Crystal structures of TM0549 and NE1324--two orthologs of E. coli AHAS isozyme III small regulatory subunit

Preparation of a whole cell catalyst overexpressing acetohydroxyacid synthase of Thermotoga maritima and its application in the syntheses of α-hydroxyketones

The large catalytic subunit of acetohydroxyacid synthase (AHAS, EC 2.2.1.6) of Thermotoga maritima (TmcAHAS) was prepared in this study. It possesses high specific activity and excellent stability. The protein and a whole cell catalyst overexpressing the protein were applied to the preparation of α-hydroxyketones including acetoin (AC), 3-hydroxy-2-pentanone (HP), and (R)-phenylacetylcarbinol (R-PAC). The results show that AC and HP could be produced in high yields (84% and 62%, respectively), while R-PAC could be synthesized in a high yield (about 78%) with an R/S ratio of 9:1. Therefore, TmcAHAS and the whole cell catalyst overexpressing the protein could be practically useful bio-catalysts in the preparation of α-hydroxyketones including AC, HP, and R-PAC. To the best of our knowledge, this is the first time that bacterial AHAS was used as a catalyst to prepare HP with a good yield, and also the first time that TmcAHAS was employed to synthesize AC and R-PAC.

Structures of fungal and plant acetohydroxyacid synthases

Acetohydroxyacid synthase (AHAS), also known as acetolactate synthase, is a flavin adenine dinucleotide-, thiamine diphosphate- and magnesium-dependent enzyme that catalyses the first step in the biosynthesis of branched-chain amino acids¹. It is the target for more than 50 commercial herbicides². AHAS requires both catalytic and regulatory subunits for maximal activity and functionality. Here we describe structures of the hexadecameric AHAS complexes of Saccharomyces cerevisiae and dodecameric AHAS complexes of Arabidopsis thaliana. We found that the regulatory subunits of these AHAS complexes form a core to which the catalytic subunit dimers are attached, adopting the shape of a Maltese cross. The structures show how the catalytic and regulatory subunits communicate with each other to provide a pathway for activation and for feedback inhibition by branched-chain amino acids. We also show that the AHAS complex of Mycobacterium tuberculosis adopts a similar structure, thus demonstrating that the overall AHAS architecture is conserved across kingdoms.

Crystallographic Structures of IlvN·Val/Ile Complexes: Conformational Selectivity for Feedback Inhibition of Aceto Hydroxy Acid Synthases

Conformational factors that predicate selectivity for valine or isoleucine binding to IlvN leading to the regulation of aceto hydroxy acid synthase I (AHAS I) of Escherichia coli have been determined for the first time from high-resolution (1.9-2.43 Å) crystal structures of IlvN·Val and IlvN·Ile complexes. The valine and isoleucine ligand binding pockets are located at the dimer interface. In the IlvN·Ile complex, among residues in the binding pocket, the side chain of Cys43 is 2-fold disordered (χ1 angles of gauche- and trans). Only one conformation can be observed for the identical residue in the IlvN·Val complexes. In a reversal, the side chain of His53, located at the surface of the protein, exhibits two conformations in the IlvN·Val complex. The concerted conformational switch in the side chains of Cys43 and His53 may play an important role in the regulation of the AHAS I holoenzyme activity. A significant result is the establishment of the subunit composition in the AHAS I holoenzyme by analytical ultracentrifugation. Solution nuclear magnetic resonance and analytical ultracentrifugation experiments have also provided important insights into the hydrodynamic properties of IlvN in the ligand-free and -bound states. The structural and biophysical data unequivocally establish the molecular basis for differential binding of the ligands to IlvN and a rationale for the resistance of IlvM to feedback inhibition by the branched-chain amino acids.

Molecular evolution of acetohydroxyacid synthase in bacteria

Acetohydroxyacid synthase (AHAS) is the key enzyme in the biosynthetic pathways of branched chain amino acids in bacteria. Since it does not exist in animal and plant cells, AHAS is an attractive target for developing antimicrobials and herbicides. In some bacteria, there is a single copy of AHAS, while in others there are multiple copies. Therefore, it is necessary to investigate the origin and evolutionary pathway of various AHASs in bacteria. In this study, all the available protein sequences of AHAS in bacteria were investigated, and an evolutionary model of AHAS in bacteria is proposed, according to gene structure, organization and phylogeny. Multiple copies of AHAS in some bacteria might be evolved from the single copy of AHAS, the ancestor. Gene duplication, domain deletion and horizontal gene transfer might occur during the evolution of this enzyme. The results show the biological significance of AHAS, help to understand the functions of various AHASs in bacteria, and would be useful for developing industrial production strains of branched chain amino acids or novel antimicrobials.

Elucidating modes of activation and herbicide resistance by sequence assembly and molecular modelling of the Acetolactate synthase complex in sugarcane

Acetolactate synthase (ALS) catalyzes the first portion of the biosynthetic pathway leading to the generation of branched-chain amino acids. As such it is essential for plant health and is a major target for herbicides. ALS is a very poorly characterized molecule in sugarcane. The enzyme is activated and inhibited by a regulatory subunit (known as VAT1 in plants) whose mode of action is entirely unknown. Using Saccharum halepense as a template we have assembled the ALS gene of sugarcane (Saccharum hybrid) and have modelled the structure of ALS based on an Arabidopsis template (the first ALS model for a monocot). We have also assembled the ALS regulatory proteins (VAT1 and VAT2) from sugarcane and show that VAT2 is specific to true grasses. Employing a bacterial model, we have generated a structural model for VAT1, which explains why the separate domains of the proteins bind to either leucine or valine but not both. Using co-evolution studies we have determined molecular contacts by which we modelled the docking of VAT1 to ALS. In conclusion, we demonstrate how the binding of VAT1 to ALS activates ALS and show how VAT1 can also confer feedback inhibition to ALS. We validate our ALS model against biochemical data and employ this model to explain the function of a novel herbicide binding mutant in sugarcane.

Characterization of acetohydroxyacid synthase from the hyperthermophilic bacterium Thermotoga maritima

Acetohydroxyacid synthase (AHAS) is the key enzyme in branched chain amino acid biosynthesis pathway. The enzyme activity and properties of a highly thermostable AHAS from the hyperthermophilic bacterium Thermotoga maritima is being reported. The catalytic and regulatory subunits of AHAS from T. maritima were over-expressed in Escherichia coli. The recombinant subunits were purified using a simplified procedure including a heat-treatment step followed by chromatography. A discontinuous colorimetric assay method was optimized and used to determine the kinetic parameters. AHAS activity was determined to be present in several Thermotogales including T. maritima. The catalytic subunit of T. maritima AHAS was purified approximately 30-fold, with an AHAS activity of approximately 160±27 U/mg and native molecular mass of 156±6 kDa. The regulatory subunit was purified to homogeneity and showed no catalytic activity as expected. The optimum pH and temperature for AHAS activity were 7.0 and 85 °C, respectively. The apparent K_m and V_max for pyruvate were 16.4±2 mM and 246±7 U/mg, respectively. Reconstitution of the catalytic and regulatory subunits led to increased AHAS activity. This is the first report on characterization of an isoleucine, leucine, and valine operon (ilv operon) enzyme from a hyperthermophilic microorganism and may contribute to our understanding of the physiological pathways in Thermotogales. The enzyme represents the most active and thermostable AHAS reported so far.

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First report of AHAS from a hyperthermophilic bacterium.

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Catalytic and regulatory subunits of AHAS of T. maritima was expressed in E. coli.

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Recombinant proteins were purified using a simplified procedure.

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Enzyme represents the most active and thermostable AHAS reported so far.

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Kinetic parameters were determined for the purified recombinant enzyme

The minimum activation peptide from ilvH can activate the catalytic subunit of AHAS from different species

Acetohydroxyacid synthases (AHASs), which catalyze the first step in the biosynthesis of branched-chain amino acids, are composed of a catalytic subunit (CSU) and a regulatory subunit (RSU). The CSU harbors the catalytic site, and the RSU is responsible for the activation and feedback regulation of the CSU. Previous results from Chipman and co-workers and our lab have shown that heterologous activation can be achieved among isozymes of Escherichia coli AHAS. It would be interesting to find the minimum peptide of ilvH (the RSU of E. coli AHAS III) that could activate other E. coli CSUs, or even those of ## species. In this paper, C-terminal, N-terminal, and C- and N-terminal truncation mutants of ilvH were constructed. The minimum peptide to activate ilvI (the CSU of E. coli AHAS III) was found to be ΔN 14-ΔC 89. Moreover, this peptide could not only activate its homologous ilvI and heterologous ilvB (CSU of E. coli AHAS I), but also heterologously activate the CSUs of AHAS from Saccharomyces cerevisiae, Arabidopsis thaliana, and Nicotiana plumbaginifolia. However, this peptide totally lost its ability for feedback regulation by valine, thus suggesting different elements for enzymatic activation and feedback regulation. Additionally, the apparent dissociation constant (Kd ) of ΔN 14-ΔC 89 when binding CSUs of different species was found to be 9.3-66.5 μM by using microscale thermophoresis. The ability of this peptide to activate different CSUs does not correlate well with its binding ability (Kd ) to these CSUs, thus implying that key interactions by specific residues is more important than binding ability in promoting enzymatic reactions. The high sequence similarity of the peptide ΔN 14-ΔC 89 to RSUs across species hints that this peptide represents the minimum activation motif in RSU and that it regulates all AHASs.

Expression, purification and preliminary crystallographic analysis of Rv3002c, the regulatory subunit of acetolactate synthase (IlvH) from Mycobacterium tuberculosis

Branched amino-acid biosynthesis is important to bacterial pathogens such as Mycobacterium tuberculosis (Mtb), a microorganism that presently causes more deaths in humans than any other prokaryotic pathogen (http://www.who.int/tb). In this study, the molecular cloning, expression, purification, crystallization and preliminary crystallographic analysis of recombinant IlvH, the small regulatory subunit of acetohydroxylic acid synthase (AHAS) in Mtb, are reported. AHAS carries out the first common reaction in the biosynthesis of valine, leucine and isoleucine. AHAS is an essential enzyme in Mtb and its inactivation leads to a lethal phenotype [Sassetti et al. (2001), Proc. Natl Acad. Sci. USA, 98, 12712-12717]. Thus, inhibitors of AHAS could potentially be developed into novel anti-Mtb therapies.

Subunit-subunit interactions are weakened in mutant forms of acetohydroxy acid synthase insensitive to valine inhibition

In acetohydroxy acid synthase from Streptomyces cinnamonensis mutants affected in valine regulation, the impact of mutations on interactions between the catalytic and the regulatory subunits was examined using yeast two-hybrid system. Mutations in the catalytic and the regulatory subunits were projected into homology models of the respective proteins. Two changes in the catalytic subunit, E139A (alpha domain) and DeltaQ217 (beta domain), both located on the surface of the catalytic subunit dimer, lowered the interaction with the regulatory subunit. Three consecutive changes in the N-terminal part of the regulatory subunit were examined. Changes G16D and V17D in a loop and adjacent alpha-helix of ACT domain affected the interaction considerably, indicating that this region might be in contact with the catalytic subunit during allosteric regulation. In contrast, the adjacent mutation L18F did not influence the interaction at all. Thus, L18 might participate in valine binding or conformational change transfer within the regulatory subunits. Shortening of the regulatory subunit to 107 residues reduced the interaction essentially, suggesting that the C-terminal part of the regulatory subunit is also important for the catalytic subunit binding.

Structure and mechanism of inhibition of plant acetohydroxyacid synthase

Plants and microorganisms synthesize valine, leucine and isoleucine via a common pathway in which the first reaction is catalysed by acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This enzyme is of substantial importance because it is the target of several herbicides, including all members of the popular sulfonylurea and imidazolinone families. However, the emergence of resistant weeds due to mutations that interfere with the inhibition of AHAS is now a worldwide problem. Here we summarize recent ideas on the way in which these herbicides inhibit the enzyme, based on the 3D structure of Arabidopsis thaliana AHAS. This structure also reveals important clues for understanding how various mutations can lead to herbicide resistance.