Structural analysis of substrate-mimicking inhibitors in complex with Neisseria meningitidis 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase – The importance of accommodating the active site water

Role of Half-of-Sites Reactivity and Inter-Subunit Communications in DAHP Synthase Catalysis and Regulation

α-Carboxyketose synthases, including 3-deoxy-d-arabinoheptulosonate 7-phosphate synthase (DAHPS), are long-standing targets for inhibition. They are challenging targets to create tight-binding inhibitors against, and inhibitors often display half-of-sites binding and partial inhibition. Half-of-sites inhibition demonstrates the existence of inter-subunit communication in DAHPS. We used X-ray crystallography and spatially resolved hydrogen-deuterium exchange (HDX) to reveal the structural and dynamic bases for inter-subunit communication in Escherichia coli DAHPS(Phe), the isozyme that is feedback-inhibited by phenylalanine. Crystal structures of this homotetrameric (dimer-of-dimers) enzyme are invariant over 91% of its sequence. Three variable loops make up 8% of the sequence and are all involved in inter-subunit contacts across the tight-dimer interface. The structures have pseudo-twofold symmetry indicative of inter-subunit communication across the loose-dimer interface, with the diagonal subunits B and C always having the same conformation as each other, while subunits A and D are variable. Spatially resolved HDX reveals contrasting responses to ligand binding, which, in turn, affect binding of the second substrate, erythrose-4-phosphate (E4P). The N-terminal peptide, M1-E12, and the active site loop that binds E4P, F95-K105, are key parts of the communication network. Inter-subunit communication appears to have a catalytic role in all α-carboxyketose synthase families and a regulatory role in some members.

Investigation of Enzymes in the Phthalide Biosynthetic Pathway in Angelica sinensis Using Integrative Metabolite Profiles and Transcriptome Analysis

The roots of Angelica sinensis (Oliv.) Diels are well known for their efficacy in promoting blood circulation. Although many studies have indicated that phthalides are the main chemical components responsible for the pharmacological properties of A. sinensis, the phthalide biosynthetic pathway and enzymes that transform different phthalides are still poorly understood. We identified 108 potential candidate isoforms for phthalide accumulation using transcriptome and metabolite profile analyses. Then, six enzymes, including phospho-2-dehydro-3-deoxyheptonate aldolase 2, shikimate dehydrogenase, primary amine oxidase, polyphenol oxidase, tyrosine decarboxylase, and shikimate O-hydroxycinnamoyl transferase, were identified and proven to be involved in phthalide accumulation by heterologously expressing these proteins in Escherichia coli. We proposed a possible mechanism underlying phthalide transformation and biosynthetic pathways in A. sinensis based on our findings. The results of our study can provide valuable information for understanding the mechanisms underlying phthalide accumulation and transformation and enable further development of quality control during the cultivation of A. sinensis.

Computational investigations of allostery in aromatic amino acid biosynthetic enzymes

Allostery, in which binding of ligands to remote sites causes a functional change in the active sites, is a fascinating phenomenon observed in enzymes. Allostery can occur either with or without significant conformational changes in the enzymes, and the molecular basis of its mechanism can be difficult to decipher using only experimental techniques. Computational tools for analyzing enzyme sequences, structures, and dynamics can provide insights into the allosteric mechanism at the atomic level. Combining computational and experimental methods offers a powerful strategy for the study of enzyme allostery. The aromatic amino acid biosynthesis pathway is essential in microorganisms and plants. Multiple enzymes involved in this pathway are sensitive to feedback regulation by pathway end products and are known to use allostery to control their activities. To date, four enzymes in the aromatic amino acid biosynthesis pathway have been computationally investigated for their allosteric mechanisms, including 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase, anthranilate synthase, chorismate mutase, and tryptophan synthase. Here we review the computational studies and findings on the allosteric mechanisms of these four enzymes. Results from these studies demonstrate the capability of computational tools and encourage future computational investigations of allostery in other enzymes of this pathway.

Isolation and biochemical characterization of a metagenome-derived 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase gene from subtropical marine mangrove wetland sediments

3-Deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS) is a key rate-limiting enzyme in aromatic amino acid anabolism. A new I_β-type DAHPS gene (aro1A) was identified in a metagenomic library from subtropical marine mangrove sediment. The gene encoded a polypeptide composed of 272 amino acids and had a maximum similarity of 52.4% to a known DAHPS at the amino acid level. Multiple sequence alignment, homologous modeling, and molecular docking showed that Aro1A had the typical (β/α)₈ barrel-shaped catalytic structural domain of DAHPS. The motifs and amino acid residues involved in the combination of substrates and metal ligand were highly conservative with the known DAHPS. The putative DAHPS gene was subcloned into a pET-30a(+) vector and was overexpressed in Escherichia coli Rosetta (DE3) cells. The recombinant protein was purified to homogeneity. The maximum activity for the recombinant Aro1A protein occurred at pH 8.0 and 40 °C. Ba²⁺ and Ca²⁺ stimulated the activity of Aro1A protein. The enzyme showed high affinity and catalytic efficiency (K _m ^PEP  = 19.58 μM, V _max ^PEP   = 29.02 μM min^-1, and k _cat ^PEP /K _m ^PEP  = 0.88 s^-1 μM^-1) under optimal reaction conditions. The enzymatic property of Aro1A indicates its potential in aromatic amino acid industrial production.