Flux through trehalose synthase flows from trehalose to the alpha anomer of maltose in mycobacteria

Abstract

Trehalose synthase (TreS) was thought to catalyze flux from maltose to trehalose, a precursor of essential trehalose mycolates in mycobacterial cell walls. However, we now show, using a genetic approach, that TreS is not required for trehalose biosynthesis in Mycobacterium smegmatis, whereas two alternative trehalose-biosynthetic pathways (OtsAB and TreYZ) are crucial. Consistent with this direction of flux, trehalose levels in Mycobacterium tuberculosis decreased when TreS was overexpressed. In addition, TreS was shown to interconvert the α anomer of maltose and trehalose using (1)H and (19)F-nuclear magnetic resonance spectroscopies using its normal substrates and deoxyfluoromaltose analogs, with the nonenzymatic mutarotation of α/β-maltose being slow. Therefore, flux through TreS in mycobacteria flows from trehalose to α-maltose, which is the appropriate anomer for maltose kinase of the GlgE α-glucan pathway, which in turn contributes to intracellular and/or capsular polysaccharide biosynthesis.

Graphical abstract

Figure 1

Metabolism of Trehalose in Mycobacteria and Proposed Mechanism of TreS (A) All known metabolic pathways associated with trehalose in mycobacteria are shown, except for its hydrolysis by trehalase to form glucose as a carbon source for growth (Carroll et al., 2007). The questions addressed by this work are indicated in boxes. (B) Proposed catalytic mechanism of TreS with the most likely relative orientations of the glucose rings of trehalose and maltose. Hydrolysis would be expected to occur when water attacks the glucosyl-enzyme intermediate, generating a second glucose molecule with an α anomeric configuration.

Figure 2

The TreS Pathway Does Not Contribute to the De Novo Biosynthesis of Trehalose in Mycobacteria (A) Trehalose growth requirements in liquid culture of Mycobacterium smegmatis gene deletion mutants (see Figure S1 for how they were generated). Cultures were incubated for 48 hr at 37°C in Middlebrook 7H9 medium containing 0.5% (v/v) glycerol and 10% (v/v) albumin-dextrose-saline (ADS) enrichment containing either 0 or 50 μM trehalose. Values are means of triplicates ± SD. (B) Trehalose growth requirements on solid media of the trehalose-auxotrophic Mycobacterium smegmatis mutants ΔotsA(u)ΔtreYZ and ΔtreS(u)ΔotsA(u)ΔtreYZ. Cells were cultivated for 72 hr on Middlebrook 7H10 agar containing 0.5% (v/v) glycerol and 10% (v/v) ADS enrichment in the presence or absence of 100 μM trehalose. (C) Trehalose growth requirements in liquid culture of the trehalose-auxotrophic Mycobacterium smegmatis mutants ΔotsA(u)ΔtreYZ and ΔtreS(u)ΔotsA(u)ΔtreYZ. Growth conditions were essentially as described for (A) with 0–50 μM trehalose. (D) Maltose supplementation of trehalose auxotrophic Mycobacterium smegmatis gene deletion mutants. Growth conditions were essentially as described for (A) without trehalose but with 0–10 mM maltose. Values are means of triplicates ± SD. (E) Effect of treS deficiency and overexpression on intracellular trehalose concentration in Mycobacterium tuberculosis. Cultures of Mycobacterium tuberculosis H37Rv wild-type, the ΔtreS mutant and a treS overexpressing strain (wild type [WT] + treS) were incubated for 14 days in Middlebrook 7H9 medium containing 0.5% (v/v) glycerol and 10% (v/v) oleic acid-albumin-dextrose-catalase enrichment before enzymatically determining trehalose concentrations. Values are means of sextuplicates ± SD.

Figure 3

Mycobacterium Tuberculosis TreS Interconverts the α Anomer of Maltose (A) The conversion of trehalose (10 mM) into maltose by TreS (2 μM) according to ¹H-NMR spectroscopy (see Figures S2A–S2C for representative spectra and nonenzymic mutarotation controls). Each component was quantified by signal integration using citrate as an internal standard. Reaction mixtures contained 10% D₂O to assist spectrum acquisition without significant risk of introducing solvent kinetic and/or equilibrium isotope effects. This necessitated solvent suppression and the introduction of experimentally determined correction factors for the cosuppression of resonances that were close to the solvent resonance. (B) The conversion of pre-equilibrated α/β-maltose (10 mM) into trehalose by TreS (2 μM) according to ¹H-NMR spectroscopy. Note that enzyme was added immediately after the t = 0 data were acquired, resulting in a small and reproducible change in the apparent concentration of starting materials at the second recorded time point. See Figure S2D for the fit of the β-maltose curve (Hoops et al., 2006). The times taken to produce 2 mM trehalose and consume 50% of the α/β-maltose were 14 and 55 min, respectively. (C) Time courses of the ratios between the β and α anomers of maltose with different TreS concentrations during the conversion of pre-equilibrated α/β-maltose. The broken line indicates the equilibrium between the two anomers in these conditions. (D) Proposed reaction scheme to account for TreS-catalyzed reactions. Protons used to quantify components of the reaction mixtures by ¹H-NMR spectroscopy (Figure S2A) are indicated.

Figure 4

Mycobacterium Tuberculosis TreS Converts the α Anomers of Deoxyfluoromaltose Analogs (A) The conversion of pre-equilibrated 2-deoxy-2-fluoro-α/β-maltose (10 mM) by TreS (2 μM) was monitored using ¹⁹F-NMR spectroscopy (see Figure S3A for spectra). The times taken to produce 2 mM 2-deoxy-2-fluorotrehalose and consume 50% of the maltose analog were 25 and 125 min, respectively, both ∼2-fold longer than with maltose. (B) Corresponding data with 3-deoxy-3-fluoro-α/β-maltose (Figure S3B). The time taken to consume 50% of the maltose analog was 1,100 min, 20-fold longer than with maltose. No 3-deoxy-3-fluorotrehalose was detected. (C) Corresponding data with 6-deoxy-6-fluoro-α/β-maltose (Figure S3C). The times taken to produce 2 mM 6-deoxy-6-fluorotrehalose and consume 50% of the maltose analog were 2,500 and 1,500 min, respectively, 180- and 27-fold longer than with maltose. (D) Proposed reaction scheme to account for the conversion of deoxyfluoromaltoses by TreS.