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

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.

NADP/H binding nearly doubles the stability of a Mycobacterium drug target: an unfolding study

Mycobacterium Aspartate beta semialdehyde dehydrogenase (ASADH) was studied using various spectroscopic techniques and size exclusion chromatography to examine the unfolding of free (apo) and NADP/H-bound (holo) forms of ASADH. Non-cooperative guanidinium chloride (GdnHCl)-induced unfolding of the apo ASADH was discovered, and no partially folded intermediate structures were stabilized. On the other hand, it was observed that GdnHCl's unfolding of holoenzyme was a cooperative process without any stable intermediate structure. The native form of holoenzyme is found to be stable against the lower concentration of GdnHCl only (namely up to 1.25 M GdnHCl). The tryptophan environment appears to unfold cooperatively in case of the holoenzyme and is in well coordination with the overall unfolding of the holoenzyme. The presence of NADP/H shows a stabilizing effect on the tryptophan environment as well as on the native NADP/H-bound enzyme. Δ G Solvent o values reveal nearly two-fold (∼1.9) conformationally more stable folded holoenzyme compared to its native apo state. The Cm for the apo and holo forms of ASADH are 1.3 and 1.9 M, respectively. Novel drug leads targeting the NADP/H binding domain of ASADH could offer promising drugs against extremely infective Mycobacterium tuberculosis.Communicated by Ramaswamy H. Sarma.

The coordinated action of the enzymes in the L-lysine biosynthetic pathway and how to inhibit it for antibiotic targets

BACKGROUND

Antimicrobial resistance is a global health issue that requires immediate attention in terms of new antibiotics and new antibiotic targets. The l-lysine biosynthesis pathway (LBP) is a promising avenue for drug discovery as it is essential for bacterial growth and survival and is not required by human beings.

SCOPE OF REVIEW

The LBP involves a coordinated action of fourteen different enzymes distributed over four distinct sub-pathways. The enzymes involved in this pathway belong to different classes, such as aspartokinase, dehydrogenase, aminotransferase, epimerase, etc. This review provides a comprehensive account of the secondary and tertiary structure, conformational dynamics, active site architecture, mechanism of catalytic action, and inhibitors of all enzymes involved in LBP of different bacterial species.

MAJOR CONCLUSIONS

LBP offers a wide scope for novel antibiotic targets. The enzymology of a majority of the LBP enzymes is well understood, although these enzymes are less widely studied in the critical pathogens (according to the 2017 WHO report) that require immediate attention. In particular, the enzymes in the acetylase pathway, DapAT, DapDH, and Aspartokinase in critical pathogens have received little attention. High throughput screening for inhibitor design against the enzymes of lysine biosynthetic pathway is rather limited, both in number and in the extent of success.

GENERAL SIGNIFICANCE

This review can serve as a guide for the enzymology of LBP and help in identifying new drug targets and designing potential inhibitors.

Structural characterization of aspartate-semialdehyde dehydrogenase from Pseudomonas aeruginosa and Neisseria gonorrhoeae

Gonorrhoea infection rates and the risk of infection from opportunistic pathogens including P. aeruginosa have both risen globally, in part due to increasing broad-spectrum antibiotic resistance. Development of new antimicrobial drugs is necessary and urgent to counter infections from drug resistant bacteria. Aspartate-semialdehyde dehydrogenase (ASADH) is a key enzyme in the aspartate biosynthetic pathway, which is critical for amino acid and metabolite biosynthesis in most microorganisms including important human pathogens. Here we present the first structures of two ASADH proteins from N. gonorrhoeae and P. aeruginosa solved by X-ray crystallography. These high-resolution structures present an ideal platform for in silico drug design, offering potential targets for antimicrobial drug development as emerging multidrug resistant strains of bacteria become more prevalent.

Aspartate semialdehyde dehydrogenase inhibition suppresses the growth of the pathogenic fungus Candida albicans

Potent inhibitors of an essential microbial enzyme have been shown to be effective growth inhibitors of Candida albicans, a pathogenic fungus. C. albicans is the main cause of oropharyngeal candidiasis, and also causes invasive fungal infections, including systemic sepsis, leading to serious complications in immunocompromised patients. As the rates of drug-resistant fungal infections continue to rise novel antifungal treatments are desperately needed. The enzyme aspartate semialdehyde dehydrogenase (ASADH) is critical for the functioning of the aspartate biosynthetic pathway in microbes and plants. Because the aspartate pathway is absent in humans, ASADH has the potential to be a promising new target for antifungal research. Deleting the asd gene encoding for ASADH significantly decreases the survival of C. albicans, establishing this enzyme as essential for this organism. Previously developed ASADH inhibitors were tested against several strains of C. albicans to measure their possible therapeutic impact. The more potent inhibitors show a good correlation between enzyme inhibitor potency and fungal growth inhibition. Growth curves generated by incubating different C. albicans strains with varying enzyme inhibitor levels show significant slowing of fungal growth by these inhibitors against each of these strains, similar to the effect observed with a clinical antifungal drug. The most effective inhibitors also demonstrated relatively low cytotoxicity against a human epithelial cell line. Taken together, these results establish that the ASADH enzyme is a promising new target for further development as a novel antifungal treatment against C. albicans and related fungal species.

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 aspartate β-semialdehyde dehydrogenase from Francisella tularensis

Aspartate β-semialdehyde dehydrogenase (ASADH) is an enzyme involved in the diaminopimelate pathway of lysine biosynthesis. It is essential for the viability of many pathogenic bacteria and therefore has been the subject of considerable research for the generation of novel antibiotic compounds. This manuscript describes the first structure of ASADH from Francisella tularensis, the causative agent of tularemia and a potential bioterrorism agent. The structure was determined at 2.45 Å resolution and has a similar biological assembly to other bacterial homologs. ASADH is known to be dimeric in bacteria and have extensive interchain contacts, which are thought to create a half-sites reactivity enzyme. ASADH from higher organisms shows a tetrameric oligomerization, which also has implications for both reactivity and regulation. This work analyzes the apo form of F. tularensis ASADH, as well as the binding of the enzyme to its cofactor NADP+.