ATP-phosphoribosyltransferase (ATPPRT) catalyses the first step in histidine biosynthesis, the condensation of ATP and 5-phospho--D-ribosyl-1-pyrophosphate to generate N 1 -(5-phospho--D-ribosyl)-ATP and inorganic pyrophosphate. The enzyme is allosterically inhibited by histidine. Two forms of ATPPRT, encoded by the hisG gene, exist in nature, depending on the species. The long form, HisGL, is a single polypeptide chain with catalytic and regulatory domains. The short form, HisGS, lacks a regulatory domain, and cannot bind histidine. HisGS instead is found in complex with a regulatory protein, HisZ, constituting the ATPPRT holoenzyme. HisZ triggers HisGS catalytic activity while rendering it sensitive to allosteric inhibition by histidine. Until recently, HisGS was thought to be catalytically inactive without HisZ. Here, recombinant HisGS and HisZ from the psychrophilic bacterium Psychrobacter arcticus were independently overexpressed and purified. The crystal structure of P. arcticus ATPPRT was solved at 2.34-Å resolution, revealing an equimolar HisGS-HisZ hetero-octamer. Steady-state kinetics indicate that both ATPPRT holoenzyme and HisGS are catalytically active. Surprisingly, HisZ confers only a modest 2-to 4-fold increase in kcat.Reaction profiles for both enzymes are indistinguishable by 31 P-NMR, indicating that the same reaction is catalysed. Temperature dependence of kcat shows deviation from Arrhenius behaviour at 308 K with the holoenzyme. Interestingly, such deviation is detected only at 313 K with HisGS. Thermal denaturation by CD spectroscopy resulted in Tm's of 312 K and 316 K for HisZ and HisGS, respectively, suggesting that HisZ renders the ATPPRT complex more thermolabile. This is the first characterisation of a psychrophilic ATPPRT. 4Adenosine 5ʹ-triphosphate phosphoribosyltransferase (ATPPRT) (EC 2.4.2.17) catalyses the reversible Mg 2+ -dependent reaction between adenosine 5ʹ-triphosphate (ATP) and(PR-ATP) and inorganic pyrophosphate (PPi) (Scheme 1), the first step in histidine biosynthesis. 1 The chemical equilibrium of the reaction strongly favours reactants, 2 and the enzyme is allosterically inhibited by histidine. 1 In addition to being a model for understanding allostery, 2-4 ATPPRT is of biotechnological interest as a tool for histidine production, provided that histidine feedback inhibition can be overcome. [5][6][7] Two forms of ATPPRT, encoded by the hisG gene, are found in nature. Fungi, plants, and most bacteria possess a long, homo-hexameric protein, HisGL, each subunit consisting of two domains that make up the catalytic core and a C-terminal regulatory domain that mediates feedback inhibition by histidine. 8 Some bacteria and archaea have a short version of the protein, HisGS, which lacks the C-terminal regulatory domain and is insensitive to histidine. In these organisms, a catalytically inactive regulatory protein, HisZ, the product of the hisZ gene, is present. 9 HisZ, which shares a common ancestry with histidyl-tRNA synthetase (HisRS), binds HisGS to form ...

Allosteric modulation of catalysis is a common regulatory strategy of flux-controlling biosynthetic enzymes. The enzyme ATP phosphoribosyltransferase (ATPPRT) catalyses the first reaction in histidine biosynthesis, the magnesium-dependent condensation of ATP and 5phospho--D-ribosyl-1-pyrophosphate (PRPP) to generate N 1-(5-phospho--D-ribosyl)-ATP (PRATP) and pyrophosphate (PPi). ATPPRT is allosterically inhibited by the final product of the pathway, histidine. Hetero-octameric ATPPRT consists of four catalytic subunits (HisGS) and four regulatory subunits (HisZ) engaged in intricate catalytic regulation. HisZ enhances HisGS catalysis in the absence of histidine while mediating allosteric inhibition in its presence. Here we report HisGS structures for the apoenzyme and complexes with substrates (PRPP, PRPP-ATP, PRPP-ADP), product (PRATP), and inhibitor (AMP), along with ATPPRT holoenzyme structures in complexes with substrates (PRPP, PRPP-ATP, PRPP-ADP) and product (PRATP). These ten crystal structures provide an atomic view of the catalytic cycle and allosteric activation of Psychrobacter arcticus ATPPRT. In both ternary complexes with PRPP-ATP, the adenine ring is found in an anticatalytic orientation, rotated 180° from the catalytic rotamer. Arg32 interacts with phosphate groups of ATP and PRPP, bringing the substrates in proximity for catalysis. The negative charge repulsion is further attenuated by a magnesium ion sandwiched between the and -phosphate groups of both substrates. HisZ binding to form the hetero-octamer brings HisGS subunits closer together in a tighter dimer in the Michaelis complex, which poises Arg56 from the adjacent HisGS molecule for crosssubunit stabilisation of the PPi leaving group at the transition state. The more electrostatically pre-organised active site of the holoenzyme likely minimises the reorganisation energy required to accommodate the transition state. This provides a structural basis for allosteric activation in which chemistry is accelerated by facilitating leaving group departure.

Current drugs to treat African sleeping sickness are inadequate and new therapies are urgently required. As part of a medicinal chemistry programme based upon the simplification of acetogenin-type ether scaffolds, we previously reported the promising trypanocidal activity of compound 1, a bis-tetrahydropyran 1,4-triazole (B-THP-T) inhibitor. This study aims to identify the protein target(s) of this class of compound in Trypanosoma brucei to understand its mode of action and aid further structural optimisation. We used compound 3, a diazirine- and alkyne-containing bi-functional photo-affinity probe analogue of our lead B-THP-T, compound 1, to identify potential targets of our lead compound in the procyclic form T. brucei. Bi-functional compound 3 was UV cross-linked to its target(s) in vivo and biotin affinity or Cy5.5 reporter tags were subsequently appended by Cu(II)-catalysed azide-alkyne cycloaddition. The biotinylated protein adducts were isolated with streptavidin affinity beads and subsequent LC-MSMS identified the FoF1-ATP synthase (mitochondrial complex V) as a potential target. This target identification was confirmed using various different approaches. We show that (i) compound 1 decreases cellular ATP levels (ii) by inhibiting oxidative phosphorylation (iii) at the FoF1-ATP synthase. Furthermore, the use of GFP-PTP-tagged subunits of the FoF1-ATP synthase, shows that our compounds bind specifically to both the α- and β-subunits of the ATP synthase. The FoF1-ATP synthase is a target of our simplified acetogenin-type analogues. This mitochondrial complex is essential in both procyclic and bloodstream forms of T. brucei and its identification as our target will enable further inhibitor optimisation towards future drug discovery. Furthermore, the photo-affinity labeling technique described here can be readily applied to other drugs of unknown targets to identify their modes of action and facilitate more broadly therapeutic drug design in any pathogen or disease model.

A series of novel bis-tetrahydropyran 1,4-triazole analogues based on the acetogenin framework display low micromolar trypanocidal activities towards both bloodstream and insect forms of Trypanosoma brucei, the causative agent of African sleeping sickness. A divergent synthetic strategy was adopted for the synthesis of the key tetrahydropyran intermediates to enable rapid access to diastereochemical variation either side of the 1,4-triazole core. The resulting diastereomeric analogues displayed varying degrees of trypanocidal activity and selectivity in structure activity relationship studies.

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