Structural insights into the antifungal drug target guanosine monophosphate synthase from Aspergillus fumigatus

Natural product guvermectin inhibits guanosine 5'-monophosphate synthetase and confers broad-spectrum antibacterial activity

Bacterial diseases caused substantial yield losses worldwide, with the rise of antibiotic resistance, there is a critical need for alternative antibacterial compounds. Natural products (NPs) from microorganisms have emerged as promising candidates due to their potential as cost-effective and environmentally friendly bactericides. However, the precise mechanisms underlying the antibacterial activity of many NPs, including Guvermectin (GV), remain poorly understood. Here, we sought to explore how GV interacts with Guanosine 5'-monophosphate synthetase (GMPs), an enzyme crucial in bacterial guanine synthesis. We employed a combination of biochemical and genetic approaches, enzyme activity assays, site-directed mutagenesis, bio-layer interferometry, and molecular docking assays to assess GV's antibacterial activity and its mechanism targeting GMPs. The results showed that GV effectively inhibits GMPs, disrupting bacterial guanine synthesis. This was confirmed through drug-resistant assays and direct enzyme inhibition studies. Bio-layer interferometry assays demonstrated specific binding of GV to GMPs, with dependency on Xanthosine 5'-monophosphate. Site-directed mutagenesis identified key residues crucial for the GV-GMP interaction. This study elucidates the antibacterial mechanism of GV, highlighting its potential as a biocontrol agent in agriculture. These findings contribute to the development of novel antibacterial agents and underscore the importance of exploring natural products for agricultural disease management.

GMP Synthetase: Allostery, Structure, and Function

Glutamine amidotransferases (GATs) catalyze the hydrolysis of glutamine and transfer the generated ammonia to diverse metabolites. The two catalytic activities, glutaminolysis and the subsequent amination of the acceptor substrate, happen in two distinct catalytic pockets connected by a channel that facilitates the movement of ammonia. The de novo pathway for the synthesis of guanosine monophosphate (GMP) from xanthosine monophosphate (XMP) is enabled by the GAT GMP synthetase (GMPS). In most available crystal structures of GATs, the ammonia channel is evident in their native state or upon ligand binding, providing molecular details of the conduit. In addition, conformational changes that enable the coordination of the two catalytic chemistries are also informed by the available structures. In contrast, despite the first structure of a GMPS being published in 1996, the understanding of catalysis in the acceptor domain and inter-domain crosstalk became possible only after the structure of a glutamine-bound mutant of Plasmodium falciparum GMPS was determined. In this review, we present the current status of our understanding of the molecular basis of catalysis in GMPS, becoming the first comprehensive assessment of the biochemical function of this intriguing enzyme.

Tertiary and Quaternary Structure Organization in GMP Synthetases: Implications for Catalysis

Glutamine amidotransferases, enzymes that transfer nitrogen from Gln to various cellular metabolites, are modular, with the amidotransferase (GATase) domain hydrolyzing Gln, generating ammonia and the acceptor domain catalyzing the addition of nitrogen onto its cognate substrate. GMP synthetase (GMPS), an enzyme in the de novo purine nucleotide biosynthetic pathway, is a glutamine amidotransferase that catalyzes the synthesis of GMP from XMP. The reaction involves activation of XMP though adenylation by ATP in the ATP pyrophosphatase (ATPPase) active site, followed by channeling and attack of NH₃ generated in the GATase pocket. This complex chemistry entails co-ordination of activity across the active sites, allosteric activation of the GATase domain to modulate Gln hydrolysis and channeling of ammonia from the GATase to the acceptor active site. Functional GMPS dimers associate through the dimerization domain. The crystal structure of the Gln-bound complex of Plasmodium falciparum GMPS (PfGMPS) for the first time revealed large-scale domain rotation to be associated with catalysis and leading to the juxtaposition of two otherwise spatially distal cysteinyl (C113/C337) residues. In this manuscript, we report on an unusual structural variation in the crystal structure of the C89A/C113A PfGMPS double mutant, wherein a larger degree of domain rotation has led to the dissociation of the dimeric structure. Furthermore, we report a hitherto overlooked signature motif tightly related to catalysis.