A cautionary tale of structure-guided inhibitor development against an essential enzyme in the aspartate-biosynthetic pathway

Structural-functional analysis of drug target aspartate semialdehyde dehydrogenase

Aspartate β-semialdehyde dehydrogenase (ASADH) is a key enzyme in the biosynthesis of essential amino acids in microorganisms and some plants. Inhibition of ASADHs can be a potential drug target for developing novel antimicrobial and herbicidal compounds. This review covers up-to-date information about sequence diversity, ligand/inhibitor-bound 3D structures, potential inhibitors, and key pharmacophoric features of ASADH useful in designing novel and target-specific inhibitors of ASADH. Most reported ASADH inhibitors have two highly electronegative functional groups that interact with two key arginyl residues present in the active site of ASADHs. The structural information, active site binding modes, and key interactions between the enzyme and inhibitors serve as the basis for designing new and potent inhibitors against the ASADH family.

A Fragment Library Screening Approach to Identify Selective Inhibitors against an Essential Fungal Enzyme

Pathogenic fungi represent a growing threat to human health, with an increase in the frequency of drug-resistant fungal infections. Identifying targets from among the selected metabolic pathways that are unique to microbial species presents an opportunity to develop new antifungal agents against new and untested targets to combat this growth threat. Aspartate semialdehyde dehydrogenase (ASADH) catalyzes a key step in a uniquely microbial amino acid biosynthetic pathway and is essential for microbial viability. This enzyme, purified from four pathogenic fungal organisms ( Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans, and Blastomyces dermatitidis), has been screened against fragment libraries to identify initial enzyme inhibitors. The binding of structural analogs of the most promising lead compounds was measured against these fungal ASADHs to establish important structure-activity relationships among these different inhibitor classes. The most potent of these inhibitors have been docked into structures of this fungal enzyme target to identify important structural elements that serve as critical binding determinants. Several inhibitors with low micromolar inhibition constants have been identified that showed selectivity against these related enzymes from different fungal species. Subsequent screening against a library of drugs and drug candidates identified some additional inhibitors containing a consistent set of functional groups required for fungal ASADH inhibition. Additional elaboration of these core structures will likely lead to more potent and selective inhibitors.

Structure of a fungal form of aspartate-semialdehyde dehydrogenase from Aspergillus fumigatus

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. The absence of this entire pathway in humans and other mammals will allow the selective targeting of pathogenic microorganisms for antimicrobial development. Here, the X-ray structure of a new form of ASADH from the pathogenic fungal species Aspergillus fumigatus has been determined. The overall structure of this enzyme is similar to those of its bacterial orthologs, but there are 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 infectious fungal organisms.

Crystal Structure of the LysY·LysW Complex from Thermus thermophilus

Several bacteria and archaea utilize the amino group-carrier protein, LysW, for lysine biosynthesis, in which an isopeptide bond is formed between the C-terminal Glu of LysW and an amino group of α-aminoadipate (AAA). The resulting LysW-γ-AAA is phosphorylated by LysZ to form LysW-γ-AAA phosphate, which is subsequently reduced to LysW-γ-aminoadipic semialdehyde (LysW-γ-AASA) through a reaction catalyzed by LysY. In this study, we determined the crystal structures of LysY from Thermus thermophilus HB27 (TtLysY) complexed with TtLysW-γ-AASA and TtLysW-γ-AAA, respectively. In both structures, the globular domain of TtLysW was recognized by positively charged residues on helix α9 and the β11-α10 loop of TtLysY through conformational changes. A mutational analysis confirmed that the interactions observed between TtLysY and TtLysW are important for the function of TtLysY. The extended LysW recognition loop and conserved arginine residue were identified as signatures to discriminate LysY from ArgC, which is involved in arginine biosynthesis. Combined with the previously determined TtLysZ·TtLysW complex structure, TtLysW may simultaneously bind TtLysZ and TtLysY. These structural insights suggest the formation of a TtLysWZY ternary complex, in which the flexible C-terminal extension of TtLysW promotes the efficient transfer of the labile intermediate from the active site of TtLysZ to that of TtLysY during the sequential reactions catalyzed by TtLysZY.

Structure of a fungal form of aspartate semialdehyde dehydrogenase from Cryptococcus neoformans

Aspartate semialdehyde dehydrogenase (ASADH) functions at a critical junction in the aspartate-biosynthetic pathway and represents a valid 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 in the biosynthesis of the essential amino acids methionine, isoleucine and threonine in fungi, and also the amino acid lysine in bacteria. 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 those 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.

Elaboration of a fragment library hit produces potent and selective aspartate semialdehyde dehydrogenase inhibitors

Aspartate-β-semialdehyde dehydrogenase (ASADH) lies at the first branch point in the aspartate metabolic pathway which leads to the biosynthesis of several essential amino acids and some important metabolites. This pathway is crucial for many metabolic processes in plants and microbes like bacteria and fungi, but is absent in mammals. Therefore, the key microbial enzymes involved in this pathway are attractive potential targets for development of new antibiotics with novel modes of action. The ASADH enzyme family shares the same substrate binding and active site catalytic groups; however, the enzymes from representative bacterial and fungal species show different inhibition patterns when previously screened against low molecular weight inhibitors identified from fragment library screening. In the present study several approaches, including fragment based drug discovery (FBDD), inhibitor docking, kinetic, and structure-activity relationship (SAR) studies have been used to guide ASADH inhibitor development. Elaboration of a core structure identified by FBDD has led to the synthesis of low micromolar inhibitors of the target enzyme, with high selectivity introduced between the Gram-negative and Gram-positive orthologs of ASADH. This new set of structures open a novel direction for the development of inhibitors against this validated drug-target enzyme.