Poster Presentation The 48th Lorne Conference on Protein Structure and Function 2023

Phosphoribosylaminoimidazole (AIR) carboxylase: reversing evolution to dissect a promising antifungal drug target in Cryptococcus neoformans (#347)

Tristan Villamor 1 , Sheena Chua 1 , Weixi Gu 1 , Jeffrey Nanson 1 , Bostjan Kobe 1 , James Fraser 1
  1. School of Chemistry and Molecular Biosciences , The University of Queensland , Brisbane , Queensland , Australia

The World Health Organisation has recently emphasised how infections caused by opportunistic fungal pathogens have increased in prevalence due to the rise of the immunocompromised population. One such pathogen on the critical priority group of the WHO is Cryptococcus neoformans, causing 180,000 deaths annually through meningoencephalitis. Commonly found in purine-rich environments, such as pigeon guano, the fungus must rely on the de novo synthesis of its own purines during infection of the purine-poor human host making this pathway a promising drug target. In fungi, a two-domain enzyme in the purine pathway named Ade2 converts phosphoribosylaminoimidazole to phosphoribosylcarboxyaminoimidazole in a two-step procedure with an intermediate, N(5)-phosphoribosyl-carboxyaminoimidazole, being produced. In humans, this enzymatic activity is simplified with a single enzyme domain performing the conversion in a single step with no intermediate being created; a difference that has strong potential as an antimicrobial drug target, as Ade2 is required for virulence. Building on our genetic studies, structural characterisation show the full-length Ade2 enzyme exists as an octamer in solution and yielded rod-shaped crystals, which diffracted to 3.7 Å with 8 molecules in the asymmetric unit. The phase problem has been solved by molecular replacement using an AlphaFold2 predicted dimer as a search model revealing a structure that mimics the shape of a Maltese cross. Structural refinement is currently being conducted. The solved structure of the Ade2 enzyme from C. neoformans will provide a strong candidate as a drug target, especially as no homolog for the full-length fungal Ade2 enzyme has been published.  

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