Structural and inhibitor screening studies of a fungal form of aspartate semialdehyde dehydrogenase from Cryptococcus neoformans

Abstract

Aspartate semialdehyde dehydrogenase (ASADH) functions at a critical junction in the aspartate biosynthetic pathway and represents a validated target for antimicrobial drug design. This enzyme catalyzes the NADPH‐dependent reductive dephosphorylation of β‐aspartyl phosphate to produce the key intermediate aspartate semialdehyde. Production of this intermediate represents the first committed step for the biosynthesis of essential amino acids in fungi and in bacteria. The absence of this enzyme in humans and other mammals will allow selective targeting of pathogenic microorganisms. We have accumulated significant structural and mechanistic information about the bacterial ASADHs, but have only limited knowledge of their fungal counterparts. The structure of a new fungal form of ASADH from Cryptococcus neoformans has been determined to 2.6 Å resolution. The overall structure of CnASADH is similar to that of its bacterial orthologs, but with some critical differences both in biological assembly and in secondary structural features that can potentially be exploited for the development of species selective drugs with selective toxicity against only certain infectious organisms. A customized fragment library has been screened against several fungal ASADHs and initial hits have been identified with inhibition constants (Ki) in the low millimolar range. The ASADH obtained from this fungal species have shown different inhibition selectivity when compared to this enzyme from bacterial species and also differences between fungal species. Docking studies with the CnASADH structure are being used to guide the design and development of potent inhibitors. The long term goal of this project is to develop drugs that selectively target this enzyme and block the aspartate biosynthetic pathway.Support or Funding InformationThis work is supported by funding from the NIH (AI077720).