Phomoxanthones A (1) and B (2), two novel xanthone dimers, were isolated from the endophytic fungus Phomopsis sp. BCC 1323. Structures of 1 and 2 were elucidated by spectroscopic methods. These compounds exhibited significant in vitro antimalarial and antitubercular activities and cytotoxicity.
Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), currently infects one third of the world's population in its latent form. The emergence of multidrug resistant strains, MDR-TB and XDR-TB has highlighted the need for new pharmacological targets within M. tuberculosis. The Class IIa fructose 1,6-bisphosphate aldolase (FBA) enzyme from M. tuberculosis (MtFBA) has been proposed as one such target since its upregulated in latent TB. Since the structure of MtFBA had not been determined and since there was little information available on its reaction mechanism, we sought to determine the X-ray structure of MtFBA in complex with its substrates. By lowering the pH of the enzyme in the crystalline state, we were able to determine a series of high-resolution X-ray structures of MtFBA bound to dihydroxyacetonephosphate (DHAP), glyceraldehyde-3-phosphate (G3P), and fructose 1,6-bisphosphate (FBP) at 1.5 Å, 2.1 Å, and 1.3 Å respectively. Through these structures it was discovered that MtFBA belongs to a novel tetrameric class of the type IIa FBAs. The molecular details at the interface of the tetramer revealed important information for being able to better predict the quaternary structures among the FBAs based on their primary sequences. These X-ray structures also provide interesting and new details on the reaction mechanism of class II FBAs. Not only were the substrates and products observed in geometries poised for catalysis, but unexpectedly the hydroxyl enolate intermediate of DHAP was also captured and resolved structurally. These concise new details provide a better understanding of the reaction mechanisms for FBAs in general and provide a structural basis for inhibitor design efforts aimed at this class of enzymes.
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