcPYK and TbPYK (81% sequence identity) have been expressed and purified from E. coli, and their kinetics have been fully characterised. X-ray crystal structures of apoenzyme TcPYK (apo TcPYK), and of TbPYK in complex with fructose 2,6-bisphosphate (F26BP) (TbPYK/F26BP/Mg) have been determined, and each possesses a tetrameric architecture composed of four identical protein chains. Each chain contains four domains which are A-domain, B-domain, C-domain and N-terminal domain. The active site is located in the cleft between the A- and B-domains, while the F26BP-bound effector site is within the C-domain. The conformational transition between inactive T-state and active R-state for both enzymes requires a concerted 8o rigid-body rotation of each of the four AC-cores (Aand C-domains) in the tetramer. During the T- to R-state transition induced by F26BP binding, the side chain of Arg311 is re-orientated to stabilise the short Aα6′ helix at the active site, and the flexible loop at the effector site is stabilised by F26BP. In this active conformation additional salt bridges form across the C-C interface to lock the enzyme in a more stable R-state. TbPYK/F26BP/Mg is the first ‘effector only’ PYK structure and identifies a third Mg2+ binding site (Mg-3) which is distinct from the two canonical Mg2+ binding sites. The substrate PEP was soaked into crystals of TbPYK/F26BP/Mg resulting in an ‘in crystallo’ 23° B-domain rotation forming a partially closed active site. This is accompanied by active site side-chain reorientations, and the movement of Mg2+ from its ‘priming’ position Mg-3 to its canonical position Mg-1. It is plausible that Mg2+ is retained in its ‘priming’ position after product release to act as a co-activator with F26BP to maintain the enzyme in its R-state conformation, as long as F26BP is present. The inherent oxaloacetate decarboxylase activity of PYK was reported over 30 years ago and has been further characterised by 1H NMR studies in this thesis. In addition, a series of TbPYK structures in complex with product (pyruvate), with analogues of the decarboxylase substrate oxaloacetate (D-malate and α-ketoglutarate), or with the competitive inhibitor oxalate have been determined by crystal soaking, and indicate that both decarboxylase activity and kinase activity share a common active site. A proposed mechanism explains the conserved decarboxylase activity of PYK where the active-site Mg2+ and Lys239 in TbPYK (which is conserved between species) play essential roles in the decarboxylation reaction. Three strategies for designing novel inhibitors against trypanosomatid PYKs have been proposed in this thesis. (1) Develop selective modulators to increase the binding affinity of inhibitors. As an example, F16BP has been shown to regulate the inhibitory effect of PEP analogues (oxalate, D-malate, α-ketoglutarate, malonate and L-tartrate) on TbPYK activity. (2) Develop allosteric inhibitors in order to lock trypanosomatid PYKs in an inactive state where the enzyme has low affinity for substrate binding. (3) A third strategy is to combine multiple modulators and inhibitors to increase the inhibition efficiency and selectivity.