Functional analysis of the acetic acid resistance (aar) gene cluster in Acetobacter aceti strain 1023

Vinegar production requires acetic acid bacteria that produce, tolerate, and conserve high levels of acetic acid. When ethanol is depleted, aerobic acetate overoxidation to carbon dioxide ensues. The resulting diauxic growth pattern has two logarithmic growth phases, the first associated with ethanol oxidation and the second associated with acetate overoxidation. The vinegar factory isolate Acetobacter aceti strain 1023 has a long intermediate stationary phase that persists at elevated acetic acid levels. Strain 1023 conserves acetic acid despite possessing a complete set of citric acid cycle (CAC) enzymes, including succinyl-CoA:acetate CoA-transferase (SCACT), the product of the acetic acid resistance (aar) gene aarC. In this study, cell growth and acid production were correlated with the functional expression of aargenes using reverse transcription-polymerase chain reaction, Western blotting, and enzyme activity assays. Citrate synthase (AarA) and SCACT (AarC) were abundant in A. aceti strain 1023 during both log phases, suggesting the transition to acetate overoxidation was not a simple consequence of CAC enzyme induction. A mutagenized derivative of strain 1023 lacking functional AarC readily oxidized ethanol but was unable to overoxidize acetate, indicating that the CAC is required for acetate overoxidation but not ethanol oxidation. The primary role of the aar genes in the metabolically streamlined industrial strain A. aceti 1023 appears to be to harvest energy via acetate overoxidation in otherwise depleted medium

You are lost without a map: Navigating the sea of protein structures

X-ray crystal structures propel biochemistry research like no other experimental method, since they answer many questions directly and inspire new hypotheses. Unfortunately, many users of crystallographic models mistake them for actual experimental data. Crystallographic models are interpretations, several steps removed from the experimental measurements, making it difficult for nonspecialists to assess the quality of the underlying data. Crystallographers mainly rely on “global” measures of data and model quality to build models. Robust validation procedures based on global measures now largely ensure that structures in the Protein Data Bank (PDB) are largely correct. However, global measures do not allow users of crystallographic models to judge the reliability of “local” features in a region of interest. Refinement of a model to fit into an electron density map requires interpretation of the data to produce a single “best” overall model. This process requires inclusion of most probable conformations in areas of poor density. Users who misunderstand this can be misled, especially in regions of the structure that are mobile, including active sites, surface residues, and especially ligands. This article aims to equip users of macromolecular models with tools to critically assess local model quality. Structure users should always check the agreement of the electron density map and the derived model in all areas of interest, even if the global statistics are good. We provide illustrated examples of interpreted electron density as a guide for those unaccustomed to viewing electron density

Metal stopping reagents facilitate discontinuous activity assays of the de novo purine biosynthesis enzyme PurE

The conversion of 5-aminoimidazole ribonucleotide (AIR) to 4-carboxy-AIR (CAIR) represents an unusual divergence in purine biosynthesis: microbes and nonmetazoan eukaryotes use class I PurEs while animals use class II PurEs. Class I PurEs are therefore a potential antimicrobial target; however, no enzyme activity assay is suitable for high throughput screening (HTS). Here we report a simple chemical quench that fixes the PurE substrate/product ratio for 24 h, as assessed by the Bratton-Marshall assay (BMA) for diazotizable amines. The ZnSO4 stopping reagent is proposed to chelate CAIR, enabling delayed analysis of this acid-labile product by BMA or other HTS method

Draft Genome Sequence of Acetobacter aceti Strain 1023, a Vinegar Factory Isolate

The genome sequence of Acetobacter aceti 1023, an acetic acid bacterium adapted to traditional vinegar fermentation, comprises 3.0 Mb (chromosome plus plasmids). A. aceti 1023 is closely related to the cocoa fermenter Acetobacter pasteurianus 386B but possesses many additional insertion sequence elements

Structural studies of tri-functional human GART

Human purine de novo synthesis pathway contains several multi-functional enzymes, one of which, tri-functional GART, contains three enzymatic activities in a single polypeptide chain. We have solved structures of two domains bearing separate catalytic functions: glycinamide ribonucleotide synthetase and aminoimidazole ribonucleotide synthetase. Structures are compared with those of homologous enzymes from prokaryotes and analyzed in terms of the catalytic mechanism. We also report small angle X-ray scattering models for the full-length protein. These models are consistent with the enzyme forming a dimer through the middle domain. The protein has an approximate seesaw geometry where terminal enzyme units display high mobility owing to flexible linker segments. This resilient seesaw shape may facilitate internal substrate/product transfer or forwarding to other enzymes in the pathway

Crystal Structures of <i>Acetobacter aceti</i> Succinyl-Coenzyme A (CoA):Acetate CoA-Transferase Reveal Specificity Determinants and Illustrate the Mechanism Used by Class I CoA-Transferases

Coenzyme A (CoA)-transferases catalyze transthioesterification reactions involving acyl-CoA substrates, using an active-site carboxylate to form covalent acyl anhydride and CoA thioester adducts. Mechanistic studies of class I CoA-transferases suggested that acyl-CoA binding energy is used to accelerate rate-limiting acyl transfers by compressing the substrate thioester tightly against the catalytic glutamate [White, H., and Jencks, W. P. (1976) <i>J. Biol. Chem. 251</i>, 1688–1699]. The class I CoA-transferase succinyl-CoA:acetate CoA-transferase is an acetic acid resistance factor (AarC) with a role in a variant citric acid cycle in <i>Acetobacter aceti</i>. In an effort to identify residues involved in substrate recognition, X-ray crystal structures of a C-terminally His₆-tagged form (AarCH6) were determined for several wild-type and mutant complexes, including freeze-trapped acetylglutamyl anhydride and glutamyl-CoA thioester adducts. The latter shows the acetate product bound to an auxiliary site that is required for efficient carboxylate substrate recognition. A mutant in which the catalytic glutamate was changed to an alanine crystallized in a closed complex containing dethiaacetyl-CoA, which adopts an unusual curled conformation. A model of the acetyl-CoA Michaelis complex demonstrates the compression anticipated four decades ago by Jencks and reveals that the nucleophilic glutamate is held at a near-ideal angle for attack as the thioester oxygen is forced into an oxyanion hole composed of Gly388 NH and CoA N2″. CoA is nearly immobile along its entire length during all stages of the enzyme reaction. Spatial and sequence conservation of key residues indicates that this mechanism is general among class I CoA-transferases

Functional dissection of the bipartite active site of the class I coenzyme A (CoA)-transferase succinyl-CoA:acetate CoA-transferase

Coenzyme A (CoA)-transferases catalyze the reversible transfer of CoA from acyl-CoA thioesters to free carboxylates. Class I CoA-transferases produce acylglutamyl anhydride intermediates that undergo attack by CoA thiolate on either the internal or external carbonyl carbon atoms, forming distinct tetrahedral intermediates less than 3 Å apart. In this study, crystal structures of succinyl-CoA:acetate CoA-transferase (AarC) from Acetobacter aceti are used to examine how the Asn347 carboxamide stabilizes the internal oxyanion intermediate. A structure of the active mutant AarC-N347A bound to CoA revealed both solvent replacement of the missing contact and displacement of the adjacent Glu294, indicating that Asn347 both polarizes and orients the essential glutamate. AarC was crystallized with the nonhydrolyzable acetyl-CoA (AcCoA) analogue dethiaacetyl-CoA (1a) in an attempt to trap a closed enzyme complex containing a stable analogue of the external oxyanion intermediate. One active site contained an acetylglutamyl anhydride adduct and truncated 1a, an unexpected result hinting at an unprecedented cleavage of the ketone moiety in 1a. Solution studies confirmed that 1a decomposition is accompanied by production of near-stoichiometric acetate, in a process that seems to depend on microbial contamination but not AarC. A crystal structure of AarC bound to the postulated 1a truncation product (2a) showed complete closure of one active site per dimer but no acetylglutamyl anhydride, even with acetate added. These findings suggest that an activated acetyl donor forms during 1a decomposition; a working hypothesis involving ketone oxidation is offered. The ability of 2a to induce full active site closure furthermore suggests that it subverts a system used to impede inappropriate active site closure on unacylated CoA